Functional Reactive Bindings

In their simplest form, bindings provide the illusion that two objects have the same property. Changing the property on one object causes the same change in the other. This is useful for coordinating state between views and models, among other entangled objects. For example, if you enter text into a text field, the same text might be added to the corresponding database record.

bind(object, "a.b", {"<->": "c.d"});

Functional Reactive Bindings go farther. They can gracefully bind long property paths and the contents of collections. They can also incrementally update the results of chains of queries including maps, flattened arrays, sums, and averages. They can also add and remove elements from sets based on the changes to a flag. FRB makes it easy to incrementally ensure consistent state.

bind(company, "payroll", {"<-": "departments.map{employees.sum{salary}}.sum()"});
bind(document, "body.classList.has('dark')", {"<-": "darkMode", source: viewModel});

FRB is built from a combination of powerful functional and generic building blocks, making it reliable, easy to extend, and easy to maintain.

Getting Started

frb is a CommonJS package, with JavaScript modules suitable for use with Node.js on the server side or Mr on the client side.

❯ npm install frb

Tutorial

In this example, we bind model.content to document.body.innerHTML.

var bind = require("frb/bind");
var model = {content: "Hello, World!"};
var cancelBinding = bind(document, "body.innerHTML", {
    "<-": "content",
    "source": model
});

When a source property is bound to a target property, the target gets reassigned to the source any time the source changes.

model.content = "Farewell.";
expect(document.body.innerHTML).toBe("Farewell.");

Bindings can be recursively detached from the objects they observe with the returned cancel function.

cancelBinding();
model.content = "Hello again!"; // doesn't take
expect(document.body.innerHTML).toBe("Farewell.");

Two-way Bindings

Bindings can go one way or in both directions. Declare one-way bindings with the <- property, and two-way bindings with the <-> property.

In this example, the "foo" and "bar" properties of an object will be inexorably intertwined.

var object = {};
var cancel = bind(object, "foo", {"<->": "bar"});

// <-
object.bar = 10;
expect(object.foo).toBe(10);

// ->
object.foo = 20;
expect(object.bar).toBe(20);

Right-to-left

Note that even with a two-way binding, the right-to-left binding precedes the left-to-right. In this example, "foo" and "bar" are bound together, but both have initial values.

var object = {foo: 10, bar: 20};
var cancel = bind(object, "foo", {"<->": "bar"});
expect(object.foo).toBe(20);
expect(object.bar).toBe(20);

The right-to-left assignment of bar to foo happens first, so the initial value of foo gets lost.

Properties

Bindings can follow deeply nested chains, on both the left and the right side.

In this example, we have two object graphs, foo, and bar, with the same structure and initial values. This binds bar.a.b to foo.a.b and also the other way around.

var foo = {a: {b: 10}};
var bar = {a: {b: 10}};
var cancel = bind(foo, "a.b", {
    "<->": "a.b",
    source: bar
});
// <-
bar.a.b = 20;
expect(foo.a.b).toBe(20);
// ->
foo.a.b = 30;
expect(bar.a.b).toBe(30);

Structure changes

Changes to the structure of either side of the binding are no matter. All of the orphaned event listeners will automatically be canceled, and the binders and observers will reattach to the new object graph.

Continuing from the previous example, we store and replace the a object from one side of the binding. The old b property is now orphaned, and the old b property adopted in its place.

var a = foo.a;
expect(a.b).toBe(30); // from before

foo.a = {}; // orphan a and replace
foo.a.b = 40;
// ->
expect(bar.a.b).toBe(40); // updated

bar.a.b = 50;
// <-
expect(foo.a.b).toBe(50); // new one updated
expect(a.b).toBe(30); // from before it was orphaned

Strings

String concatenation is straightforward.

var object = {name: "world"};
bind(object, "greeting", {"<-": "'hello ' + name + '!'"});
expect(object.greeting).toBe("hello world!");

Sum

Some advanced queries are possible with one-way bindings from collections. FRB updates sums incrementally. When values are added or removed from the array, the sum of only those values is taken and added or removed from the last known sum.

var object = {array: [1, 2, 3]};
bind(object, "sum", {"<-": "array.sum()"});
expect(object.sum).toEqual(6);

Average

The arithmetic mean of a collection can be updated incrementally. Each time the array changes, the added and removed values adjust the last known sum and count of values in the array.

var object = {array: [1, 2, 3]};
bind(object, "average", {"<-": "array.average()"});
expect(object.average).toEqual(2);

Rounding

The round, floor, and ceil methods operate on numbers and return the nearest integer, the nearest integer toward -infinity, and the nearest integer toward infinity respectively.

var object = {number: -0.5};
Bindings.defineBindings(object, {
    "round": {"<-": "number.round()"},
    "floor": {"<-": "number.floor()"},
    "ceil": {"<-": "number.ceil()"}
});
expect(object.round).toBe(0);
expect(object.floor).toBe(-1);
expect(object.ceil).toBe(0);

Last

FRB provides an operator for watching the last value in an Array.

var array = [1, 2, 3];
var object = {array: array, last: null};
Bindings.defineBinding(object, "last", {"<-": "array.last()"});
expect(object.last).toBe(3);

array.push(4);
expect(object.last).toBe(4);

When the dust settles, array.last() is equivalent to array[array.length - 1], but the last observer guarantees that it will not jitter between the ultimate value and null or the penultimate value of the collection. With array[array.length], the underlying may not change its content and length atomically.

var changed = jasmine.createSpy();
PropertyChanges.addOwnPropertyChangeListener(object, "last", changed);
array.unshift(0);
array.splice(3, 0, 3.5);
expect(object.last).toBe(4);
expect(changed).not.toHaveBeenCalled();

array.pop();
expect(object.last).toBe(3);

array.clear();
expect(object.last).toBe(null);

Only

FRB provides an only operator, which can either observe or bind the only element of a collection. The only observer watches a collection for when there is only one value in that collection and emits that value.. If there are multiple values, it emits null.

var object = {array: [], only: null};
Bindings.defineBindings(object, {
    only: {"<->": "array.only()"}
});

object.array = [1];
expect(object.only).toBe(1);

object.array.pop();
expect(object.only).toBe(undefined);

object.array = [1, 2, 3];
expect(object.only).toBe(undefined);

The only binder watches a value. When the value is null, it does nothing. Otherwise, it will update the bound collection such that it only contains that value. If the collection was empty, it adds the value. Otherwise, if the collection did not have the value, it replaces the collection's content with the one value. Otherwise, it removes everything but the value it already contains. Regardless of the means, the end result is the same. If the value is non-null, it will be the only value in the collection.

object.only = 2;
expect(object.array.slice()).toEqual([2]);
// Note that slice() is necessary only because the testing scaffold
// does not consider an observable array equivalent to a plain array
// with the same content

object.only = null;
object.array.push(3);
expect(object.array.slice()).toEqual([2, 3]);

One

Like the only operator, there is also a one operator. The one operator will observe one value from a collection, whatever value is easiest to obtain. For an array, it's the first value; for a sorted set, it's whatever value was most recently found or added; for a heap, it's whatever is on top. However, if the collection is null, undefined, or empty, the result is undefined.

var object = {array: [], one: null};
Bindings.defineBindings(object, {
    one: {"<-": "array.one()"}
});

expect(object.one).toBe(undefined);

object.array.push(1);
expect(object.one).toBe(1);

// Still there...
object.array.push(2);
expect(object.one).toBe(1);

Unlike only, one is not bindable.

Map

You can also create mappings from one array to a new array and an expression to evaluate on each value. The mapped array is bound once, and all changes to the source array are incrementally updated in the target array.

var object = {objects: [
    {number: 10},
    {number: 20},
    {number: 30}
]};
bind(object, "numbers", {"<-": "objects.map{number}"});
expect(object.numbers).toEqual([10, 20, 30]);
object.objects.push({number: 40});
expect(object.numbers).toEqual([10, 20, 30, 40]);

Any function, like sum or average, can be applied to the result of a mapping. The straight-forward path would be objects.map{number}.sum(), but you can use a block with any function as a short hand, objects.sum{number}.

Filter

A filter block generates an incrementally updated array filter. The resulting array will contain only those elements from the source array that pass the test deescribed in the block. As values of the source array are added, removed, or changed such that they go from passing to failing or failing to passing, the filtered array gets incrementally updated to include or exclude those values in their proper positions, as if the whole array were regenerated with array.filter by brute force.

var object = {numbers: [1, 2, 3, 4, 5, 6]};
bind(object, "evens", {"<-": "numbers.filter{!(%2)}"});
expect(object.evens).toEqual([2, 4, 6]);
object.numbers.push(7, 8);
object.numbers.shift();
object.numbers.shift();
expect(object.evens).toEqual([4, 6, 8]);

Scope

In a binding, there is always a value in scope. It is the implicit value for looking up properties and for applying operators, like methods. The value in scope can be called out explicitly as this. On the left side, the value in scope is called the target, on the right it is called the source.

Each scope has a this value and may have a parent scope. Inside a map block, like the number in numbers.map{number}, the value in scope is one of the numbers, and the value in the parent scope is an object with a numbers property. To access the value in a parent scope, use the parent scope operator, ^.

Suppose you have an object with numbers and maxNumber properties. In this example, we bind a property, smallNumbers to an array of all the numbers less than or equal to the maxNumber.

var object = Bindings.defineBindings({
    numbers: [1, 2, 3, 4, 5],
    maxNumber: 3
}, {
    smallNumbers: {
        "<-": "numbers.filter{this <= ^maxNumber}"
    }
});

Keywords like this overlap with the notation normally used for properties of this. If an object has a this property, you may use the notation .this, this.this, or this['this']. .this is the normal form.

var object = Bindings.defineBindings({
    "this": 10
}, {
    that: {"<-": ".this"}
});
expect(object.that).toBe(object["this"]);

The only other FRB keywords that collide with propery names are true, false, and null, and the same technique for disambiguation applies.

Some and Every

A some block incrementally tracks whether some of the values in a collection meet a criterion.

var object = Bindings.defineBindings({
    options: [
        {checked: true},
        {checked: false},
        {checked: false}
    ]
}, {
    anyChecked: {
        "<-": "options.some{checked}"
    }
});
expect(object.anyChecked).toBe(true);

An every block incrementally tracks whether all of the values in a collection meet a criterion.

var object = Bindings.defineBindings({
    options: [
        {checked: true},
        {checked: false},
        {checked: false}
    ]
}, {
    allChecked: {
        "<-": "options.every{checked}"
    }
});
expect(object.allChecked).toBe(false);

You can use a two-way binding on some and every blocks.

var object = Bindings.defineBindings({
    options: [
        {checked: true},
        {checked: false},
        {checked: false}
    ]
}, {
    allChecked: {
        "<->": "options.every{checked}"
    },
    noneChecked: {
        "<->": "!options.some{checked}"
    }
});

object.noneChecked = true;
expect(object.options.every(function (option) {
    return !option.checked
}));

object.allChecked = true;
expect(object.noneChecked).toBe(false);

The caveat of an equals binding applies. If the condition for every element of the collection is set to true, the condition will be bound incrementally to true on each element. When the condition is set to false, the binding will simply be canceled.

object.allChecked = false;
expect(object.options.every(function (option) {
    return option.checked; // still checked
}));

Sorted

A sorted block generates an incrementally updated sorted array. The resulting array will contain all of the values from the source except in sorted order.

var object = {numbers: [5, 2, 7, 3, 8, 1, 6, 4]};
bind(object, "sorted", {"<-": "numbers.sorted{}"});
expect(object.sorted).toEqual([1, 2, 3, 4, 5, 6, 7, 8]);

The block may specify a property or expression by which to compare values.

var object = {arrays: [[1, 2, 3], [1, 2], [], [1, 2, 3, 4], [1]]};
bind(object, "sorted", {"<-": "arrays.sorted{-length}"});
expect(object.sorted.map(function (array) {
    return array.slice(); // to clone
})).toEqual([
    [1, 2, 3, 4],
    [1, 2, 3],
    [1, 2],
    [1],
    []
]);

The sorted binding responds to changes to the sorted property by removing them at their former place and adding them back at their new position.

object.arrays[0].push(4, 5);
expect(object.sorted.map(function (array) {
    return array.slice(); // to clone
})).toEqual([
    [1, 2, 3, 4, 5], // new
    [1, 2, 3, 4],
    // old
    [1, 2],
    [1],
    []
]);

Unique and Sorted

FRB can create a sorted index of unique values using sortedSet blocks.

var object = Bindings.defineBindings({
    folks: [
        {id: 4, name: "Bob"},
        {id: 2, name: "Alice"},
        {id: 3, name: "Bob"},
        {id: 1, name: "Alice"},
        {id: 1, name: "Alice"} // redundant
    ]
}, {
    inOrder: {"<-": "folks.sortedSet{id}"},
    byId: {"<-": "folks.map{[id, this]}.toMap()"},
    byName: {"<-": "inOrder.toArray().group{name}.toMap()"}
});

expect(object.inOrder.toArray()).toEqual([
    object.byId.get(1),
    object.byId.get(2),
    object.byId.get(3),
    object.byId.get(4)
]);

expect(object.byName.get("Alice")).toEqual([
    object.byId.get(1),
    object.byId.get(2)
]);

The outcome is a SortedSet data structure, not an Array. The sorted set is useful for fast lookups, inserts, and deletes on sorted, unique data. If you would prefer a sorted array of unique values, you can combine other operators to the same effect.

var object = Bindings.defineBindings({
    folks: [
        {id: 4, name: "Bob"},
        {id: 2, name: "Alice"},
        {id: 3, name: "Bob"},
        {id: 1, name: "Alice"},
        {id: 1, name: "Alice"} // redundant
    ]
}, {
    index: {"<-": "folks.group{id}.sorted{.0}.map{.1.last()}"}
});

expect(object.index).toEqual([
    {id: 1, name: "Alice"},
    {id: 2, name: "Alice"},
    {id: 3, name: "Bob"},
    {id: 4, name: "Bob"}
]);

Min and Max

A binding can observe the minimum or maximum of a collection. FRB uses a binary heap internally to incrementally track the minimum or maximum value of the collection.

var object = Bindings.defineBindings({}, {
    min: {"<-": "values.min()"},
    max: {"<-": "values.max()"}
});

expect(object.min).toBe(undefined);
expect(object.max).toBe(undefined);

object.values = [2, 3, 2, 1, 2];
expect(object.min).toBe(1);
expect(object.max).toBe(3);

object.values.push(4);
expect(object.max).toBe(4);

Min and max blocks accept an expression on which to compare values from the collection.

var object = Bindings.defineBindings({}, {
    loser: {"<-": "rounds.min{score}.player"},
    winner: {"<-": "rounds.max{score}.player"}
});

object.rounds = [
    {score: 0, player: "Luke"},
    {score: 100, player: "Obi Wan"},
    {score: 250, player: "Vader"}
];
expect(object.loser).toEqual("Luke");
expect(object.winner).toEqual("Vader");

object.rounds[1].score = 300;
expect(object.winner).toEqual("Obi Wan");

Group

FRB can incrementally track equivalence classes within in a collection. The group block accepts an expression that determines the equivalence class for each object in a collection. The result is a nested data structure: an array of [key, class] pairs, where each class is itself an array of all members of the collection that have the corresponding key.

var store = Bindings.defineBindings({}, {
    "clothingByColor": {"<-": "clothing.group{color}"}
});
store.clothing = [
    {type: 'shirt', color: 'blue'},
    {type: 'pants', color: 'red'},
    {type: 'blazer', color: 'blue'},
    {type: 'hat', color: 'red'}
];
expect(store.clothingByColor).toEqual([
    ['blue', [
        {type: 'shirt', color: 'blue'},
        {type: 'blazer', color: 'blue'}
    ]],
    ['red', [
        {type: 'pants', color: 'red'},
        {type: 'hat', color: 'red'}
    ]]
]);

Tracking the positions of every key and every value in its equivalence class can be expensive. Internally, group blocks are implemented with a groupMap block followed by an entries() observer. The groupMap produces a Map data structure and does not waste any time, but does not produce range change events. The entries() observer projects the map of classes into the nested array data structure.

You can use the groupMap block directly.

Bindings.cancelBinding(store, "clothingByColor");
Bindings.defineBindings(store, {
    "clothingByColor": {"<-": "clothing.groupMap{color}"}
});
var blueClothes = store.clothingByColor.get('blue');
expect(blueClothes).toEqual([
    {type: 'shirt', color: 'blue'},
    {type: 'blazer', color: 'blue'}
]);

store.clothing.push({type: 'gloves', color: 'blue'});
expect(blueClothes).toEqual([
    {type: 'shirt', color: 'blue'},
    {type: 'blazer', color: 'blue'},
    {type: 'gloves', color: 'blue'}
]);

The group and groupMap blocks both respect the type of the source collection. If instead of an array you were to use a SortedSet, the equivalence classes would each be sorted sets. This is useful because replacing values in a sorted set can be performed with much less waste than with a large array.

View

Suppose that your source is a large data store, like a SortedSet from the Collections package. You might need to view a sliding window from that collection as an array. The view binding reacts to changes to the collection and the position and length of the window.

var SortedSet = require("collections/sorted-set");
var controller = {
    index: SortedSet([1, 2, 3, 4, 5, 6, 7, 8]),
    start: 2,
    length: 4
};
var cancel = bind(controller, "view", {
    "<-": "index.view(start, length)"
});

expect(controller.view).toEqual([3, 4, 5, 6]);

// change the window length
controller.length = 3;
expect(controller.view).toEqual([3, 4, 5]);

// change the window position
controller.start = 5;
expect(controller.view).toEqual([6, 7, 8]);

// add content behind the window
controller.index.add(0);
expect(controller.view).toEqual([5, 6, 7]);

Enumerate

An enumeration observer produces [index, value] pairs. You can bind to the index or the value in subsequent stages. The prefix dot distinguishes the zeroeth property from the literal zero.

var object = {letters: ['a', 'b', 'c', 'd']};
bind(object, "lettersAtEvenIndexes", {
    "<-": "letters.enumerate().filter{!(.0 % 2)}.map{.1}"
});
expect(object.lettersAtEvenIndexes).toEqual(['a', 'c']);
object.letters.shift();
expect(object.lettersAtEvenIndexes).toEqual(['b', 'd']);

Range

A range observes a given length and produces and incrementally updates an array of consecutive integers starting with zero with that given length.

var object = Bindings.defineBinding({}, "stack", {
    "<-": "&range(length)"
});
expect(object.stack).toEqual([]);

object.length = 3;
expect(object.stack).toEqual([0, 1, 2]);

object.length = 1;
expect(object.stack).toEqual([0]);

Flatten

You can flatten nested arrays. In this example, we have an array of arrays and bind it to a flat array.

var arrays = [[1, 2, 3], [4, 5, 6]];
var object = {};
bind(object, "flat", {
    "<-": "flatten()",
    source: arrays
});
expect(object.flat).toEqual([1, 2, 3, 4, 5, 6]);

Note that changes to the inner and outer arrays are both projected into the flattened array.

arrays.push([7, 8, 9]);
arrays[0].unshift(0);
expect(object.flat).toEqual([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

Also, as with all other bindings that produce arrays, the flattened array is never replaced, just incrementally updated.

var flat = object.flat;
arrays.splice(0, arrays.length);
expect(object.flat).toBe(flat); // === same object

Concat

You can observe the concatenation of collection of dynamic arrays.

var object = Bindings.defineBinding({
    head: 10,
    tail: [20, 30]
}, "flat", {
    "<-": "[head].concat(tail)"
});
expect(object.flat).toEqual([10, 20, 30]);

The underlying mechanism is equivalent to [[head], tail].flatten().

Reversed

You can bind the reversal of an array.

var object = {forward: [1, 2, 3]};
bind(object, "backward", {
    "<->": "forward.reversed()"
});
expect(object.backward.slice()).toEqual([3, 2, 1]);
object.forward.push(4);
expect(object.forward.slice()).toEqual([1, 2, 3, 4]);
expect(object.backward.slice()).toEqual([4, 3, 2, 1]);

Note that you can do two-way bindings, <-> with reversed arrays. Changes to either side are updated to the opposite side.

object.backward.pop();
expect(object.backward.slice()).toEqual([4, 3, 2]);
expect(object.forward.slice()).toEqual([2, 3, 4]);

Has

You can bind a property to always reflect whether a collection contains a particular value.

var object = {
    haystack: [1, 2, 3],
    needle: 3
};
bind(object, "hasNeedle", {"<-": "haystack.has(needle)"});
expect(object.hasNeedle).toBe(true);
object.haystack.pop(); // 3 comes off
expect(object.hasNeedle).toBe(false);

The binding also reacts to changes to the value you seek.

// Continued from above...
object.needle = 2;
expect(object.hasNeedle).toBe(true);

has bindings are not incremental, but with the right data-structure, updates are cheap. The Collections package contains Lists, Sets, and OrderedSets that all can send ranged content change notifications and thus can be bound.

// Continued from above...
var Set = require("collections/set");
object.haystack = new Set([1, 2, 3]);
expect(object.hasNeedle).toBe(true);

Likewise, Maps implement addMapChangeListener, so you can use a has binding to observe whether an entry exists with the given key.

// Continued from above...
var Map = require("collections/map");
object.haystack = new Map([[1, "a"], [2, "b"]]);
object.needle = 2;
expect(object.hasNeedle).toBe(true);
object.needle = 3;
expect(object.hasNeedle).toBe(false);

has bindings can also be left-to-right and bi-directional.

bind(object, "hasNeedle", {"<->": "haystack.has(needle)"});
object.hasNeedle = false;
expect(object.haystack.has(2)).toBe(false);

The collection on the left-hand-side must implement has or contains, add, and delete or remove. FRB shims Array to have has, add, and delete, just like all the collections in Collections. It happens that the classList properties of DOM elements, when they are supported, implement add, remove, and contains.

var model = {darkMode: false};
bind(document.body, "classList.has('dark')", {
    "<-": "darkMode",
    source: model
});

The DOM classList does not however implement addRangeChangeListener or removeRangeChangeListener, so it cannot be used on the right-hand-side of a binding, and such bindings cannot be bidirectional. With some DOM Mutation Observers, you might be able to help FRB overcome this limitation in the future.

Get

A binding can observe changes in key-to-value mappings in arrays and map Collections.

var object = {
    array: [1, 2, 3],
    second: null
};
var cancel = bind(object, "second", {
    "<->": "array.get(1)"
});
expect(object.array.slice()).toEqual([1, 2, 3]);
expect(object.second).toBe(2);

object.array.shift();
expect(object.array.slice()).toEqual([2, 3]);
expect(object.second).toBe(3);

object.second = 4;
expect(object.array.slice()).toEqual([2, 4]);

cancel();
object.array.shift();
expect(object.second).toBe(4); // still

The source collection can be a Map, Dict, MultiMap, SortedMap, SortedArrayMap, or anything that implements get and addMapChangeListener as specified in Collections. The key can also be a variable.

var Map = require("collections/map");
var a = {id: 0}, b = {id: 1};
var object = {
    source: new Map([[a, 10], [b, 20]]),
    key: null,
    selected: null
};

var cancel = bind(object, "selected", {
    "<-": "source.get(key)"
});
expect(object.selected).toBe(undefined);

object.key = a;
expect(object.selected).toBe(10);

object.key = b;
expect(object.selected).toBe(20);

object.source.set(b, 30);
expect(object.selected).toBe(30);

var SortedMap = require("collections/sorted-map");
object.source = SortedMap();
expect(object.selected).toBe(undefined);

object.source.set(b, 40);
expect(object.selected).toBe(40);

cancel();
object.key = a; // no effect
expect(object.selected).toBe(40);

You can also bind the entire content of a map-like collection to the content of another. Bear in mind that the content of the source replaces the content of the target initially.

var Map = require("collections/map");
var object = {
    a: new Map({a: 10}),
    b: new Map()
};
var cancel = bind(object, "a.mapContent()", {"<->": "b.mapContent()"});
expect(object.a.toObject()).toEqual({});
expect(object.b.toObject()).toEqual({});

object.a.set('a', 10);
expect(object.a.toObject()).toEqual({a: 10});
expect(object.b.toObject()).toEqual({a: 10});

object.b.set('b', 20);
expect(object.a.toObject()).toEqual({a: 10, b: 20});
expect(object.b.toObject()).toEqual({a: 10, b: 20});

In this case, the source of the binding is a different object than the target, so the binding descriptor specifies the alternate source.

Keys, Values, Entries

If the source of a binding is a map, FRB can also translate changes to the map into changes on an array. The keys, values, and entries observers produce incrementally updated projections of the key-value-mappings onto an array.

var Map = require("collections/map");
var object = Bindings.defineBindings({}, {
    keys: {"<-": "map.keysArray()"},
    values: {"<-": "map.valuesArray()"},
    entries: {"<-": "map.entriesArray()"}
});
object.map = new Map({a: 10, b: 20, c: 30});
expect(object.keys).toEqual(['a', 'b', 'c']);
expect(object.values).toEqual([10, 20, 30]);
expect(object.entries).toEqual([['a', 10], ['b', 20], ['c', 30]]);

object.map.set('d', 40);
object.map.delete('a');
expect(object.keys).toEqual(['b', 'c', 'd']);
expect(object.values).toEqual([20, 30, 40]);
expect(object.entries).toEqual([['b', 20], ['c', 30], ['d', 40]]);

Coerce to Map

Records (Objects with a fixed shape), arrays of entries, and Maps themselves can be coerced to an incrementally updated Map with the toMap operator.

var object = Bindings.defineBindings({}, {
    map: {"<-": "entries.toMap()"}
});

// map property will persist across changes to entries
var map = object.map;
expect(map).not.toBe(null);

object.entries = {a: 10};
expect(map.keysArray()).toEqual(['a']);
expect(map.has('a')).toBe(true);
expect(map.get('a')).toBe(10);

The toMap observer maintains the insertion order of the keys.

// Continued...
object.entries = [['b', 20], ['c', 30]];
expect(map.keysArray()).toEqual(['b', 'c']);

object.entries.push(object.entries.shift());
expect(map.keysArray()).toEqual(['c', 'b']);

If the entries do not have unique keys, the last entry wins. This is managed internally by observing, entries.group{.0}.map{.1.last()}.

// Continued...
object.entries = [['a', 10], ['a', 20]];
expect(map.get('a')).toEqual(20);
object.entries.pop();
expect(map.get('a')).toEqual(10);

toMap binds the content of the output map to the content of the input map and will clear and repopulate the output map if the input map is replaced.

// Continued...
object.entries = new Map({a: 10});
expect(map.keysArray()).toEqual(['a']);

Equals

You can bind to whether expressions are equal.

var fruit = {apples: 1, oranges: 2};
bind(fruit, "equal", {"<-": "apples == oranges"});
expect(fruit.equal).toBe(false);
fruit.orange = 1;
expect(fruit.equal).toBe(true);

Equality can be bound both directions. In this example, we do a two-way binding between whether a radio button is checked and a corresponding value in our model.

var component = {
    orangeElement: {checked: false},
    appleElement: {checked: true}
};
Bindings.defineBindings(component, {
    "orangeElement.checked": {"<->": "fruit == 'orange'"},
    "appleElement.checked": {"<->": "fruit == 'apple'"},
});

component.orangeElement.checked = true;
expect(component.fruit).toEqual("orange");

component.appleElement.checked = true;
expect(component.fruit).toEqual("apple");

Because equality and assignment are interchanged in this language, you can use either = or ==.

FRB also supports a comparison operator, <=>, which uses Object.compare to determines how two operands should be sorted in relation to each other.

Array and Map Content

In JavaScript, arrays behave both like objects (in the sense that every index is a property, but also like a map collection of index-to-value pairs. The Collections package goes so far as to patch up the Array prototype so arrays can masquerade as maps, with the caveat that delete(value) behaves like a Set instead of a Map.

This duplicity is reflected in FRB. You can access the values in an array using the object property notation or the mapped key notation.

var object = {
    array: [1, 2, 3]
};
Bindings.defineBindings(object, {
    first: {"<-": "array.0"},
    second: {"<-": "array.get(1)"}
});
expect(object.first).toBe(1);
expect(object.second).toBe(2);

To distinguish a numeric property of the source from a number literal, use a dot. To distingish a mapped index from an array literal, use an empty expression.

var array = [1, 2, 3];
var object = {};
Bindings.defineBindings(object, {
    first: {
        "<-": ".0",
        source: array
    },
    second: {
        "<-": "get(1)",
        source: array
    }
});
expect(object.first).toBe(1);
expect(object.second).toBe(2);

Unlike property notation, map notation can observe a variable index.

var object = {
    array: [1, 2, 3],
    index: 0
};
Bindings.defineBinding(object, "last", {
    "<-": "array.get(array.length - 1)"
});
expect(object.last).toBe(3);

object.array.pop();
expect(object.last).toBe(2);

You can also bind all of the content of an array by range or by mapping. The notation for binding ranged content is rangeContent(). Every change to an Array or SortedSet dispatches range changes and any collection that implements splice and swap can be a target for such changes.

var SortedSet = require("collections/sorted-set");
var object = {
    set: SortedSet(),
    array: []
};
Bindings.defineBindings(object, {
    "array.rangeContent()": {"<-": "set"}
});
object.set.addEach([5, 2, 6, 1, 4, 3]);
expect(object.array).toEqual([1, 2, 3, 4, 5, 6]);

The notation for binding the content of any mapping collection using map changes is mapContent(). On the target of a binding, this will note when values are added or removed on each key of the source collection and apply the same change to the target. The target and source can be arrays or map collections.

var Map = require("collections/map");
var object = {
    map: new Map(),
    array: []
};
Bindings.defineBinding(object, "map.mapContent()", {
    "<-": "array"
});
object.array.push(1, 2, 3);
expect(object.map.toObject()).toEqual({
    0: 1,
    1: 2,
    2: 3
});

Value

A note about the source value: an empty path implies the source value. Using empty paths and empty expressions is useful in some situations.

If a value is ommitted on either side of an operator, it implies the source value. The expression sorted{} indicates a sorted array, where each value is sorted by its own numeric value. The expression filter{!!} would filter falsy values. The operand is implied. Similarly, filter{!(%2)} produces only even values.

This is why you can use .0 to get the zeroth property of an array, to distingiush the form from 0 which would be a numeric literal, and why you can use ()[0] to map the zeroeth key of a map or array, to distinguish the form from [0] which would be an array literal.

With Context Value

Expressions can be evaluated in the context of another value using a variant of property notation. A parenthesized expression can follow a path.

var object = {
    context: {a: 10, b: 20}
};
Bindings.defineBinding(object, "sum", {
    "<-": "context.(a + b)"
});
expect(object.sum).toBe(30);

Bindings.cancelBinding(object, "sum");
object.context.a = 20;
expect(object.sum).toBe(30); // unchanged

To observe a constructed array or object literal, the expression does not need parentheses.

var object = {
    context: {a: 10, b: 20}
};
Bindings.defineBindings(object, {
    "duple": {"<-": "context.[a, b]"},
    "pair": {"<-": "context.{key: a, value: b}"}
});
expect(object.duple).toEqual([10, 20]);
expect(object.pair).toEqual({key: 10, value: 20});

Bindings.cancelBindings(object);

Operators

FRB can also recognize many operators. These are in order of precedence unary - negation, + numeric coercion, and ! logical negation and then binary ** power, // root, %% logarithm, *, /, % modulo, %% remainder, +, -, <, >, <=, >=, = or ==, !=, && and ||.

var object = {height: 10};
bind(object, "heightPx", {"<-": "height + 'px'"});
expect(object.heightPx).toEqual("10px");

The unary + operator coerces a value to a number. It is handy for binding a string to a number.

var object = {
    number: null,
    string: null,
};
Bindings.defineBinding(object, "+number", {
    "<-": "string"
});
object.string = '10';
expect(object.number).toBe(10);

Functions

FRB supports some common functions. startsWith, endsWith, and contains all operate on strings. join concatenates an array of strings with a given delimiter (or empty string). split breaks a string between every delimiter (or just between every character). join and split are algebraic and can be bound as well as observed.

Conditional

FRB supports the ternary conditional operator, if ? then : else.

var object = Bindings.defineBindings({
    condition: null,
    consequent: 10,
    alternate: 20
}, {
    choice: {"<->": "condition ? consequent : alternate"}
});

expect(object.choice).toBe(undefined); // no choice made

object.condition = true;
expect(object.choice).toBe(10);

object.condition = false;
expect(object.choice).toBe(20);

The ternary operator can bind in both directions.

object.choice = 30;
expect(object.alternate).toBe(30);

object.condition = true;
object.choice = 40;
expect(object.consequent).toBe(40);

And

The logical and operator, &&, observes either the left or right argument depending on whether the first argument is both defined and true. If the first argument is null, undefined, or false, it will stand for the whole expression. Otherwise, the second argument will stand for the whole expression.

If we assume that the first and second argument are always defined and either true or false, the and operator serves strictly as a logical combinator. However, with bindings, it is common for a value to at least initially be null or undefined. Logical operators are the exception to the rule that an expression will necessarily terminate if any operand is null or undefined.

In this example, the left and right sides are initially undefined. We set the right operand to 10 and the bound value remains undefined.

var object = Bindings.defineBindings({
    left: undefined,
    right: undefined
}, {
    and: {"<-": "left && right"}
});

object.right = 10;
expect(object.and).toBe(undefined);

We set the left operand to 20. The bound value becomes the value of the right operand, 10.

// Continued...
object.left = 20;
expect(object.and).toBe(10);

Interestingly, logical and is bindable. The objective of the binding is to do whatever is necessary, if possible, to make the logical expression equal the bound value.

Supposing that both the left and right operands are false, and the result is or becomes true, to satisfy the equality left && right == true, both left and right must be set and bound to true.

var object = Bindings.defineBindings({}, {
    "left && right": {
        "<-": "leftAndRight"
    }
});

object.leftAndRight = true;
expect(object.left).toBe(true);
expect(object.right).toBe(true);

As with the equals binder, logic bindings will prefer to alter the left operand if altering either operand would suffice to validate the expression. So, if the expression then becomes false, it is sufficient to set the left side to false to satisfy the equality.

// Continued...
object.leftAndRight = false;
expect(object.left).toBe(false);
expect(object.right).toBe(true);

This can facilitate some interesting, tri-state logic. For example, if you have a checkbox that can be checked, unchecked, or disabled, and you want it to be unchecked if it is disabled, you can use logic bindings to ensure this.

var controller = Bindings.defineBindings({
    checkbox: {
        checked: false,
        disabled: false
    },
    model: {
        expanded: false,
        children: [1, 2, 3]
    }
}, {
    "checkbox.checked": {"<->": "model.expanded && expandable"},
    "checkbox.disabled": {"<-": "!expandable"},
    "expandable": {"<-": "model.children.length > 0"}
});

expect(controller.checkbox.checked).toBe(false);
expect(controller.checkbox.disabled).toBe(false);

// check the checkbox
controller.checkbox.checked = true;
expect(controller.model.expanded).toBe(true);

// alter the model such that the checkbox is unchecked and disabled
controller.model.children.clear();
expect(controller.checkbox.checked).toBe(false);
expect(controller.checkbox.disabled).toBe(true);

Or

As with the and operator, the logical or is an exception to the rule that an expression is null, undefined, or empty if any of the operands are null or undefined. If both operands are defined and boolean, or expressions behave strictly within the realm of logic. However, if the values are non-boolean or even non-values, they serve to select either the left or right side based on whether the left side is defined and true.

If the first argument is undefined or false, the aggregate expression will evaluate to the second argument, even if that argument is null or undefined.

Suppose we bind or to left || right on some object. or will be undefined initially, but if we set the right to 10, or will become 10, bypassing the still undefined left side.

var object = Bindings.defineBindings({
    left: undefined,
    right: undefined
}, {
    or: {"<-": "left || right"}
});

object.right = 10;
expect(object.or).toBe(10);

However, the left hand side takes precedence over the right if it is defined and true.

// Continued...
object.left = 20;
expect(object.or).toBe(20);

And it will remain bound, even if the right hand side becomes undefined.

object.right = undefined;
expect(object.or).toBe(20);

Aside: JavaScript’s delete operator performs a configuration change, and desugars to Object.defineProperty, and is not interceptable with an ES5 setter. So, don't use it on any property that is involved in a binding. Setting to null or undefined should suffice.

Logical or is bindable. As with logical and, the binding performs the minimum operation necessary to ensure that the expression is equal. If the expression becomes true, and either of the operands are true, the nothing needs to change. If the expression becomes false, however, both operands must be bound to false. If the expression becomes true again, it is sufficient to bind the left operand to true to ensure that the expression as a whole is true. Rather than belabor the point, I leave as an exercise to the reader to apply DeMorgan’s Theorem to the documentation for logical and bindings.

Default

The default operator, ??, is similar to the or, || operator, except that it decides whether to use the left or right solely based on whether the left is defined. If the left is null or undefined, the aggregate expression will evaluate to the right expression. If the left is defined, even if it is false, the result will be the left expression.

var object = Bindings.defineBindings({
    left: undefined,
    right: undefined
}, {
    or: {"<-": "left ?? right"}
});

object.right = 10;
expect(object.or).toBe(10);

object.left = false;
expect(object.or).toBe(false);

The default operator is not bindable, but weirder things have happened.

Defined

The defined() operator serves a similar role to the default operator. If the value in scope is null or undefined, it the result will be false, and otherwise it will be true. This will allow a term that may be undefined to propagate.

var object = Bindings.defineBindings({}, {
    ready: {
        "<-": "value.defined()"
    }
});
expect(object.ready).toBe(false);

object.value = 10;
expect(object.ready).toBe(true);

The defined operator is also bindable. If the source is or becomes false, the target will be bound to null. If the source is null or false, the binding has no effect.

var object = Bindings.defineBindings({
    value: 10,
    operational: true
}, {
    "value.defined()": {"<-": "operational"}
});
expect(object.value).toBe(10);

object.operational = false;
expect(object.value).toBe(undefined);

If the source becomes null or undefined, it will cancel the previous binding but does not set or restore the bound value. Vaguely becoming “defined” is not enough information to settle on a particular value.

object.operational = true;
expect(object.value).toBe(undefined);

However, another binding might settle the issue.

Bindings.defineBindings(object, {
    "value == 10": {
        "<-": "operational"
    }
});
expect(object.value).toBe(10);

Algebra

FRB can automatically invert algebraic operators as long as they operate strictly on the left-most expressions on both the source and target are bindable properties.

In this example, the primary binding is notToBe <- !toBe, and the inverse binding is automatically computed toBe <- !notToBe.

var caesar = {toBe: false};
bind(caesar, "notToBe", {"<->": "!toBe"});
expect(caesar.toBe).toEqual(false);
expect(caesar.notToBe).toEqual(true);

caesar.notToBe = false;
expect(caesar.toBe).toEqual(true);

FRB does algebra by rotating the expressions on one side of a binding to the other until only one independent property remains (the left most expression) on the target side of the equation.

convert: y <- !x
revert: x <- !y

convert: y <- x + a
revert: x <- y - a

The left-most independent variable on the right hand side becomes the dependent variable on the inverted binding. At present, this only works for numbers and when the left-most expression is a bindable property because it cannot assign a new value to the literal 10. For example, FRB cannot yet implicitly revert y <-> 10 + x.

Literals

You may have noticed literals in the previous examples. String literals take the form of any characters between single quotes. Any character can be escaped with a back slash.

var object = {};
bind(object, "greeting", {"<-": "'Hello, World!'"});
expect(object.greeting).toBe("Hello, World!");

Number literals are digits with an optional mantissa.

bind(object, 'four', {"<-": "2 + 2"});

Tuples

Bindings can produce fixed-length arrays. These are most useful in conjunction with mappings. Tuples are comma-delimited and parantheses-enclosed.

var object = {array: [[1, 2, 3], [4, 5]]};
bind(object, "summary", {"<-": "array.map{[length, sum()]}"});
expect(object.summary).toEqual([
    [3, 6],
    [2, 9]
]);

Records

Bindings can also produce fixed-shape objects. The notation is comma-delimited, colon-separated entries, enclosed by curly-braces.

var object = {array: [[1, 2, 3], [4, 5]]};
bind(object, "summary", {
    "<-": "array.map{{length: length, sum: sum()}}"
});
expect(object.summary).toEqual([
    {length: 3, sum: 6},
    {length: 2, sum: 9}
]);

The left hand side of an entry in a record is any combination of letters or numbers. The right side is any expression.

Parameters

Bindings can also involve parameters. The source of parameters is by default the same as the source. The source, in turn, defaults to the same as the target object. It can be specified on the binding descriptor. Parameters are declared by any expression following a dollar sign.

var object = {a: 10, b: 20, c: 30};
bind(object, "foo", {
    "<-": "[$a, $b, $c]"},
    parameters: object
});

Bindings also react to changes to the parameters.

object.a = 0;
object.b = 1;
object.c = 2;
expect(object.foo).toEqual([0, 1, 2]);

The degenerate case of the property language is an empty string. This is a valid property path that observes the value itself. So, as an emergent pattern, a $ expression by itself corresponds to the whole parameters object.

var object = {};
bind(object, "ten", {"<-": "$", parameters: 10});
expect(object.ten).toEqual(10);

Elements and Components

FRB provides a # notation for reaching into the DOM for an element. This is handy for binding views and models on a controller object.

The defineBindings method accepts an optional final argument, parameters, which is shared by all bindings (unless shadowed by a more specific parameters object on an individual descriptor).

The parameters can include a document. The document may be any object that implements getElementById.

Additionally, the frb/dom is an experiment that monkey-patches the DOM to make some properties of DOM elements observable, like the value or checked attribute of an input or textarea element.

var Bindings = require("frb");
require("frb/dom");

var controller = Bindings.defineBindings({}, {

    "fahrenheit": {"<->": "celsius * 1.8 + 32"},
    "celsius": {"<->": "kelvin - 272.15"},

    "#fahrenheit.value": {"<->": "+fahrenheit"},
    "#celsius.value": {"<->": "+celsius"},
    "#kelvin.value": {"<->": "+kelvin"}

}, {
    document: document
});

controller.celsius = 0;

One caveat of this approach is that it can cause a lot of DOM repaint and reflow events. The Montage framework uses a synchronized draw cycle and a component object model to minimize the cost of computing CSS properties on the DOM and performing repaints and reflows, deferring such operations to individual animation frames.

For a future release of Montage, FRB provides an alternate notation for reaching into the component object model, using its deserializer. The @ prefix refers to another component by its label. Instead of providing a document, Montage provides a serialization, which in turn implements getObjectForLabel.

var Bindings = require("frb");

var controller = Bindings.defineBindings({}, {

    "fahrenheit": {"<->": "celsius * 1.8 + 32"},
    "celsius": {"<->": "kelvin - 272.15"},

    "@fahrenheit.value": {"<->": "+fahrenheit"},
    "@celsius.value": {"<->": "+celsius"},
    "@kelvin.value": {"<->": "+kelvin"}

}, {
    serializer: serializer
});

controller.celsius = 0;

Observers

FRB’s bindings use observers and binders internally. You can create an observer from a property path with the observe function exported by the frb/observe module.

var results = [];
var object = {foo: {bar: 10}};
var cancel = observe(object, "foo.bar", function (value) {
    results.push(value);
});

object.foo.bar = 10;
expect(results).toEqual([10]);

object.foo.bar = 20;
expect(results).toEqual([10, 20]);

For more complex cases, you can specify a descriptor instead of the callback. For example, to observe a property’s value before it changes, you can use the beforeChange flag.

var results = [];
var object = {foo: {bar: 10}};
var cancel = observe(object, "foo.bar", {
    change: function (value) {
        results.push(value);
    },
    beforeChange: true
});

expect(results).toEqual([10]);

object.foo.bar = 20;
expect(results).toEqual([10, 10]);

object.foo.bar = 30;
expect(results).toEqual([10, 10, 20]);

If the product of an observer is an array, that array is always updated incrementally. It will only get emitted once. If you want it to get emitted every time its content changes, you can use the contentChange flag.

var lastResult;
var array = [[1, 2, 3], [4, 5, 6]];
observe(array, "map{sum()}", {
    change: function (sums) {
        lastResult = sums.slice();
        // 1. [6, 15]
        // 2. [6, 15, 0]
        // 3. [10, 15, 0]
    },
    contentChange: true
});

expect(lastResult).toEqual([6, 15]);

array.push([0]);
expect(lastResult).toEqual([6, 15, 0]);

array[0].push(4);
expect(lastResult).toEqual([10, 15, 0]);

Nested Observers

To get the same effect as the previous example, you would have to nest your own content change observer.

var i = 0;
var array = [[1, 2, 3], [4, 5, 6]];
var cancel = observe(array, "map{sum()}", function (array) {
    function contentChange() {
        if (i === 0) {
            expect(array.slice()).toEqual([6, 15]);
        } else if (i === 1) {
            expect(array.slice()).toEqual([6, 15, 0]);
        } else if (i === 2) {
            expect(array.slice()).toEqual([10, 15, 0]);
        }
        i++;
    }
    contentChange();
    array.addRangeChangeListener(contentChange);
    return function cancelRangeChange() {
        array.removeRangeChangeListener(contentChange);
    };
});
array.push([0]);
array[0].push(4);
cancel();

This illustrates one crucial aspect of the architecture. Observers return cancelation functions. You can also return a cancelation function inside a callback observer. That canceler will get called each time a new value is observed, or when the parent observer is canceled. This makes it possible to nest observers.

var object = {foo: {bar: 10}};
var cancel = observe(object, "foo", function (foo) {
    return observe(foo, "bar", function (bar) {
        expect(bar).toBe(10);
    });
});

Bindings

FRB provides utilities for declaraing and managing multiple bindings on objects. The frb (frb/bindings) module exports this interface.

var Bindings = require("frb");

The Bindings module provides defineBindings and cancelBindings, defineBinding and cancelBinding, as well as binding inspector methods getBindings and getBinding. All of these take a target object as the first argument.

The Bindings.defineBinding(target, descriptors) method returns the target object for convenience.

var target = Bindings.defineBindings({}, {
    "fahrenheit": {"<->": "celsius * 1.8 + 32"},
    "celsius": {"<->": "kelvin - 272.15"}
});
target.celsius = 0;
expect(target.fahrenheit).toEqual(32);
expect(target.kelvin).toEqual(272.15);

Bindings.getBindings in that case would return an object with fahrenheit and celsius keys. The values would be identical to the given binding descriptor objects, like {"<->": "kelvin - 272.15"}, but it also gets annotated with a cancel function and the default values for any ommitted properties like source (same as target), parameters (same as source), and others.

Bindings.cancelBindings cancels all bindings attached to an object and removes them from the bindings descriptors object.

Bindings.cancelBindings(target);
expect(Bindings.getBindings(object)).toEqual({});

Binding Descriptors

Binding descriptors describe the source of a binding and additional parameters. Bindings.defineBindings can set up bindings (<- or <->), computed (compute) properties, and falls back to defining ES5 properties with permissive defaults (enumerable, writable, and configurable all on by default).

If a descriptor has a <- or <->, it is a binding descriptor. FRB creates a binding, adds the canceler to the descriptor, and adds the descriptor to an internal table that tracks all of the bindings defined on that object.

var object = Bindings.defineBindings({
    darkMode: false,
    document: document
}, {
    "document.body.classList.has('dark')": {
        "<-": "darkMode"
    }
});

You can get all the binding descriptors with Bindings.getBindings, or a single binding descriptor with Bindings.getBinding. Bindings.cancel cancels all the bindings to an object and Bindings.cancelBinding will cancel just one.

// Continued from above...
var bindings = Bindings.getBindings(object);
var descriptor = Bindings.getBinding(object, "document.body.classList.has('dark')");
Bindings.cancelBinding(object, "document.body.classList.has('dark')");
Bindings.cancelBindings(object);
expect(Object.keys(bindings)).toEqual([]);

Converters

A binding descriptor can have a convert function, a revert function, or alternately a converter object. Converters are useful for transformations that cannot be expressed in the property language, or are not reversible in the property language.

In this example, a and b are synchronized such that a is always half of b, regardless of which property gets updated.

var object = Bindings.defineBindings({
    a: 10
}, {
    b: {
        "<->": "a",
        convert: function (a) {
            return a * 2;
        },
        revert: function (b) {
            return b / 2;
        }
    }
});
expect(object.b).toEqual(20);

object.b = 10;
expect(object.a).toEqual(5);

Converter objects are useful for reusable or modular converter types and converters that track additional state.

function Multiplier(factor) {
    this.factor = factor;
}
Multiplier.prototype.convert = function (value) {
    return value * this.factor;
};
Multiplier.prototype.revert = function (value) {
    return value / this.factor;
};

var doubler = new Multiplier(2);

var object = Bindings.defineBindings({
    a: 10
}, {
    b: {
        "<->": "a",
        converter: doubler
    }
});
expect(object.b).toEqual(20);

object.b = 10;
expect(object.a).toEqual(5);

Reusable converters have an implied direction, from some source type to a particular target type. Sometimes the types on your binding are the other way around. For that case, you can use the converter as a reverter. This merely swaps the convert and revert methods.

var uriConverter = {
    convert: encodeURI,
    revert: decodeURI
};
var model = Bindings.defineBindings({}, {
    "title": {
        "<->": "location",
        reverter: uriConverter
    }
});

model.title = "Hello, World!";
expect(model.location).toEqual("Hello,%20World!");

model.location = "Hello,%20Dave.";
expect(model.title).toEqual("Hello, Dave.");

Computed Properties

A computed property is one that gets updated with a function call when one of its arguments changes. Like a converter, it is useful in cases where a transformation or computation cannot be expressed in the property language, but can additionally accept multiple arguments as input. A computed property can be used as the source for another binding.

In this example, we create an object as the root of multiple bindings. The object synchronizes the properties of a "form" object with the window’s search string, effectively navigating to a new page whenever the "q" or "charset" values of the form change.

Bindings.defineBindings({
    window: window,
    form: {
        q: "",
        charset: "utf-8"
    }
}, {
    queryString: {
        args: ["form.q", "form.charset"],
        compute: function (q, charset) {
            return "?" + QS.stringify({
                q: q,
                charset: charset
            });
        }
    },
    "window.location.search": {
        "<-": "queryString"
    }
});