es6-summary

A short summary of ES6 features and their ES5 equivalents.

Access in-depth ES6 articles here.

Table of contents

Arrow Functions
Functions
Scope
Default Argument Values
Classes
Object Property Shorthand Notation
Destructuring
Rest Parameters
Spread Operator
Objects
Tail call optimization
Symbols
Sets
Maps
Weak Sets
Weak Maps
for...of loop
Iterable Interface
Generator functions
Strings
Template literals
Tag functions for Template Literals
Promises
Modules
Reflect API
Proxies
59 Exercises

Arrow Functions

describes a mapping between the argument list and return value
the return value may be in a block (see 2, 3, and 4)
only use it when implicit context binding is acceptable
don't use arrow functions in classes, constructors, prototype extensions

// 1.
a => value;

// 2.
a => { return value; }

// 3.
( a, b ) => { 
    statements;
    return value;
}

// 4.
( ...args ) => {
    statements;
    return value;
}

// 1.
// 2.
function( a ) { return value; }.bind( this );

// 3.
function( a, b ) {
    statements;
}.bind( this );

// 4.
function() {
    var args = arguments;
    statements;
}.bind( this );

Functions

Name property

const f = () => null;

f.name
> "f"

`new.target`

const C = function() {
    console.log( new.target === C );
}

const o = new C();
> true

Scope

var: function scope

(function() {
    // first and second are accessible
    // first = undefined, second = undefined
    var first = 1;
    {
        var second = 2;
    } 
    // first and second are accessible
    // first = 1, second = 2
});

After hoisting:

(function() {
    var first, second;
    first = 1;
    {
        second = 2;
    } 
    // first and second are accessible
    // first = 1, second = 2
});

let, const: block scope

let and const declarations are hoisted just like vars. However, their values are not accessible before their first assignment.

{
    // accessing radius or PI throws an error
    const PI = 3.14;
    let radius = 2;
    // radius and PI are accessible
    // radius may be modified
}
// accessing radius or PI throws an error

Default argument values

ES6:

function( name, phone = '-' ) {
    // ...
}

ES5:

function( name, phone ) {
    phone = typeof phone === 'undefined' ? '-' : phone;
    // ...
}

Warning: default arguments does not affect the arguments array

const f = ( x = 0, y = 0 ) => {
    console.log( arguments.length );
};

f( 0 )
> 1

Classes

Basics

ES6:

concise method syntax has to be used

class Shape {
    constructor( color ) {
        this.color = color;
    }
    getColor() {
        return this.color;
    }
}

ES5:

function Shape( color ) {
    this.color = color;
}

Shape.prototype.getColor = function() {*
    return this.color;
}

Inheritance

Rectangle is the child class, Shape is the parent class
redefining a method with the same name in the child class shadows the method in the parent class
super has to be called in the constructor in the child class
in the absence of a constructor, a constructor including a super call is implicitly assumed

ES6:

class Rectangle extends Shape {
    constructor( color, width, height ) {
        super( color );
        this.width = width;
        this.height = height;
    }
}

ES5:

function Rectangle( color, width, height ) {
    Shape.call( this, color );
    this.width = width;
    this.height = height;
}

Rectangle.prototype = Object.create( Shape.prototype ); 
Rectangle.prototype.constructor = Rectangle;

Getters and Setters

class Rectangle extends Shape {
    constructor( color, width, height ) {
        super( color );
        this.width = width;
        this.height = height;
    }
    get area() {
        return this.width * this.height;
    }
    set area( value ) {
        console.log( 'Attempting to set area to ' + value );
        throw 'Setting area is not allowed';
    }
}

const myRectangle = new Rectangle( 'red', 5, 3 );

myRectangle.area
> 15

myRectangle.area = 25
> Attempting to set area to 25

Static methods

class level methods
cannot be accessed from instances

class Die {
    static cast() { return Math.round( Math.random() * 6 ) + 1; }
}

Die.cast()
> 3

Object property shorthand notation

ES6:

var language = 'Markdown';
var extension = 'md';

var file = {
    language,
    extension
};

ES5:

var language = 'Markdown';
var extension = 'md';

var file = {
    language: language,
    extension: extension
};

Destructuring

null or undefined on the right hand side of a destructuring expression yields a JavaScript error

Arrays

ES6:

var a = 1, b = 2;

// 1.
[a, b] = [b, a + b];

// 2.
var [,c] = [a, b];

// 3. 
var [,,d] = [1, 2];

ES5:

var a = 1, b = 2;

// 1.
var temp_a = a, temp_b = b;
a = temp_b;
b = temp_a + temp_b;

// 2.
var temp_b2 = b;
var c = temp_b2;

// 3.
var d = undefined;

Objects

ES6:

var file = {
    filename: 'es6.md',
    metadata: {
        language: 'Markdown',
        extension: 'md'
    }
};

var { filename, metadata: { language } } = file;
// same as:
// var { filename, metadata: { language: language } } = file;

ES5:

var file = {
    filename: 'es6.md',
    metadata: {
        language: 'Markdown',
        extension: 'md'
    }
};

var temp_filename = file.filename;
var temp_language = file.metadata.language;
var filename = temp_filename;
var language = temp_language;

Return value of a destructuring expression

LEFT = RIGHT returns RIGHT

{ a } = { b } = { a: 1, b: 2 }   

// is evaluated as:
// b becomes 2
{ a } = { a: 1, b: 2 }

// a becomes 1
{ a: 1, b: 2 }

Destructuring function arguments

function f( L1, L2 ) { ... }

f( R1, R2 );

executes

L1 = R1;
L2 = R2;

Rest parameters

ES6:

( function( first, ...rest ) {
    console.log( 'first:', first );
    console.log( 'rest:', rest );
} )( 1, 'Second', 3 );
> first: 1
> rest: ['Second', 3]

ES5:

note: arguments is not an array in JavaScript, it has to be converted to an array with Array.from to be able to slice it

( function() {
    var first = arguments[0];
    var rest = Array.from( arguments ).slice( 1 );
    console.log( 'first:', first );
    console.log( 'rest:', rest );
} )( 1, 'Second', 3 );
> first: 1
> rest: ["Second", 3]

Spread operator

const arr = [1, 2, 3];

// ...arr spreads the elements of arr into comma separated values
[ ...arr, ...arr ]
> [1, 2, 3, 1, 2, 3]

Math.max( ...arr ) // same as Math.max( 1, 2, 3 )
> 3

Strings are spread as comma separated strings of one character each.

[...'ES6']
> ["E", "S", "6"]

ES5 equivalents: loops, utility functions, call / apply function for calling functions with a variable number of arguments

Objects

Object.assign

ES6:

Object.assign( target, ...sourceList )

copies all key-value pairs of sourceList to target, mutating target
copying is done from left to right
returns target

Example:

var target = { a: 1 };
Object.assign( target, { b: 1}, { a: 2, b: 2 } );
// target becomes { a: 2, b: 2} AND returns target

ES5:

assume all arguments are objects, and they are not null
in practice, error handling should be taken care of, this implementation is for illustration purposes only

Object.assign = function( target /*, ...sourceList */ ) {
    var source;
    for ( var i = 1; i < arguments.length; ++i ) {
        source = arguments[i]; 
        for ( var key in source ) {
            if ( source.hasOwnProperty( key ) ) target[key] = source[key]; 
        }
    }

Computed object keys

Place any JavaScript expression as an object key between [ and ]
The JavaScript expression is stringified and converted into an object key

{
    [ 3 + 2]: 1,
    [ [] ]: 2
}

becomes

{
    "5": 1,
    "[object Object]": 2
}

Shorthand method notation for objects

const publicInterface = {
    operation() { return 'ES6'; }
}

Object prototype extensions

Object.getPrototypeOf( o );
Object.setPrototypeOf( o, newProto );

Tail call optimization

Tail call optimization avoids using the stack for a function call
Function calls in tail position are tail-call optimized
A function is in tail position if it's the last action in the function
Compatibility: currently low support ( https://kangax.github.io/compat-table/es6/ )

Regular recursion:

function sumToN( n ) { 
    if ( n <= 1 ) return n;
    return n + sumToN( n - 1 ); 
};

Tail call optimization with accumulator variables:

function sumToN( n, sum = 0 ) { 
    if ( n <= 1 ) return sum; 
    let result = sum + n; 
    return sumToN( n - 1, result ); 
};

Symbols

Symbol() creates a unique symbol
Objects can have string or symbol keys
Node.js does not print Symbol keys
JSON.stringify excludes Symbol keys
Object.assign copies Symbol keys over to target

const s1 = Symbol();
const s2 = Symbol();

s1 === s2 
> false

typeof s1
> "symbol"

const o = {
    [s2]: 1,
    [Symbol()]: 2
};

o[s1] = 3;

JSON.stringify( o )
> "{}"

Global Symbol Registry

const s1 = Symbol.for( 'key' );
const s2 = Symbol.for( 'key' );

s1 === s2
> true

Symbol.keyFor( s1 ) 
> "key"

Sets

arbitrary values
values can be enumerated using the iterable interface

let colors = new Set();
colors.add( 'red' );
colors.add( 'green' );

colors.size
> 2

colors.has( 'green' )
> true

colors.delete( 'green' )
> true

Maps

arbitrary keys
arbitrary values
keys can be enumerated using the iterable interface

let horses = new Map();
horses.set( 8, 'Chocolate' );

horses.size
> 1

horses.has( 8 )
> true

horses.get( 8 )
> 'Chocolate'

horses.delete( 8 )
> true

Alternative map constructor

let horses = new Map( [[8, 'Chocolate'][3, 'Filippone]] );

Weak sets

sets hold weak reference to their values: if the only reference to a value is in the set, the value is garbage collected
size of the set is unknown
weak sets may only store objects
weak sets are not iterable

let firstElement = { order: 1 }, secondElement = { order: 2 }; 
let ws = new WeakSet( [ firstElement, secondElement ] );

ws.has( firstElement ) 
> true

firstElement = {};
ws.has( firstElement ) 
> false

Weak maps

object keys, arbitrary values
only the keys of the weak map are weak: in case the map holds the only reference to its key, the key is garbage collected

let firstElement = { order: 1 }, secondElement = { order: 2 }; 
let wm = new WeakMap();

wm.set( firstElement, 1 ); 
wm.set( secondElement, {} );

wm.get( secondElement ) 
> {}

secondElement = {};
wm.get( secondElement ) 
> undefined

`for...of` loop

works on iterables

const arr = [1, 2, 3];
const message = 'hello';

for ( let i of arr ) console.log( i );
> 1
> 2
> 3

for ( let ch of message ) {
    console.log( ch );
}
> 'h'
> 'e'
> 'l'
> 'l'
> 'o'

let colors = new Set();
colors.add( 'red' );
colors.add( 'green' );
for ( let color of colors ) console.log( color );
> 'red'
> 'green'

let horses = new Map( [[8, 'Chocolate'][3, 'Filippone]] );
for ( let [key, value] of horses ) console.log( key, value );
> 8 'Chocolate'
> 3 'Filippone'

Iterable interface

Properties of iterables and iterators

Iterable object: has a [Symbol.iterator] method returning an iterator object
Iterator object: have a next() method returning { done: <Boolean>, value: <NextValue> } of the iteration
when done is truthy, the iteration ends
when done is falsy, value is the upcoming element of the iteration

Consumer constructs

for...of loop consumes iterables
... (spread) operator consumes iterables

Built-in iterables:

Arrays
Strings
DOM data structures
Maps
Sets

Example:

let colors = new Set( [ 'red', 'yellow', 'green' ] ); 
let horses = new Map( [ [5, 'QuickBucks'], [8, 'Chocolate'], [3, 'Filippone'] ] );

console.log( colors.entries() ); 
> SetIterator {["red", "red"], ["yellow", "yellow"], ["green", "green"]}

console.log( colors.keys() ); 
> SetIterator {"red", "yellow", "green"}

console.log( colors.values() ); 
> SetIterator {"red", "yellow", "green"}

console.log( horses.entries() );
> MapIterator {[5, "QuickBucks"], [8, "Chocolate"], [3, "Filippone"]}

console.log( horses.keys() ); 
> MapIterator {5, 8, 3}

console.log( horses.values() ); 
> MapIterator {"QuickBucks", "Chocolate", "Filippone"}

Generators functions

Return an iterable that is also an iterator
function* keyword creates a generator function
yield keyword yields the next value returned by the iterator, with done: false
return v exits the generator with { done: true, value: v }. v is not consumed by data consumers

function *getLampIterator() { 
    yield 'red'; 
    yield 'green'; 
    return 'lastValue'; 
}

let lampIterator = getLampIterator();
console.log( lampIterator.next() ); 
> Object {value: "red", done: false}

console.log( lampIterator.next() ); 
> Object {value: "green", done: false}

console.log( lampIterator.next() ); 
> Object {value: "lastValue", done: true}

Combining generators

let generator1 = function *() { ... };
let generator2 = function *() { ... };

let combinedGenerator = function *() {
    yield *generator1();
    yield *generator2();
}

Passing parameters to iterables

let greetings = function *() { 
    let name = yield 'Hi!'; 
    yield `Hello, ${ name }!`; 
}

let greetingIterator = greetings();

console.log( greetingIterator.next() ); 
> Object {value: "Hi!", done: false}

console.log( greetingIterator.next( 'Lewis' ) ); 
> Object {value: "Hello, Lewis!", done: false}

Strings

const s = 'hello'

s.startsWith( 'he' );
> true

s.endsWith( 'lo' );
> true

s.includes( 'll' );
> true

s.repeat( 3 );
> 'hellohellohello'

// codePointAt returns the character code regardless of whether it's a 2, 3, or 4 byte character
'\u{1f600}\u{1f600}'.codePointAt( 0 );
> 128512
'\u{1f600}\u{1f600}'.codePointAt( 1 ); // garbage: second half of the first character
> 56832
'\u{1f600}\u{1f600}'.codePointAt( 2 );
> 128512

Template literals

evaluate JavaScript expressions inside ${ and }
can span multiple lines
return a string

const x = '3 + 2';

console.log( `${x} is ${3 + 2}
.
.
.` );
> 3 + 2 is 5
> .
> .
> .

Tag functions for template literals

tagFunction( literalFragmentArray, ...expressionSubstitutions ) {
    // returns string returned by the template literal
}

Example:

const toUpperExpressionsTag = ( literals, ...expressions ) => {
    let str = '';
    for ( let i = 0; i < expressions.length; ++i ) {
        str += literals[i] + expressions[i].toUpperCase();
    }
    str += literals[ literals.length - 1];
    return str;
}
const dave = 'dave';

toUpperExpressionsTag`Hello ${ dave }!`
> "Hello DAVE!"

Promises

promises are immutable
they are either kept or broken
when a promise is kept, we are guaranteed to receive a value
when a promise is broken, we are guaranteed to receive a reason (error)

Promise states

pending: may transition to fulfilled or rejected
fulfilled: must have a value
rejected: must have a reason for rejection

Creating promises

let promise = new Promise( function( resolve, reject ) { 
    // call function resolve( value ) to resolve a promise 
    // call function reject( reason ) to reject a promise 
} );
let onFulfilled = function() { /* success handler */ }
let onRejected = function() { /* rejection handler */ }

promise.then( onFulfilled, onRejected );

Then is chainable: it returns a promise

promise.then( s1, r1 ).then( s2, r2 );

On success, it returns s2( s1( v1 ) ) in case promise is resolved with v1
If promise is resolved with return value v1, and s1( v1 ) returns a value, then the promise calling s2 will be fulfilled with value s1( v1 )

Errors can be caught with promise.catch()

promise.then( onFulfilled1 ).then( onFulfilled2 ).catch( onReject );

Creating automatically resolved promises

let promise = Promise.resolve( v );

Example

p.then( ( value ) => console.log( 'Value:', value ) ) 
 .then( () => { throw new Error('Error in second handler' ) } ) 
 .catch( ( error ) => console.log( 'Error: ', error.toString() ) );

`Promise.all`

handles an iterable of promises
onFulfilled is called once all promises in the iterable are resolved
onRejected is called once any promise in the iterable is rejected

Promise.all( [ promise1, promise2 ] ).then( onFulfilled, onRejected );

Modules

Importing module from ./moduleName.js and ./folder/moduleName2.js

import module from 'moduleName';
import { key1, key2 } from 'folder/moduleName2';  // notice the destructuring on the left

Exporting objects in ./moduleName.js:

class C { ... }

export default C;

Reflect API

The Reflect API is far too complex to be summarized in a couple of paragraphs. It is also an advanced topic, requiring in-depth understanding. For this reason, I highly recommend the below article.

Reflect API

Proxies

Proxies are not only complex, but they heavily build on the Reflect API. Check out this article to learn them.

Proxies in Practice

59 Exercises

Check out ES6 in Practice, and put theory into practice by solving 59 exercises, and checking out their reference solutions.

zsolt-nagy/es6-summary

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Repository Details

es6-summary

Arrow Functions

Functions

Name property

new.target

Scope

Default argument values

Classes

Basics

Inheritance

Getters and Setters

Static methods

Object property shorthand notation

Destructuring

Arrays

Objects

Return value of a destructuring expression

Destructuring function arguments

Rest parameters

Spread operator

Objects

Object.assign

Computed object keys

Shorthand method notation for objects

Object prototype extensions

Tail call optimization

Symbols

Global Symbol Registry

Sets

Maps

Weak sets

Weak maps

for...of loop

Iterable interface

Properties of iterables and iterators

Consumer constructs

Built-in iterables:

Generators functions

Combining generators

Passing parameters to iterables

Strings

Template literals

Tag functions for template literals

Promises

Promise states

Creating promises

Creating automatically resolved promises

Example

Promise.all

Modules

Reflect API

Proxies

59 Exercises

More Repositories

`new.target`

`for...of` loop

`Promise.all`