Filtrex

A simple, safe, JavaScript expression engine, allowing end-users to enter arbitrary expressions without p0wning you.

category == "meal" and (calories * weight > 2000.0 or subcategory in ("cake", "pie"))

Why?

There are many cases where you want a user to be able enter an arbitrary expression through a user interface. e.g.

• Plot a chart (example)
• Filter/searching across items using multiple fields (example)
• Colorize items based on values (example)
• Implement a browser based spreadsheet

Sure, you could do that with JavaScript and eval(), but I'm sure I don't have to tell you how stupid that would be.

Filtrex defines a really simple expression language that should be familiar to anyone who's ever used a spreadsheet and compile it into a JavaScript function at runtime.

Features

• Simple! End user expression language looks like this transactions <= 5 and abs(profit) > 20.5
• Fast! Expressions get compiled into JavaScript functions, offering the same performance as if it had been hand coded. e.g. function(item) { return item.transactions <=5 && Math.abs(item.profit) > 20.5; }
• Safe! You as the developer have control of which data can be accessed and the functions that can be called. Expressions cannot escape the sandbox.
• Pluggable! Add your own data and functions.
• Predictable! Because users can't define loops or recursive functions, you know you won't be left hanging.

Get it

• DOWNLOAD filtrex.js

10 second tutorial

// Input from user (e.g. search filter)
var expression = 'transactions <= 5 and abs(profit) > 20.5';

// Compile expression to executable function
var myfilter = compileExpression(expression);

// Execute function
myfilter({transactions: 3, profit:-40.5}); // returns 1
myfilter({transactions: 3, profit:-14.5}); // returns 0

Under the hood, the above expression gets compiled to a clean and fast JavaScript function, looking something like this:

// Resulting function
function(item) {
  return item.transactions <= 5 && Math.abs(item.profit) > 20.5;
}

Expressions

There are only 2 types: numbers and strings. Numbers may be floating point or integers. Boolean logic is applied on the truthy value of values (e.g. any non-zero number is true, any non-empty string is true, otherwise false).

Operator precedence follows that of any sane language.

Adding custom functions

When integrating in to your application, you can add your own custom functions.

// Custom function: Return string length.
function strlen(s) {
  return s.length;
}

// Compile expression to executable function
var myfilter = compileExpression(
                    'strlen(firstname) > 5',
                    {strlen:strlen}); // custom functions

myfilter({firstname:'Joe'});    // returns 0
myfilter({firstname:'Joseph'}); // returns 1

FAQ

Why the name?

Because it was originally built for FILTeR EXpressions.

What's Jison?

Jison is bundled with Filtrex – it's a JavaScript parser generator that does the underlying hard work of understanding the expression. It's based on Flex and Bison.

License?

MIT

Unit tests?

Here: Source, Results

What happens if the expression is malformed?

Calling compileExpression() with a malformed expression will throw an exception. You can catch that and display feedback to the user. A good UI pattern is to attempt to compile on each keystroke and continuously indicate whether the expression is valid.

Contributors

• @joewalnes Joe Walnes – the author of this repository
• @msantos Michael Santos – quoted symbols, regex matches and numerous fixes
• @alexgorbatchev Alex Gorbatchev – NPM package and housekeeping
• @m93a Michal Grňo – critical fixes of security bug

Like this? Want other thingies?

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• ReconnectingWebSocket – Simplest way to add some robustness to your WebSocket connections.
• Smoothie Charts – JavaScript charts for streaming data.
• Visit The Igloo Lab to see and subscribe to other thingies I make.

And follow @joewalnes!