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 3 types: numbers, strings and arrays of these. 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).

Okay, I lied to you, there are also objects whose properties can be accessed by the of operator. And there's undefined. But everything else is just numbers, strings and arrays!

Values | Description --- | --- 43, -1.234 | Numbers "hello" | String " \" \\ " | Escaping of double-quotes and blackslash in string foo, a.b.c, 'foo-bar' | External data variable defined by application (may be numbers or strings)

Numeric arithmetic | Description --- | --- x + y | Add x - y | Subtract x * y | Multiply x / y | Divide x % y | Modulo x ^ y | Power

Comparisons | Description --- | --- x == y | Equals x != y | Does not equal x < y | Less than x <= y | Less than or equal to x > y | Greater than x >= y | Greater than or equal to x ~= y | Regular expression match x in (a, b, c) | Equivalent to (x == a or x == b or x == c) x not in (a, b, c) | Equivalent to (x != a and x != b and x != c)

Boolean logic | Description --- | --- x or y | Boolean or x and y | Boolean and not x | Boolean not x ? y : z | If boolean x, value y, else z ( x ) | Explicity operator precedence

Objects and arrays | Description --- | --- (a, b, c) | Array a in b | Array a is a subset of array b x of y | Property x of object y

Built-in functions | Description --- | --- abs(x) | Absolute value ceil(x) | Round floating point up floor(x) | Round floating point down log(x) | Natural logarithm max(a, b, c...) | Max value (variable length of args) min(a, b, c...) | Min value (variable length of args) random() | Random floating point from 0.0 to 1.0 round(x) | Round floating point sqrt(x) | Square root

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;
}

let options = {
  extraFunctions: { strlen }
};

// Compile expression to executable function
let myfilter = compileExpression('strlen(firstname) > 5', options);

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

Custom property function

If you want to do some more magic with your filtrex, you can supply a custom function that will resolve the identifiers used in expressions and assign them a value yourself. This is called a property function and has the following signature:

function propFunction(
  propertyName: string, // name of the property being accessed
  get: (name: string) => obj[name], // safe getter that retrieves the property from obj
  obj: any // the object passed to compiled expression
)

For example, this can be useful when you're filtering based on whether a string contains some words or not:

function containsWord(string, word) {
  // your optimized code
}

let options = {
  customProp: (word, _, string) => containsWord(string, word)
};

let myfilter = compileExpression('Bob and Alice or Cecil', options);

myfilter("Bob is boring"); // returns 0
myfilter("Bob met Alice"); // returns 1
myfilter("Cecil is cool"); // returns 1

Safety note: The get function returns undefined for properties that are defined on the object's prototype, not on the object itself. This is important, because otherwise the user could access things like toString.constructor and maybe do some nasty things with it. Bear this in mind if you decide not to use get and access the properties yourself.

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
• @m93a Michal Grňo – maintainer of the NPM package and the current main developer
• @msantos Michael Santos – quoted symbols, regex matches and numerous fixes
• @bradparks Brad Parks – extensible prop function
• @arendjr Arend van Beelen jr. – quote escaping in string literals
• @alexgorbatchev Alex Gorbatchev – the original maintainer of the NPM package

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