A simple, safe, JavaScript Filter Expression compiler for end-users
JavaScript HTML Other
Latest commit 218c5be Mar 6, 2015 @joewalnes Merge pull request #11 from msantos/fix-zero-arity-fun
Fix parsing of 0-arity functions



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"))


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.


  • 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

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;


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).

Values Description
43, -1.234 Numbers
"hello" 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 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
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;

// 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


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.



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.

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  • Visit The Igloo Lab to see and subscribe to other thingies I make.

And follow @joewalnes!