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Javascript Expression Language: Powerful context-based expression parser and evaluator
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rehandalal Merge pull request #23 from rehandalal/fix-string-parsing
Fix issue with escaped quotes at the end of string
Latest commit 3d09092 Mar 27, 2019

Mozjexl CircleCI

Javascript Expression Language: Powerful context-based expression parser and evaluator Mozjexl is a fork of Jexl for use at Mozilla, specifically as a part of SHIELD and Normandy.

Quick start

Use it with promises or callbacks:

var context = {
    name: {first: 'Sterling', last: 'Archer'},
    assoc: [
        {first: 'Lana', last: 'Kane'},
        {first: 'Cyril', last: 'Figgis'},
        {first: 'Pam', last: 'Poovey'}
    age: 36

// Filter an array
mozjexl.eval('assoc[.first == "Lana"].last', context).then(function(res) {
    console.log(res); // Output: Kane

// Do math
mozjexl.eval('age * (3 - 1)', context, function(err, res) {
    console.log(res); // Output: 72

// Concatenate
mozjexl.eval('name.first + " " + name["la" + "st"]', context).then(function(res) {
    console.log(res); // Output: Sterling Archer

// Compound
mozjexl.eval('assoc[.last == "Figgis"].first == "Cyril" && assoc[.last == "Poovey"].first == "Pam"', context)
    .then(function(res) {
        console.log(res); // Output: true

// Use array indexes
mozjexl.eval('assoc[1]', context, function(err, res) {
    console.log(res.first + ' ' + res.last); // Output: Cyril Figgis

// Use conditional logic
mozjexl.eval('age > 62 ? "retired" : "working"', context).then(function(res) {
    console.log(res); // Output: working

// Transform
mozjexl.addTransform('upper', function(val) {
    return val.toUpperCase();
mozjexl.eval('"duchess"|upper + " " + name.last|upper', context).then(function(res) {
    console.log(res); // Output: DUCHESS ARCHER

// Transform asynchronously, with arguments
mozjexl.addTransform('getStat', function(val, stat) {
    return dbSelectByLastName(val, stat); // Returns a promise
mozjexl.eval('name.last|getStat("weight")', context, function(err, res) {
    if (err) console.log('Database Error', err.stack);
    else console.log(res); // Output: 184

// Add your own (a)synchronous operators
// Here's a case-insensitive string equality
mozjexl.addBinaryOp('_=', 20, function(left, right) {
    return left.toLowerCase() === right.toLowerCase();
mozjexl.eval('"Guest" _= "gUeSt"').then(function(val) {
    console.log(res); // Output: true


For Node.js or Web projects, type this in your project folder:

yarn add mozjexl

Access Mozjexl the same way, backend or front:

import mozjexl from 'mozjexl';

All the details

Unary Operators

Operation Symbol
Negate !

Binary Operators

Operation Symbol
Add, Concat +
Subtract -
Multiply *
Divide /
Divide and floor //
Modulus %
Power of ^
Logical AND &&
Logical OR ||


Comparison Symbol
Equal ==
Not equal !=
Greater than >
Greater than or equal >=
Less than <
Less than or equal <=
Element in array or string in

A note about in:

The in operator can be used to check for a substring: "Cad" in "Ron Cadillac", and it can be used to check for an array element: "coarse" in ['fine', 'medium', 'coarse']. However, the == operator is used behind-the-scenes to search arrays, so it should not be used with arrays of objects. The following expression returns false: {a: 'b'} in [{a: 'b'}].

Ternary operator

Conditional expressions check to see if the first segment evaluates to a truthy value. If so, the consequent segment is evaluated. Otherwise, the alternate is. If the consequent section is missing, the test result itself will be used instead.

Expression Result
"" ? "Full" : "Empty" Empty
"foo" in "foobar" ? "Yes" : "No" Yes
{agent: "Archer"}.agent ?: "Kane" Archer

Native Types

Type Examples
Booleans true, false
Strings "Hello "user"", 'Hey there!'
Numerics 6, -7.2, 5, -3.14159
Objects {hello: "world!"}
Arrays ['hello', 'world!']


Parentheses work just how you'd expect them to:

Expression Result
(83 + 1) / 2 42
1 < 3 && (4 > 2 || 2 > 4) true


Access variables in the context object by just typing their name. Objects can be traversed with dot notation, or by using brackets to traverse to a dynamic property name.

Example context:

    name: {
        first: "Malory",
        last: "Archer"
    exes: [
        "Nikolai Jakov",
        "Len Trexler",
        "Burt Reynolds"
    lastEx: 2
Expression Result
name.first Malory
name['la' + 'st'] Archer
exes[2] Burt Reynolds
exes[lastEx - 1] Len Trexler


Collections, or arrays of objects, can be filtered by including a filter expression in brackets. Properties of each collection can be referenced by prefixing them with a leading dot. The result will be an array of the objects for which the filter expression resulted in a truthy value.

Example context:

    employees: [
        {first: 'Sterling', last: 'Archer', age: 36},
        {first: 'Malory', last: 'Archer', age: 75},
        {first: 'Lana', last: 'Kane', age: 33},
        {first: 'Cyril', last: 'Figgis', age: 45},
        {first: 'Cheryl', last: 'Tunt', age: 28}
    retireAge: 62
Expression Result
employees[.first == 'Sterling'] [{first: 'Sterling', last: 'Archer', age: 36}]
employees[.last == 'Tu' + 'nt'].first Cheryl
employees[.age >= 30 && .age < 40] [{first: 'Sterling', last: 'Archer', age: 36},{first: 'Lana', last: 'Kane', age: 33}]
employees[.age >= 30 && .age < 40][.age < 35] [{first: 'Lana', last: 'Kane', age: 33}]
employees[.age >= retireAge].first Malory


The power of Mozjexl is in transforming data, synchronously or asynchronously. Transform functions take one or more arguments: The value to be transformed, followed by anything else passed to it in the expression. They must return either the transformed value, or a Promise that resolves with the transformed value. Add them with mozjexl.addTransform(name, function).

mozjexl.addTransform('split', function(val, char) {
    return val.split(char);
mozjexl.addTransform('lower', function(val) {
    return val.toLowerCase();
Expression Result
"Pam Poovey"|lower|split(' ')[1] poovey
"password==guest"|split('=' + '=') ['password', 'guest']

Advanced Transforms

Using Transforms, Mozjexl can support additional string formats like embedded JSON, YAML, XML, and more. The following, with the help of the xml2json module, allows XML to be traversed just as easily as plain javascript objects:

var xml2json = require('xml2json');

mozjexl.addTransform('xml', function(val) {
    return xml2json.toJson(val, {object: true});

var context = {
        "<Employees>" +
            "<Employee>" +
                "<FirstName>Cheryl</FirstName>" +
                "<LastName>Tunt</LastName>" +
            "</Employee>" +
            "<Employee>" +
                "<FirstName>Cyril</FirstName>" +
                "<LastName>Figgis</LastName>" +
            "</Employee>" +

var expr = 'xmlDoc|xml.Employees.Employee[.LastName == "Figgis"].FirstName';

mozjexl.eval(expr, context).then(function(res) {
    console.log(res); // Output: Cyril


Variable contexts are straightforward Javascript objects that can be accessed in the expression, but they have a hidden feature: they can include a Promise object, and when that property is used, Mozjexl will wait for the Promise to resolve and use that value!



A reference to the Jexl constructor. To maintain separate instances of Jexl with each maintaining its own set of transforms, simply re-instantiate with new mozjexl.Jexl().

mozjexl.addBinaryOp({string} operator, {number} precedence, {function} fn)

Adds a binary operator to the Jexl instance. A binary operator is one that considers the values on both its left and right, such as "+" or "==", in order to calculate a result. The precedence determines the operator's position in the order of operations (please refer to lib/grammar.js to see the precedence of existing operators). The provided function will be called with two arguments: a left value and a right value. It should return either the resulting value, or a Promise that resolves to the resulting value.

mozjexl.addUnaryOp({string} operator, {function} fn)

Adds a unary operator to the Jexl instance. A unary operator is one that considers only the value on its right, such as "!", in order to calculate a result. The provided function will be called with one argument: the value to the operator's right. It should return either the resulting value, or a Promise that resolves to the resulting value.

mozjexl.addTransform({string} name, {function} transform)

Adds a transform function to this Jexl instance. See the Transforms section above for information on the structure of a transform function.

mozjexl.addTransforms({{}} map)

Adds multiple transforms from a supplied map of transform name to transform function.

mozjexl.getTransform({string} name)

Returns {function|undefined}. Gets a previously set transform function, or undefined if no function of that name exists.

mozjexl.eval({string} expression, {{}} [context], {function} [callback])

Returns {Promise<*>}. Evaluates an expression. The context map and callback function are optional. If a callback is specified, it will be called with the standard signature of {Error} first argument, and the expression's result in the second argument. Note that if a callback function is supplied, the returned Promise will already have a .catch() attached to it.

mozjexl.removeOp({string} operator)

Removes a binary or unary operator from the Jexl instance. For example, "^" can be passed to eliminate the "power of" operator.


$ yarn install
$ yarn test

Precommit hook

Mozjexl provides a config for Therapist. Install it with Pip, and then run therapist install in this repo to set it up. It will automatically format your Javascript with Prettier, and run ESLint checks before committing your code.


Mozjexl is licensed under the MIT license. Please see LICENSE.txt for full details.


Jexl was designed and created at TechnologyAdvice.

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