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Javascript Expression Language: Powerful context-based expression parser and evaluator

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  • Added toString method on Expression to turn compiled expression AST back into a parsable JEXL string.




Javascript Expression Language: Powerful context-based expression parser and evaluator

This is forked from the original work by Tom Frost due to the original repository suffering from lack of maintenance for 2 years at the time of this fork.

Additional work from Chetan Padia for conversion of compile expressions to string has also been incorporated.

Quick start

Use it with promises or synchronously:

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

// Filter an array asynchronously...
await const res = jexl.eval('assoc[.first == "Lana"].last', context)
console.log(res) // Output: Kane

// Or synchronously!
console.log(jexl.evalSync('assoc[.first == "Lana"].last')) // Output: Kane

// Do math
await jexl.eval('age * (3 - 1)', context)
// 72

// Concatenate
await jexl.eval('name.first + " " + name["la" + "st"]', context)
// "Sterling Archer"

// Compound
await jexl.eval(
  'assoc[.last == "Figgis"].first == "Cyril" && assoc[.last == "Poovey"].first == "Pam"',
// true

// Use array indexes
await jexl.eval('assoc[1]', context)
// { first: 'Cyril', last: 'Figgis' }

// Use conditional logic
await jexl.eval('age > 62 ? "retired" : "working"', context)
// "working"

// Transform
jexl.addTransform('upper', (val) => val.toUpperCase())
await jexl.eval('"duchess"|upper + " " + name.last|upper', context)

// Transform asynchronously, with arguments
jexl.addTransform('getStat', async (val, stat) => dbSelectByLastName(val, stat))
try {
  const res = await jexl.eval('name.last|getStat("weight")', context)
  console.log(res) // Output: 184
} catch (e) {
  console.log('Database Error', e.stack)

// Functions too, sync or async, args or no args
jexl.addFunction('getOldestAgent', () => db.getOldestAgent())
await jexl.eval('age == getOldestAgent().age', context)
// false

// Add your own (a)synchronous operators
// Here's a case-insensitive string equality
  (left, right) => left.toLowerCase() === right.toLowerCase()
await jexl.eval('"Guest" _= "gUeSt"')
// true

// Compile your expression once, evaluate many times!
const { expr } = jexl
const danger = expr`"Danger " + place` // Also: jexl.compile('"Danger " + place')
danger.evalSync({ place: 'zone' }) // Danger zone
danger.evalSync({ place: 'ZONE!!!' }) // Danger ZONE!!! (Doesn't recompile the expression!)

Play with it


Jexl works on the backend, and on the frontend if bundled using a bundler like Parcel or Webpack.

Install from npm:

npm install jexl --save

or yarn:

yarn add jexl

and use it:

const jexl = require('jexl')

Async vs Sync: Which to use

There is little performance difference between eval and evalSync. The functional difference is that, if eval is used, Jexl can be customized with asynchronous operators, transforms, and even wait for unresolved promises in the context object with zero additional overhead or handling on the programmer's part. evalSync eliminates those advantages, exposing the expression to raw Promise objects if any are returned as the result of a custom transform or operator. However, if your application doesn't require async methods, the evalSync API can be simpler to use.

All the details

Unary Operators


Binary Operators

Add, Concat+
Divide and floor//
Power of^
Logical AND&&
Logical OR||


Not equal!=
Greater than>
Greater than or equal>=
Less than<
Less than or equal<=
Element in array or stringin
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.

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

Native Types

Booleanstrue, false
Strings"Hello "user"", 'Hey there!'
Numerics6, -7.2, 5, -3.14159
Objects{hello: "world!"}
Arrays['hello', 'world!']


Parentheses work just how you'd expect them to:

(83 + 1) / 242
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
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
employees[.first == 'Sterling'][{first: 'Sterling', last: 'Archer', age: 36}]
employees[.last == 'Tu' + 'nt'].firstCheryl
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].firstMalory


The power of Jexl 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 jexl.addTransform(name, function).

jexl.addTransform("split", (val, char) => val.split(char));
jexl.addTransform("lower", (val) => val.toLowerCase());
"Pam Poovey"|lower|split(' ')[1]poovey
"password==guest"|split('=' + '=')['password', 'guest']
Advanced Transforms

Using Transforms, Jexl 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:

const xml2json = require("xml2json");

jexl.addTransform("xml", (val) => xml2json.toJson(val, { object: true }));

const context = {
  xmlDoc: `

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

jexl.eval(expr, context).then(console.log); // Output: Cyril


While Transforms are the preferred way to change one value into another value, Jexl also allows top-level expression functions to be defined. Use these to provide access to functions that either don't require an input, or require multiple equally-important inputs. They can be added with jexl.addFunction(name, function). Like transforms, functions can return a value, or a Promise that resolves to the resulting value.

jexl.addFunction("min", Math.min);
jexl.addFunction("expensiveQuery", async () => db.runExpensiveQuery());
min(4, 2, 19)2
counts.missions || expensiveQuery()Query only runs if needed


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, Jexl 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 jexl.Jexl().

jexl.addBinaryOp({string} operator, {number} precedence, {function} fn, {boolean} [manualEval])

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.

If manualEval is true, the left and right arguments will be wrapped in objects with an eval function. Calling left.eval() or right.eval() will return a promise that resolves to that operand's actual value. This is useful to conditionally evaluate operands, and is how && and || work.

jexl.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.

jexl.addFunction({string} name, _{function} func)

Adds an expression function to this Jexl instance. See the Functions section above for information on the structure of an expression function.

jexl.addFunctions({{}} map)

Adds multiple functions from a supplied map of function name to expression function.

jexl.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.

jexl.addTransforms({{}} map)

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

jexl.compile({string} expression)

Constructs an Expression object around the given Jexl expression string. Expression objects allow a Jexl expression to be compiled only once but evaluated many times. See the Expression API below. Note that the only difference between this function and jexl.createExpression is that this function will immediately compile the expression, and throw any errors associated with invalid expression syntax.

jexl.createExpression({string} expression)

Constructs an Expression object around the given Jexl expression string. Expression objects allow a Jexl expression to be compiled only once but evaluated many times. See the Expression API below.

jexl.getTransform({string} name)

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

jexl.eval({string} expression, {{}} [context])

Returns {Promise<*>}. Evaluates an expression. The context map is optional.

jexl.evalSync({string} expression, {{}} [context])

Returns {*}. Evaluates an expression and returns the result. The context map is optional.

jexl.expr: tagged template literal

A convenient bit of syntactic sugar for jexl.createExpression

const someNumber = 10;
const expression = jexl.expr`5 + ${someNumber}`;
console.log(expression.evalSync()); // 15

Note that expr will stay bound to its associated Jexl instance even if it's pulled out of context:

const { expr } = jexl;
jexl.addTransform("double", (val) => val * 2);
const expression = expr`2|double`;
console.log(expression.evalSync()); // 4
jexl.removeOp({string} operator)

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


Expression objects are created via jexl.createExpression, jexl.compile, or jexl.expr, and are a convenient way to ensure jexl expressions compile only once, even if they're evaluated multiple times.


Returns self {Expression}. Forces the expression to compile, even if it was compiled before. Note that each compile will happen with the latest grammar and transforms from the associated Jexl instance.

expression.eval({{}} [context])

Returns {Promise<*>}. Evaluates the expression. The context map is optional.

expression.evalSync({{}} [context])

Returns {*}. Evaluates the expression and returns the result. The context map is optional.

Other implementations

PyJEXL - A Python-based JEXL parser and evaluator.


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


Created by Tom Shawver in 2015 and contributed to by these great people.

Jexl was originally created at TechnologyAdvice in Nashville, TN.

Forked by Aaron Stephens of Firehammer Solutions in 2023.



Package last updated on 24 Oct 2023

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