@marcbachmann/cel-js

A high-performance, zero-dependency implementation of the Common Expression Language (CEL) in JavaScript.

🚀 Use the CEL JS Playground to test expressions.

Overview

CEL (Common Expression Language) is a non-Turing complete language designed for simplicity, speed, safety, and portability. This JavaScript implementation provides a fast, lightweight CEL evaluator perfect for policy evaluation, configuration, and embedded expressions.

Features

🚀 Zero Dependencies - No external packages required
⚡ High Performance - Up to 22x faster evaluation, 3x faster parsing than alternatives
📦 ES Modules - Modern ESM with full tree-shaking support
🔒 Type Safe - Environment API with type checking for variables, custom types and functions
🎯 Most of the CEL Spec - Including macros, custom functions and types, input variables, and all operators (except optional chaining)
📘 TypeScript Support - Full type definitions included

Installation

npm install @marcbachmann/cel-js

Quick Start

import {evaluate} from '@marcbachmann/cel-js'

// Simple evaluation
evaluate('1 + 2 * 3') // 7n

// With context
const allowed = evaluate(
  'user.age >= 18 && "admin" in user.roles',
  {user: {age: 30, roles: ['admin', 'user']}}
)
// true

API

Simple Evaluation

import {evaluate, parse} from '@marcbachmann/cel-js'

// Direct evaluation
evaluate('1 + 2') // 3n

// With variables
evaluate('name + "!"', {name: 'Alice'}) // "Alice!"

// Parse once, evaluate multiple times for better performance
const expr = parse('user.age >= minAge')
expr({user: {age: 25}, minAge: 18}) // true
expr({user: {age: 16}, minAge: 18}) // false

// Access parsed AST and type checking
console.log(expr.ast)        // AST representation
const typeCheck = expr.check() // Type check without evaluation

Environment API (Recommended)

For type-safe expressions with custom functions and operators:

import {Environment} from '@marcbachmann/cel-js'

const env = new Environment()
  .registerVariable('user', 'map')
  .registerVariable('minAge', 'int')
  .registerFunction('isAdult(int): bool', age => age >= 18n)
  .registerOperator('string * int', (str, n) => str.repeat(Number(n)))

// Type-checked evaluation
env.evaluate('isAdult(user.age)', {
  user: {age: 25n},
  minAge: 18n
})

// Custom operators
env.evaluate('"Hi" * 3') // "HiHiHi"

Environment Options

new Environment({
  // Treat undeclared variables as dynamic type
  unlistedVariablesAreDyn: false
})

Environment Methods

registerVariable(name, type) - Declare a variable with type checking
registerType(typename, constructor) - Register custom types
registerFunction(signature, handler) - Add custom functions
registerOperator(signature, handler) - Add custom operators
hasVariable(name) - Check if variable is registered
parse(expression) - Parse expression for reuse
evaluate(expression, context) - Evaluate with context
check(expression) - Validate expression types without evaluation

Supported Types: int, uint, double, string, bool, bytes, list, map, timestamp, duration, null_type, type, dyn, or custom types

Type Checking

Validate expressions before evaluation to catch type errors early:

import {Environment, TypeError} from '@marcbachmann/cel-js'

const env = new Environment()
  .registerVariable('age', 'int')
  .registerVariable('name', 'string')

// Check expression validity
const result = env.check('age >= 18 && name.startsWith("A")')

if (result.valid) {
  console.log(`Expression is valid, returns: ${result.type}`) // bool
  // Safe to evaluate
  const value = env.evaluate('age >= 18 && name.startsWith("A")', {
    age: 25n,
    name: 'Alice'
  })
} else {
  console.error(`Type error: ${result.error.message}`)
}

// Detect errors without evaluation
const invalid = env.check('age + name') // Invalid: can't add int + string
console.log(invalid.valid) // false
console.log(invalid.error.message) // "Operator '+' not defined for types 'int' and 'string'"

Benefits:

Catch type mismatches before runtime
Validate user-provided expressions safely
Get inferred return types for expressions
Better error messages with source location

Language Features

Operators

// Arithmetic
evaluate('10 + 5 - 3')     // 12n
evaluate('10 * 5 / 2')     // 25n
evaluate('10 % 3')         // 1n

// Comparison
evaluate('5 > 3')          // true
evaluate('5 >= 5')         // true
evaluate('5 == 5')         // true
evaluate('5 != 4')         // true

// Logical
evaluate('true && false')  // false
evaluate('true || false')  // true
evaluate('!false')         // true

// Ternary
evaluate('5 > 3 ? "yes" : "no"')  // "yes"

// Membership
evaluate('2 in [1, 2, 3]')        // true
evaluate('"ell" in "hello"')      // true

Data Types

// Numbers (default to BigInt)
evaluate('42')           // 42n
evaluate('3.14')         // 3.14
evaluate('0xFF')         // 255n

// Strings
evaluate('"hello"')      // "hello"
evaluate('r"\\n"')       // "\\n" (raw string)
evaluate('"""multi\nline"""')  // "multi\nline\n"

// Bytes
evaluate('b"hello"')     // Uint8Array
evaluate('b"\\xFF"')     // Uint8Array [255]

// Collections
evaluate('[1, 2, 3]')           // [1n, 2n, 3n]
evaluate('{name: "Alice"}')     // {name: "Alice"}

// Other
evaluate('true')         // true
evaluate('null')         // null

Built-in Functions

// Type conversion
evaluate('string(123)')           // "123"
evaluate('int("42")')             // 42n
evaluate('double("3.14")')        // 3.14
evaluate('bytes("hello")')        // Uint8Array
evaluate('dyn(42)')               // Converts to dynamic type

// Collections
evaluate('size([1, 2, 3])')       // 3n
evaluate('size("hello")')         // 5n
evaluate('size({a: 1, b: 2})')    // 2n

// Time
evaluate('timestamp("2024-01-01T00:00:00Z")')  // Date

// Type checking
evaluate('type(42)')              // int
evaluate('type("hello")')         // string

String Methods

evaluate('"hello".contains("ell")')         // true
evaluate('"hello".startsWith("he")')        // true
evaluate('"hello".endsWith("lo")')          // true
evaluate('"hello".matches("h.*o")')         // true
evaluate('"hello".size()')                  // 5n
evaluate('"hello".indexOf("ll")')           // 2n
evaluate('"hello world".indexOf("o", 5)')   // 7n (search from index 5)
evaluate('"hello".lastIndexOf("l")')        // 3n
evaluate('"hello".substring(1)')            // "ello"
evaluate('"hello".substring(1, 4)')         // "ell"

List Methods

evaluate('[1, 2, 3].size()')                // 3n
evaluate('["a", "b", "c"].join()')          // "abc"
evaluate('["a", "b", "c"].join(", ")')      // "a, b, c"

Bytes Methods

evaluate('b"hello".size()')                 // 5n
evaluate('b"hello".string()')               // "hello"
evaluate('b"hello".hex()')                  // "68656c6c6f"
evaluate('b"hello".base64()')               // "aGVsbG8="
evaluate('b"{\\"x\\": 42}".json()')         // {x: 42n}
evaluate('b"hello".at(0)')                  // 104n (byte value at index)

Timestamp Methods

All timestamp methods support an optional timezone parameter (e.g., "America/New_York", "UTC"):

const ctx = {t: new Date('2024-01-15T14:30:45.123Z')}

evaluate('t.getFullYear()', ctx)            // 2024n
evaluate('t.getMonth()', ctx)               // 0n (January, 0-indexed)
evaluate('t.getDayOfMonth()', ctx)          // 15n
evaluate('t.getDayOfWeek()', ctx)           // 1n (Monday, 0=Sunday)
evaluate('t.getDayOfYear()', ctx)           // 15n
evaluate('t.getHours()', ctx)               // 14n
evaluate('t.getMinutes()', ctx)             // 30n
evaluate('t.getSeconds()', ctx)             // 45n
evaluate('t.getMilliseconds()', ctx)        // 123n

// With timezone
evaluate('t.getHours("America/New_York")', ctx)  // 9n (UTC-5)

Macros

const ctx = {
  numbers: [1, 2, 3, 4, 5],
  users: [
    {name: 'Alice', admin: true},
    {name: 'Bob', admin: false}
  ]
}

// Check property exists
evaluate('has(user.email)', {user: {}})  // false

// All elements match
evaluate('numbers.all(n, n > 0)', ctx)   // true

// Any element matches
evaluate('numbers.exists(n, n > 3)', ctx)  // true

// Exactly one matches
evaluate('numbers.exists_one(n, n == 3)', ctx)  // true

// Transform
evaluate('numbers.map(n, n * 2)', ctx)
// [2n, 4n, 6n, 8n, 10n]

// Filter
evaluate('numbers.filter(n, n > 2)', ctx)
// [3n, 4n, 5n]

// Filter + Transform
evaluate('users.filter(u, u.admin).map(u, u.name)', ctx)

// Or using three arg form of .map
evaluate('users.map(u, u.admin, u.name)', ctx)
// ["Alice"]

Custom Types

import {Environment} from '@marcbachmann/cel-js'

class Vector {
  constructor(x, y) {
    this.x = x
    this.y = y
  }
  add(other) {
    return new Vector(this.x + other.x, this.y + other.y)
  }
}

const env = new Environment()
  .registerType('Vector', Vector)
  .registerVariable('v1', 'Vector')
  .registerVariable('v2', 'Vector')
  .registerOperator('Vector + Vector', (a, b) => a.add(b))
  .registerFunction('magnitude(Vector): double', (v) =>
    Math.sqrt(v.x * v.x + v.y * v.y)
  )

const result = env.evaluate('magnitude(v1 + v2)', {
  v1: new Vector(3, 4),
  v2: new Vector(1, 2)
})
// 7.211102550927978

Performance

Benchmark results comparing against the cel-js package on Node.js v24.8.0 (Apple Silicon):

Parsing Performance

Average: 3.1x faster (range: 0.76x - 14.8x)
Simple expressions: 7-15x faster
Array/Map creation: 8-10x faster

Evaluation Performance

Average: 22x faster (range: 5.5x - 111x)
Simple values: 64-111x faster
Collections: 46-58x faster
Complex logic: 5-14x faster

Highlights

Operation	Parsing	Evaluation
Simple number	7.3x	111x
Array creation	10.1x	57.9x
Map creation	8.6x	46x
Complex authorization	1.3x	5.5x

Run benchmarks: npm run benchmark

Error Handling

import {evaluate, ParseError, EvaluationError, TypeError} from '@marcbachmann/cel-js'

try {
  evaluate('invalid + + syntax')
} catch (error) {
  if (error instanceof ParseError) {
    console.error('Syntax error:', error.message)
  } else if (error instanceof EvaluationError) {
    console.error('Runtime error:', error.message)
  }
}

// Type checking returns errors without throwing
const env = new Environment().registerVariable('x', 'int')
const result = env.check('x + "string"')
if (!result.valid && result.error instanceof TypeError) {
  console.error('Type error:', result.error.message)
}

Examples

Authorization Rules

import {Environment} from '@marcbachmann/cel-js'

// Instantiating an environment is expensive, please do that outside hot code paths
const authEnv = new Environment()
  .registerVariable('user', 'map')
  .registerVariable('resource', 'map')

const canEdit = authEnv.parse(`
  user.isActive &&
  (user.role == "admin" ||
  user.id == resource.ownerId)
`)

canEdit({
  user: {id: 123, role: 'user', isActive: true},
  resource: {ownerId: 123}
}) // true

Data Validation

import {Environment} from '@marcbachmann/cel-js'

// Instantiating an environment is expensive, please do that outside hot code paths
const validator = new Environment()
  .registerVariable('email', 'string')
  .registerVariable('age', 'int')
  .registerFunction('isValidEmail(string): bool',
    email => /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)
  )

const valid = validator.evaluate(
  'isValidEmail(email) && age >= 18 && age < 120',
  {email: 'user@example.com', age: 25n}
)

Feature Flags

import {parse} from '@marcbachmann/cel-js'

const flags = {
  'new-dashboard': parse(
    'user.betaUser || user.id in allowedUserIds'
  ),
  'premium-features': parse(
    'user.subscription == "pro" && !user.trialExpired'
  )
}

function isEnabled(feature, context) {
  return flags[feature]?.(context) ?? false
}

TypeScript

Full TypeScript support included:

import {Environment, evaluate, ParseError} from '@marcbachmann/cel-js'

// Instantiating an environment is expensive, please do that outside hot code paths
const env = new Environment()
  .registerVariable('count', 'int')
  .registerFunction('multiplyByTwo(int): int', (x) => x * 2n)

const result: any = env.evaluate('multiplyByTwo(count)', {count: 21n})

Contributing

Contributions welcome! Please open an issue before submitting major changes.

# Run tests
npm test

# Run benchmarks
npm run benchmark

# Run in watch mode
npm run test:watch

Name		Name	Last commit message	Last commit date
Latest commit History 98 Commits
benchmark		benchmark
lib		lib
test		test
.drone.yaml		.drone.yaml
.gitignore		.gitignore
.nvmrc		.nvmrc
.prettierignore		.prettierignore
.prettierrc		.prettierrc
LICENSE		LICENSE
README.md		README.md
biome.json		biome.json
eslint.config.js		eslint.config.js
examples.js		examples.js
langdef.md		langdef.md
package-lock.json		package-lock.json
package.json		package.json
renovate.json		renovate.json

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@marcbachmann/cel-js

Overview

Features

Installation

Quick Start

API

Simple Evaluation

Environment API (Recommended)

Environment Options

Environment Methods

Type Checking

Language Features

Operators

Data Types

Built-in Functions

String Methods

List Methods

Bytes Methods

Timestamp Methods

Macros

Custom Types

Performance

Parsing Performance

Evaluation Performance

Highlights

Error Handling

Examples

Authorization Rules

Data Validation

Feature Flags

TypeScript

Contributing

License

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