Jexl

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

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

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

// Do math
const res = await jexl.eval('age * (3 - 1)', context)
console.log(res) // Output: 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"', context)
// 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)
// "DUCHESS ARCHER"

// 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)
}

// Add your own (a)synchronous operators
// Here's a case-insensitive string equality
jexl.addBinaryOp('_=', 20, (left, right) => left.toLowerCase() === right.toLowerCase())
await jexl.eval('"Guest" _= "gUeSt"')
// true

Installation

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

Operation	Symbol
Negate	!

Binary Operators

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

Comparisons

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!']

Groups

Parentheses work just how you'd expect them to:

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

Identifiers

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

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

Transforms

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())

Expression	Result
"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: `
    <Employees>
      <Employee>
        <FirstName>Cheryl</FirstName>
        <LastName>Tunt</LastName>
      </Employee>
      <Employee>
        <FirstName>Cyril</FirstName>
        <LastName>Figgis</LastName>
      </Employee>
    </Employees>`
}

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

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

Context

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!

API

jexl.Jexl

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)

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.

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.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.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.removeOp({string} operator)

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

Other implementations

PyJEXL - A Python-based JEXL parser and evaluator.

License

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

Credits

Jexl was originally created at TechnologyAdvice in Nashville, TN.