naturallymitchell / json-logic-js

Build complex rules, serialize them as JSON, and execute them in JavaScript

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This parser accepts JsonLogic rules and executes them in JavaScript.

The JsonLogic format is designed to allow you to share rules (logic) between front-end and back-end code (regardless of language difference), even to store logic along with a record in a database. JsonLogic is documented extensively at, including examples of every supported operation and a place to try out rules in your browser.

The same format can also be executed in PHP by the library json-logic-php

Table of Contents


To parse JsonLogic rules in a JavaScript backend (like Node.js), install this library via NPM:

npm install @axa-ch/json-logic-js --save

Note that this project provides:

  • an ESM build for cherry picking
  • a module loader that also makes it suitable for RequireJS projects.
  • a minified bundle ready for the browser

If that doesn't suit you, and you want to manage updates yourself, the entire library is self-contained in dist/jsonLogic.js and you can download it straight into your project as you see fit.

Cherry-picked build

If the default bundle size is too big for you or you only need certain operations, just utilize the ESM build and create your customized jsonLogic object as follows:

import createJsonLogic from './createJsonLogic';

// pick just what you need, or create your own
import { variable, equal } from './operations';

const jsonLogic = createJsonLogic({ variable, equal });

export default jsonLogic;



jsonLogic.apply( { "==" : [1, 1] } );
// true

This is a simple test, equivalent to 1 == 1. A few things about the format:

  1. The operator is always in the "key" position. There is only one key per JsonLogic rule.
  2. The values are typically an array.
  3. Each value can be a string, number, boolean, array (non-associative), or null


Here we're beginning to nest rules.

  {"and" : [
    { ">" : [3,1] },
    { "<" : [1,3] }
  ] }
// true

In an infix language (like JavaScript) this could be written as:

( (3 > 1) && (1 < 3) )


Obviously these rules aren't very interesting if they can only take static literal data. Typically jsonLogic will be called with a rule object and a data object. You can use the var operator to get attributes of the data object:

  { "var" : ["a"] }, // Rule
  { a : 1, b : 2 }   // Data
// 1

If you like, we support syntactic sugar on unary operators to skip the array around values:

  { "var" : "a" },
  { a : 1, b : 2 }
// 1

You can also use the var operator to access an array by numeric index:

  {"var" : 1 },
  [ "apple", "banana", "carrot" ]
// "banana"

Here's a complex rule that mixes literals and data. The pie isn't ready to eat unless it's cooler than 110 degrees, and filled with apples.

var rules = { "and" : [
  {"<" : [ { "var" : "temp" }, 110 ]},
  {"==" : [ { "var" : "pie.filling" }, "apple" ] }
] };

var data = { "temp" : 100, "pie" : { "filling" : "apple" } };

jsonLogic.apply(rules, data);
// true

Always and Never

Sometimes the rule you want to process is "Always" or "Never." If the first parameter passed to jsonLogic is a non-object, non-associative-array, it is returned immediately.

jsonLogic.apply(true, data_will_be_ignored);
// true

jsonLogic.apply(false, i_wasnt_even_supposed_to_be_here);
// false


This library makes use of Array.isArray, Array.forEach, and Array.reduce, so it's not exactly Internet Explorer 8 friendly.

If you want to use JsonLogic and support deprecated browsers, you could easily use BabelJS's polyfill or directly incorporate the polyfills documented on MDN for map and reduce.


It's not possible to include everyone's excellent ideas without the core library bloating, bringing in a ton of outside dependencies, or occasionally causing use case conflicts (some people need to safely execute untrusted rules, some people need to change outside state).

Check out the documentation for adding custom operations and be sure to stop by the Wiki page of custom operations to see if someone has already solved your problem or to share your solution.

Operator Meta Tags

You can specify the following meta tags as properties of your operators function:

  • op: usefull if you operator name should be a reserver keyword, like var, <, +, etc., type string
  • withApply: usefull if you need more control over operator recursion, type boolean
  • deepFirst: usefull disable default deep-first recursion, type boolean


Build complex rules, serialize them as JSON, and execute them in JavaScript

License:MIT License


Language:JavaScript 92.8%Language:HTML 7.2%