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json-logic-js

json-logic-js

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 JsonLogic.com, 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

Examples

Simple

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

Compound

Here we're beginning to nest rules.

jsonLogic.apply(
  {"and" : [
    { ">" : [3,1] },
    { "<" : [1,3] }
  ] }
);
// true 

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

( (3 > 1) && (1 < 3) )

Data-Driven

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:

jsonLogic.apply(
  { "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:

jsonLogic.apply(
  { "var" : "a" },
  { a : 1, b : 2 }
);
// 1 

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

jsonLogic.apply(
  {"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.

//Always 
jsonLogic.apply(true, data_will_be_ignored);
// true 
 
//Never 
jsonLogic.apply(false, i_wasnt_even_supposed_to_be_here);
// false 

Installation

To parse JsonLogic rules in a JavaScript frontend, install this library is via Bower:

bower install --save json-logic-js

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

npm install json-logic-js

Note that this project uses a module loader that also makes it suitable for RequireJS projects.

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

curl -O https://raw.githubusercontent.com/jwadhams/json-logic-js/master/logic.js

Compatibility

This library makes use of Array.map 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.