JsonLogic

Why use JsonLogic?

If you’re looking for a way to share logic between front-end and back-end code, and even store it in a database, JsonLogic might be a fit for you.

JsonLogic isn’t a full programming language. It’s a small, safe way to delegate one decision. You could store a rule in a database to decide later. You could send that rule from back-end to front-end so the decision is made immediately from user input. Because the rule is data, you can even build it dynamically from user actions or GUI input.

JsonLogic has no setters, no loops, no functions or gotos. One rule leads to one decision, with no side effects and deterministic computation time.

Virtues

1. Terse.
2. Consistent. {"operator" : ["values" ... ]} Always.
3. Secure. We never eval(). Rules only have read access to data you provide, and no write access to anything.
4. Flexible. Easy to add new operators, easy to build complex structures.

Examples

Simple

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

This is a simple rule, 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) )

JsonLogic is, effectively, an abstract syntax tree, so order of operations is unambiguous.

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

What next?

Check out the complete list of supported operations or try out your own rules in the web playground.