json-expression-eval

json-expression-eval

A Node.js module that evaluates a json described boolean expressions using dynamic functions and a given context.

The module is strictly typed, ensuring that passed expressions are 100% valid at compile time.

This module is especially useful if you need to serialize complex expressions (to be saved in DB for example)

Installation

npm install json-expression-eval

Or

yarn add json-expression-eval

Usage

Please see tests and examples dir for more usages and example (under /src)

import { Expression, evaluate } from 'json-expression-eval';

interface IExampleContext {
  userId: string;
  times: number;
}

const exampleContext: IExampleContext = {
  userId: '[e-mail]',
  times: 5,
};

const functionsTable = {
  countRange: ([min, max]: [number, number], context: IExampleContext): boolean => {
    return context.times >= min && context.times < max;
  },
};

const expression: Expression<IExampleContext, typeof functionsTable> = {
  or: [
    {
      userId: '[e-mail]',
    },
    {
      and: [
        {
          countRange: [2, 6],
        },
        {
          userId: '[e-mail]',
        },
      ],
    },
  ],
};
console.log(evaluate(expression, exampleContext, functionsTable)); // true

Expression

There are 4 types of operators you can use (evaluated in that order of precedence):

• and - accepts a non empty list of operators
• or - accepts a non empty list of operators
• not - accepts another operator
• <user defined funcs> - accepts any type of argument and evaluated by the user defined functions and given context.
• <compare funcs> - operates on one of the context properties and compare it to a given value.
  • {property: {op: value}}
    • available ops:
      • gt - >
      • gte - >=
      • lt - <
      • lte - <=
      • eq - ===
      • neq - !==
  • {property: value}
    • compares the property to that value (shorthand to the eq op)

Example expressions, assuming we have the user and maxCount user defined functions in place can be:

{  
   "or":[  
      {  
         "not":{  
            "user":"[e-mail]"
         }
      },
      {  
         "maxCount":1
      },
      {  
         "times": { "eq" : 5}
      },
      {  
         "country": "USA"
      }
   ]
}