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    3.0.1 • Public • Published

    UCAST JavaScript

    @ucast/js NPM version UCAST join the chat

    This package is a part of ucast ecosystem. It provides interpreter that can execute conditions AST in JavaScript against any JavaScript object.


    npm i @ucast/js
    # or
    yarn add @ucast/js
    # or
    pnpm add @ucast/js

    Getting Started

    Interpret conditions AST

    First of all, you need AST to interpret it. For the sake of an example, we will create it manually:

    import { CompoundCondition, FieldCondition } from '@ucast/core';
    import { interpret } from '@ucast/js';
    // x > 5 && y < 10
    const condition = new CompoundCondition('and', [
      new FieldCondition('gt', 'x', 5),
      new FieldCondition('lt', 'y', 10),
    interpret(condition, { x: 2, y: 1 }); // false
    interpret(condition, { x: 6, y: 7 }); // true

    The default interpret function:

    • supports the next operators, implemented according to MongoDB query language:

      • eq, ne
      • lt, lte
      • gt, gte
      • within (the same as in but in is a reserved word in JavaScript), nin
      • all
      • regex
      • or, nor, and, not
      • exists
      • size
      • mod
      • where,
      • elemMatch
    • supports dot notation to access nested object property values in conditions:

      const condition = new FieldCondition('eq', 'address.street', 'some street');
      interpret(condition, { address: { street: 'another street' } }); // false
    • compare values by strict equality, so variables that reference objects are equal only if they are references to the same object:

      const address = { street: 'test' };
      const condition = new FieldCondition('eq', 'address', address);
      interpret(condition, { address }) // true
      interpret(condition, { address: { street: 'test' } }) // false, objects are compared by strict equality

    Custom interpreter

    Sometimes you may want to reduce (or restrict) amount of supported operators (e.g., to utilize tree-shaking and reduce bundle size). To do this you can create a custom interpreter manually:

    import { FieldCondition } from '@ucast/core';
    import { createJsInterpreter, eq, lt, gt } from '@ucast/js';
    // supports only $eq, $lt and $gt operators
    const interpret = createJsInterpreter({ eq, lt, gt });
    const condition = new FieldCondition('in', 'x', [1, 2]);
    interpret(condition, { x: 1 }) // throws Error, `$in` is not supported

    Custom object matching

    You can also provide a custom get or compare function. So, you can implement custom logic to get object's property or to compare values. compare is used everywhere equality or comparison is required (e.g., in $in, $lt, $gt). This function must return 1 if a > b, -1 if a < b and 0 if a === b.

    Let's enhance our interpreter to support deep object comparison using [lodash]:

    import isEqual from 'lodash/isEqual';
    import { createJsInterpreter, allInterpreters, compare } from '@ucast/js';
    const interpret = createJsInterpreter(allInterpreters, {
      compare(a, b) {
        if (typeof a === typeof b && typeof a === 'object' && isEqual(a, b)) {
          return 0;
        return compare(a, b);
    const condition = new FieldCondition('eq', 'x', { active: true });
    interpret(condition, { x: { active: true } }); // true

    Custom Operator Interpreter

    Any operator is just a function that accepts 3 parameters and returns boolean result. To see how to implement this function let's create $type interpreter that checks object property type using typeof operator:

    import { createJsInterpreter } from '@ucast/js';
    function type(condition, object, { get }) {
      return typeof get(object, condition.field) === condition.value;
    const interpret = createJsInterpreter({ type });
    const condition = new FieldCondition('type', 'x', 'number');
    interpret(condition, { x: 1 }); // true

    Pay attention that object property is got by using get function. Make sure that you always use get function in custom operators to get object's property value, otherwise your operator will not support dot notation.

    Want to help?

    Want to file a bug, contribute some code, or improve documentation? Excellent! Read up on guidelines for contributing


    Apache License, Version 2.0


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