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    1.0.6 • Public • Published



    A conditional expression parser and interpreter. This is a safe way to evaluate boolean/logical expressions without using evil eval.

    The expressions follow the JavaScript syntax. You can check the comprehensive tests for examples on functionality. Variables are evaluated at a later stage when evaluate() function is called with a context object. The context provides an object with the variables to evaluate.

    Quick example: evaluate("user.firstName === firstNames[0]", { user: { firstName: 'Mark' }, firstNames: ['John'] }) would return false.


    npm i angel-eval


    Supported Types

    • Integer (ex: 104, ...)
    • Float (ex: 5.5, 1.2e5, ...)
    • String (ex: 'hello' or "hello" -- no backtick (``) syntax yet)

    Unary Operators

    • !expression (ex: !true, !variable, !(expression && expression), ...)

    Equality Operators

    • === (ex: 5 === 5, var === 5, ...)
    • !== (ex: var !== false, ...)

    == and != are not supported because they are usually bad practice.

    Relational Operators

    • >
    • >=
    • <
    • <=

    Logical Operators

    • &&
    • ||


    Normal paranthesis grouping can be used to group multiple conditional expressions together.

    evaluate("x === 4 && y === 4 || z", { x: 3, y: 5, z: "hello" }); // false
    evaluate("(x === 4 && y === 4) || (z)", { x: 3, y: 5, z: "hello" }); // true


    Whitespace is allowed where it would usually be expected to be allowed in JavaScript.


    angel-eval provides two public functions:


    The evaluate function evaluates the expression provided. The variables used are read from the context object.

    const { evaluate } = require("angel-eval");
     * Evaluates the given expression using the context provided.
     * @param {string} expression
     * @param {{ [key: string]: any }} [context]
     * @param {boolean} [strictBoolean] Whether to always return a boolean.
     * @returns {boolean}
    evaluate(expression: string, context?: { [key: string]: any }, strictBoolean?: boolean) => boolean


    const { parse } = require("angel-eval");
     * Parses the given expression and returns an
     * `Evaluatable` that can be evaluated with a context.
     * This function is memoized for optimization.
     * You can access the function's cache using `parse.cache`.
     * @param {string} expression
     * @returns {Evaluatable}
    parse(expression: string) => Evaluatable


    const { evaluate } = require("angel-eval");
    evaluate("foo.x !== y", { foo: { x: 5 }, y: 5 }); // false
    evaluate('gender === "female"', { gender: "female" }); // true
    evaluate("5 === potato", { potato: 5 }); // true
    evaluate("x <= 76 && (y > 5 || z === 'hello world')", {
      x: 76,
      y: 5,
      z: "hello world",
    }); // true
    evaluate("!x || y", { x: true, y: 2 }); // true
    evaluate("(x || y) === 'world'", { x: false, y: "world" }); // true

    Under the hood

    This uses nearley to generate a parser. Variables are evaluated from the context using lodash.get.

    Expression are parsed once and are memoized so that consequent operations would just evaluate and not parse those expressions again. Memoization is done on the parse() function using lodash.memoize, so if you need to reset memoization cache you can run parse.cache.clear().

    Additional Thoughts

    Some can say that using Function may be safer than eval, but to me both are equally dangerous since they have access to the JavaScript interpreter and Function can still see global variables and modify app state.

    angel-eval's method is safe because no eval() is actually happening. Expressions cannot access any variables not specified in the context object. The expression is either parsed correctly and interpreted as needed, or it will fail. No additional code can be run through this and no changes to your app can actually happen.

    lodash does come with vulnerabilities sometimes and this package uses both _.get and _.memoize, so later on it might be worth moving away from lodash (sorry lo, I still love you).


    This package uses Babel to build the source.

    If you change the grammar, you need to rebuild it. You can do so using:

    npm run build:grammar


    I have a comprehensive list of tests but I still would like to add more. To run current tests you can use:

    npm run test 

    This will not build the grammar before running. If grammar is changed, it needs to be rebuilt using npm run build:grammar.


    This project has 100% coverage. If you find the need to add more tests, contributions are greatly appreciated. Thanks!


    Any contribution is welcome and appreciated. Please create a pull request if you have changes you would like to contribute.

    If you would like to report a bug, please submit an issue.


    Thanks to Dr. Barry Wittman for introducing me to compiler work and teaching me great things. Check out his amazing programming language: Shadow.


    npm i angel-eval

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