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    1.3.6 • Public • Published

    jsep: A Tiny JavaScript Expression Parser

    jsep is a simple expression parser written in JavaScript. It can parse JavaScript expressions but not operations. The difference between expressions and operations is akin to the difference between a cell in an Excel spreadsheet vs. a proper JavaScript program.

    Why jsep?

    I wanted a lightweight, tiny parser to be included in one of my other libraries. esprima and other parsers are great, but had more power than I need and were way too large to be included in a library that I wanted to keep relatively small.

    jsep's output is almost identical to esprima's, which is in turn based on SpiderMonkey's.

    Custom Build

    While in the jsep project directory, run:

    npm install
    npm run default

    The jsep built files will be in the build/ directory.



    <script type="module">
      import jsep from '/PATH/TO/jsep.min.js';
      const parsed = jsep('1 + 1');
    <script src="/PATH/TO/jsep.iife.min.js"></script>
    let parse_tree = jsep("1 + 1");


    First, run npm install jsep. Then, in your source file:

    // ESM:
    import jsep from 'jsep';
    const parse_tree = jsep('1 + 1');
    // or:
    import { Jsep } from 'jsep';
    const parse_tree = Jsep.parse('1 + 1');
    // CJS:
    const jsep = require('jsep').default;
    const parsed = jsep('1 + 1');
    // or:
    const { Jsep } = require('jsep');
    const parse_tree = Jsep.parse('1 + 1');

    Custom Operators

    // Add a custom ^ binary operator with precedence 10
    // (Note that higher number = higher precedence)
    jsep.addBinaryOp("^", 10);
    // Add exponentiation operator (right-to-left)
    jsep.addBinaryOp('**', 11, true);
    // Add a custom @ unary operator
    // Remove a binary operator
    // Remove a unary operator

    Custom Identifiers

    You can add or remove additional valid identifier chars. ('_' and '$' are already treated like this.)

    // Add a custom @ identifier
    // Removes a custom @ identifier


    JSEP supports defining custom hooks for extending or modifying the expression parsing. Plugins are registered by calling jsep.plugins.register() with the plugin(s) as the argument(s).

    JSEP-provided plugins:

    ternary Built-in by default, adds support for ternary a ? b : c expressions
    arrow Adds arrow-function support: v => !!v
    assignment Adds assignment and update expression support: a = 2, a++
    comment Adds support for ignoring comments: a /* ignore this */ > 1 // ignore this too
    new Adds 'new' keyword support: new Date()
    numbers Adds hex, octal, and binary number support, ignore _ char
    object Adds object expression support: { a: 1, b: { c }}
    regex Adds support for regular expression literals: /[a-z]{2}/ig
    spread Adds support for the spread operator, fn(...[1, ...a]). Works with object plugin, too
    template Adds template literal support: `hi ${name}`

    How to add plugins:

    Plugins have a name property so that they can only be registered once. Any subsequent registrations will have no effect. Add a plugin by registering it with JSEP:

    import jsep from 'jsep';
    import ternary from '@jsep-plugin/ternary';
    import object from '@jsep-plugin/object';
    jsep.plugins.register(ternary, object);

    List plugins:

    Plugins are stored in an object, keyed by their name. They can be retrieved through jsep.plugins.registered.

    Writing Your Own Plugin:

    Plugins are objects with two properties: name and init. Here's a simple plugin example:

    const plugin = {
      name: 'the plugin',
      init(jsep) {
        jsep.hooks.add('gobble-expression', function myPlugin(env) {
          if (this.char === '@') {
            this.index += 1;
            env.node = {
              type: 'MyCustom@Detector',

    This example would treat the @ character as a custom expression, returning a node of type MyCustom@Detector.


    Most plugins will make use of hooks to modify the parsing behavior of jsep. All hooks are bound to the jsep instance, are called with a single argument, and return void. The this context provides access to the internal parsing methods of jsep to allow reuse as needed. Some hook types will pass an object that allows reading/writing the node property as needed.

    Hook Types
    • before-all: called just before starting all expression parsing.
    • after-all: called after parsing all. Read/Write arg.node as required.
    • gobble-expression: called just before attempting to parse an expression. Set arg.node as required.
    • after-expression: called just after parsing an expression. Read/Write arg.node as required.
    • gobble-token: called just before attempting to parse a token. Set arg.node as required.
    • after-token: called just after parsing a token. Read/Write arg.node as required.
    • gobble-spaces: called when gobbling whitespace.
    The this context of Hooks
    export interface HookScope {
        index: number;
        readonly expr: string;
        readonly char: string; // current character of the expression
        readonly code: number; // current character code of the expression
        gobbleSpaces: () => void;
        gobbleExpressions: (untilICode?: number) => Expression[];
        gobbleExpression: () => Expression;
        gobbleBinaryOp: () => PossibleExpression;
        gobbleBinaryExpression: () => PossibleExpression;
        gobbleToken: () => PossibleExpression;
        gobbleTokenProperty: (node: Expression) => Expression;
        gobbleNumericLiteral: () => PossibleExpression;
        gobbleStringLiteral: () => PossibleExpression;
        gobbleIdentifier: () => PossibleExpression;
        gobbleArguments: (untilICode: number) => PossibleExpression;
        gobbleGroup: () => Expression;
        gobbleArray: () => PossibleExpression;
        throwError: (msg: string) => void;


    jsep is under the MIT license. See LICENSE file.


    Some parts of the latest version of jsep were adapted from the esprima parser.




    npm i jsep

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