happy-ferret / meriyah

A 100% compliant, self-hosted javascript parser with high focus on both performance and stability

Meriyah

A 100% compliant, self-hosted javascript parser with high focus on both performance and stability

Features

• Conforms to the standard ECMAScript® 2020 (ECMA-262 10th Edition) language specification
• Support TC39 proposals via option
• Support for additional ECMAScript features for Web Browsers
• Optionally track syntactic node locations
• Emits an ESTree-compatible abstract syntax tree.
• No backtracking
• Reduced memory usage
• Very well tested (~81 000 unit tests with full code coverage))
• Lightweight - ~76 KB minified

ESNext features

• Decorators
• Class Public Instance Fields & Private Instance Fields
• Hashbang Grammar
• Private methods
• Static class fields and private static methods

Note: These features need to be enabled with the next option.

API

Meriyah generates AST according to ESTree AST format, and can be used to perform syntactic analysis (parsing) of a JavaScript program, and with ES2015 and later a JavaScript program can be either a script or a module.

The parse method exposed by meriyah takes an optional options object which allows you to specify whether to parse in script mode (the default) or in module mode.

Here is a quick example to parse a script:

import { parseScript } from './meriyah';

parseScript('({x: [y] = 0} = 1)');

This will return when serialized in json:

{
    type: "Program",
    sourceType: "script",
    body: [
        {
            type: "ExpressionStatement",
            expression: {
                type: "AssignmentExpression",
                left: {
                    type: "ObjectPattern",
                    properties: [
                        {
                            type: "Property",
                            key: {
                                type: "Identifier",
                                name: "x"
                            },
                            value: {
                                type: "AssignmentPattern",
                                left: {
                                    type: "ArrayPattern",
                                    elements: [
                                        {
                                            "type": "Identifier",
                                            "name": "y"
                                        }
                                    ]
                                },
                                right: {
                                    type: "Literal",
                                    value: 0
                                }
                            },
                            kind: "init",
                            computed: false,
                            method: false,
                            shorthand: false
                        }
                    ]
                },
                operator: "=",
                right: {
                    type: "Literal",
                    value: 1
                }
            }
        }
    ]
}

Options

The second argument allows you to specify various options:

Option	Description
directives	Enable directive prologue to each literal node
globalReturn	Allow return in the global scope
impliedStrict	Enable strict mode (initial enforcement)
lexical	Enable lexical binding and scope tracking
loc	Enable line/column location information to each node
module	Allow parsing with module goal
next	Allow parsing with ESNext features
parenthesizedExpr	Enable non-standard parenthesized expression node
raw	Attach raw property to each literal node
ranges	Append start and end offsets to each node
source	Adds a source attribute in every node’s loc object when the locations option is true.
webcompat	Enable web compability

ECMAScript compability

Out of the box Meriyah is 100% ECMA spec compatible, but it's main focus is on performance. Therefore, you must enable several options by your own choice to make sure your code parses with 100% ECMA spec compability.

Also note that support for additional ECMAScript features for Web Browsers (annexB) isn't enabled by default as in other parsers, but you can instead parse with and without web compability .

This is done because AnnexB is an extension of the language, and also Test262 have this off by default and has it own web compat tests separated from the main tests.

Lexical binding and scope tracking has to be enabled with the lexical option.