esutils

library1.0.1jsnpmunverified

ESUtils is a foundational utility library for ECMAScript language tools, providing a 'utility box' for analyzing JavaScript Abstract Syntax Trees (ASTs), character codes, and keywords/reserved words based on specific ECMA262 editions. It offers functions to classify AST nodes (e.g., `isExpression`, `isStatement`), identify character types (e.g., `isDecimalDigit`, `isIdentifierStart`), and check for keywords (`isKeywordES5`, `isKeywordES6`). The current stable version is 2.0.3, released in 2018. Given its core utility nature, its API is highly stable, and its release cadence is low, typically for maintenance or specification updates. It serves as a building block for various JavaScript parsing, linting, and transformation projects, providing low-level, spec-compliant checks.

testing

npm install esutils

github ↗package ↗

INSTALL

IMPORT

SIG · ESUTILS

esutils

testingjavascriptv1.0.1

Install

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Import

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Disk

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Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

esutils

✓ import esutils from 'esutils';

✗ import { ast, code, keyword } from 'esutils';

The library primarily exports a default object containing `ast`, `code`, and `keyword` sub-modules. Named imports for these sub-modules directly are not supported.

esutils.ast

✓ import esutils from 'esutils'; const isExpression = esutils.ast.isExpression;

✗ const ast = require('esutils').ast;

CommonJS users should `require('esutils')` to get the full object, then access properties like `ast`.

esutils.keyword

✓ import esutils from 'esutils'; const isKeywordES6 = esutils.keyword.isKeywordES6;

✗ import { isKeywordES6 } from 'esutils';

Individual functions like `isKeywordES6` are properties of the `keyword` sub-module, which is itself a property of the default `esutils` export. Direct named imports are incorrect.

Demonstrates basic usage of `esutils` to check AST node types, character codes, and ECMAScript keywords.

import esutils from 'esutils'; // Example AST node (simplified for demonstration) const ifStatementNode = { type: 'IfStatement', test: { type: 'Identifier', name: 'condition' }, consequent: { type: 'BlockStatement', body: [] }, alternate: null }; // 1. Using AST utilities console.log(`Is 'ifStatementNode' an Expression? ${esutils.ast.isExpression(ifStatementNode)}`); console.log(`Is 'ifStatementNode' a Statement? ${esutils.ast.isStatement(ifStatementNode)}`); // 2. Using code utilities (character codes) const charCodeA = 'A'.charCodeAt(0); const charCode5 = '5'.charCodeAt(0); console.log(`Is charCode 'A' a decimal digit? ${esutils.code.isDecimalDigit(charCodeA)}`); console.log(`Is charCode '5' a decimal digit? ${esutils.code.isDecimalDigit(charCode5)}`); console.log(`Is charCode 'A' an identifier start? ${esutils.code.isIdentifierStart(charCodeA)}`); // 3. Using keyword utilities const identifier = 'const'; const strictIdentifier = 'yield'; console.log(`Is '${identifier}' a keyword in ES6? ${esutils.keyword.isKeywordES6(identifier, false)}`); console.log(`Is '${strictIdentifier}' a keyword in ES6 strict mode? ${esutils.keyword.isKeywordES6(strictIdentifier, true)}`);

Debug

Known issues

gotchaWhen checking for keywords or reserved words, ensure you use the correct ECMA262 edition function (e.g., `isKeywordES5` vs. `isKeywordES6`) based on your target JavaScript environment or desired spec compliance. Using the wrong version can lead to incorrect parsing or validation results.

fix

Always explicitly choose `esutils.keyword.isKeywordES5`, `isKeywordES6`, `isReservedWordES5`, or `isReservedWordES6` according to the ECMAScript version you intend to target.

affects: >=2.0.0

gotchaThe `ast.isProblematicIfStatement` function identifies specific, unusual `IfStatement` structures that cannot be faithfully represented in standard JavaScript code due to ambiguity in `else` clause association (e.g., a dangling `else`). This is an edge case for advanced AST manipulation and should be understood carefully.

fix

Consult the official `esutils` documentation for `isProblematicIfStatement` to understand its precise definition and implications before relying on it for AST transformations.

affects: >=2.0.0

gotchaThe character code utilities (e.g., `isIdentifierStart`, `isIdentifierPart`) operate on raw Unicode code points. While they follow ECMA262 definitions, handling complex Unicode identifiers (e.g., those involving supplementary planes or combining characters) requires careful pre-processing or a more sophisticated Unicode-aware parser if full compliance for all possible characters is needed beyond basic ASCII/Latin-1 ranges.

fix

For basic character checks, the functions are sufficient. For full Unicode identifier validation, consider pairing `esutils` with a dedicated Unicode normalization library or a parser that handles identifier processing for all Unicode ranges.

affects: >=2.0.0

Errors

Common errors & fixes

TypeError: Cannot read properties of undefined (reading 'isExpression')

Attempting to access `esutils.ast.isExpression` when `esutils` was not correctly imported or resolved.

fix

Ensure `esutils` is properly imported: `import esutils from 'esutils';` for ESM or `const esutils = require('esutils');` for CommonJS, then access `esutils.ast.isExpression`.

ReferenceError: require is not defined

Using `require()` syntax in an ECMAScript Module (ESM) context without a bundler or transpiler set up to handle CommonJS imports.

fix

Switch to ESM `import esutils from 'esutils';` or configure your build system (e.g., Webpack, Rollup, Babel) to transpile CommonJS `require` calls in ESM environments.

TypeError: esutils.ast is not a function

Attempting to call `esutils.ast` as a function, or incorrectly assuming `ast` is directly exported.

fix

`esutils.ast` is an object containing utility functions, not a function itself. Access its methods like `esutils.ast.isExpression(node)`.

Upgrade

Version history

1.0.1latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

4 hits · last 30 days

node

Resources

githubhttps://github.com/estools/esutils ↗

packagehttps://www.npmjs.com/package/esutils ↗