is-identifier

Common errors

• TypeError: (0 , is_identifier__WEBPACK_IMPORTED_MODULE_0__.default) is not a function

  cause Incorrectly attempting to destructure the default import in some bundler configurations, or using a named import when the package only provides a default export.
  fix Ensure you are using `import isIdentifier from 'is-identifier';` (ESM) or `const isIdentifier = require('is-identifier');` (CommonJS) as it is a default export.

• ReferenceError: isIdentifier is not defined

  cause Attempting to use `isIdentifier` without properly importing or requiring it, or using an incorrect import/require syntax.
  fix Verify your import statement: `import isIdentifier from 'is-identifier';` for ESM, or `const isIdentifier = require('is-identifier');` for CommonJS environments.

Warnings

• gotcha TypeScript users upgrading from v1.0.0 might observe changes in type inference or validation. Version 1.0.1 removed an incorrect TypeScript type guard, which corrects the library's type behavior to align precisely with its runtime logic and ECMAScript identifier rules.
  Fix: No direct code change is typically required; this fix improves type correctness. Re-check any custom type assertions if issues arise.
• gotcha The library intentionally treats certain global properties (e.g., `globalThis`, `Infinity`, `NaN`, `undefined`) as invalid identifiers. While not strictly reserved keywords, using them as identifiers is generally ill-advised and this package enforces that practical restriction.
  Fix: Be aware of this specific behavior. If you require identifier validation that permits these global properties (against common best practices), this library might require a wrapper or an alternative.
• gotcha The `await` keyword is correctly recognized and treated as a reserved identifier, causing `isIdentifier('await')` to return `false`. This holds true even in contexts where `await` might be valid within an `async` function, as the check is for general identifier validity.
  Fix: Always validate context-sensitive keywords like `await` using `isIdentifier` to ensure adherence to general JavaScript identifier rules, independent of execution context.

Install

• npm install is-identifier npm
• yarn add is-identifier yarn
• pnpm add is-identifier pnpm

Imports

• isIdentifier
  wrong:

  import { isIdentifier } from 'is-identifier';

  correct:

  import isIdentifier from 'is-identifier';

  The library exports a default function. Do not use named imports.
• isIdentifier (CommonJS)
  wrong:

  const { isIdentifier } = require('is-identifier');

  correct:

  const isIdentifier = require('is-identifier');

  While Node.js v18+ supports ESM natively, CommonJS `require` is still supported, but `isIdentifier` is the default export.

Quickstart

Demonstrates how to import and use `isIdentifier` to check various strings for JavaScript identifier validity, including valid cases, invalid cases, reserved keywords, and special global properties treated as non-identifiers.

import isIdentifier from 'is-identifier';

// Basic valid identifiers
console.log(`'foo' is identifier: ${isIdentifier('foo')}`);
console.log(`'$bar' is identifier: ${isIdentifier('$bar')}`);
console.log(`'_baz' is identifier: ${isIdentifier('_baz')}`);
console.log(`'myVariable123' is identifier: ${isIdentifier('myVariable123')}`);

// Invalid identifiers
console.log(`'1kg' is identifier: ${isIdentifier('1kg')}`);
console.log(`'await' is identifier: ${isIdentifier('await')}`); // Reserved keyword
console.log(`'my-variable' is identifier: ${isIdentifier('my-variable')}`);
console.log(`'function' is identifier: ${isIdentifier('function')}`); // Reserved keyword

// Special global properties treated as non-identifiers
console.log(`'globalThis' is identifier: ${isIdentifier('globalThis')}`);
console.log(`'Infinity' is identifier: ${isIdentifier('Infinity')}`);
console.log(`'NaN' is identifier: ${isIdentifier('NaN')}`);
console.log(`'undefined' is identifier: ${isIdentifier('undefined')}`);