HAST Utility: Is Event Handler

Common errors

• Error [ERR_REQUIRE_ESM]: require() of ES Module ... hast-util-is-event-handler/index.js from ... not supported.

  cause Attempting to use `require()` to import `hast-util-is-event-handler`, which is an ES Module (ESM) only package.
  fix Change your import statement to use ESM syntax: `import { isEventHandler } from 'hast-util-is-event-handler';`. Ensure your `package.json` has `"type": "module"` or your file uses the `.mjs` extension for Node.js environments.

• TypeError: (0 , hast_util_is_event_handler__WEBPACK_IMPORTED_MODULE_0__.isEventHandler) is not a function

  cause This error often occurs in bundled or transpiled code when a named export is incorrectly imported, such as trying to destructure a default export or importing a non-existent named export.
  fix Verify that `isEventHandler` is imported as a named export from `hast-util-is-event-handler`: `import { isEventHandler } from 'hast-util-is-event-handler';`. Do not use `import isEventHandler from 'hast-util-is-event-handler';` as there is no default export.

• TS2305: Module '"hast-util-is-event-handler"' has no exported member 'IsEventHandler'.

  cause Attempting to import a type named `IsEventHandler` from the package, but the package does not explicitly export a type with that name from its root.
  fix The function `isEventHandler` is fully typed, and its return type (`boolean`) is simple. You typically do not need to import a specific type for this utility. If you're looking for a type related to AST nodes, refer to `@types/hast` or `hast` itself. For custom type definitions, you might need to declare them locally or extract them from the function's signature if complex.

Warnings

• breaking The `rehypejs` ecosystem, including `hast-util-is-event-handler`, updated to require Node.js 16 or higher in its v7 monorepo release (which applies to `hast-util-is-event-handler` v3.x line).
  Fix: Upgrade your Node.js environment to version 16 or newer. Use `nvm install 16 && nvm use 16` or update your CI/CD configuration.
• breaking The package transitioned to ES Modules (ESM) exclusively, discontinuing support for CommonJS `require()`. This change was part of the broader `rehypejs` monorepo v7 update, impacting `hast-util-is-event-handler`'s v3.x.
  Fix: Update all imports to use ESM syntax: `import { isEventHandler } from 'hast-util-is-event-handler'`. Ensure your project is configured for ESM, potentially by setting `"type": "module"` in `package.json`.
• breaking The broader `rehypejs` monorepo v7 release introduced changes to package `exports`, affecting how modules are resolved and preventing access to 'private' internal APIs.
  Fix: Always import symbols directly from the main package entry point (`'hast-util-is-event-handler'`) and avoid relying on deep imports to internal paths, as these are subject to change without major version bumps.
• gotcha The `isEventHandler` utility only checks if a property name *looks like* an event handler (starts with 'on' and has 5+ characters). It does not validate if the event handler is a known or valid HTML event handler (e.g., 'onmadeupevent' will still return true).
  Fix: Do not rely on `isEventHandler` alone for security or strict HTML validation. If you need to verify against a list of actual HTML events, combine this utility with a separate, comprehensive schema check.
• gotcha When working with `hast` trees, especially when accepting user input, there is a risk of Cross-Site Scripting (XSS) vulnerabilities if HTML is not properly sanitized. This utility itself doesn't introduce XSS but is part of an ecosystem where it's a concern.
  Fix: Always use `rehype-sanitize` or a similar sanitization plugin when processing untrusted HTML input to prevent XSS attacks in your `rehype` pipeline.

Install

• npm install hast-util-is-event-handler npm
• yarn add hast-util-is-event-handler yarn
• pnpm add hast-util-is-event-handler pnpm

Imports

• isEventHandler
  wrong:

  const isEventHandler = require('hast-util-is-event-handler')

  correct:

  import { isEventHandler } from 'hast-util-is-event-handler'

  The package is ESM-only since its major version 3 release, aligning with the broader rehypejs monorepo's v7 update. CommonJS `require()` is not supported.
• isEventHandler (TypeScript)
  wrong:

  import isEventHandler from 'hast-util-is-event-handler'

  correct:

  import { isEventHandler } from 'hast-util-is-event-handler'
  import type { Handler } from 'hast-util-is-event-handler/lib'

  This package ships with full TypeScript types. There is no default export; the function must be imported as a named export. While `Handler` is not directly exported from the root, types for internal structures may be available from subpaths if needed for advanced usage, otherwise, relying on the function's inferred types is common.

Quickstart

Demonstrates how to import and use the `isEventHandler` function to identify attribute names that conform to the basic pattern of an HTML event handler, emphasizing its `on` prefix and length criteria.

import { isEventHandler } from 'hast-util-is-event-handler';

// Basic usage to check common event handler names
console.log("Is 'onclick' an event handler?", isEventHandler('onclick'));
//=> true

console.log("Is 'onmouseover' an event handler?", isEventHandler('onmouseover'));
//=> true

// Checks for 'on' prefix and length >= 5
console.log("Is 'ones' an event handler?", isEventHandler('ones'));
//=> false (length < 5)

console.log("Is 'onfoo' an event handler?", isEventHandler('onfoo'));
//=> true (matches 'on' prefix and length criteria, even if 'onfoo' is not a real event)

console.log("Is 'class' an event handler?", isEventHandler('class'));
//=> false

// This package requires Node.js 16+ due to its ESM-only nature.
// Ensure your environment supports ES Modules for direct import usage.