babel-plugin-transform-proto-to-assign

6.26.0 verified Sat Apr 25 auth: no javascript deprecated

This Babel plugin transforms assignments to __proto__ into a shallow property copy, enabling prototype-like inheritance in environments that do not support __proto__ or Object.setPrototypeOf. The latest stable version is 6.26.0 (2017-01-30), part of Babel 6.x. It is maintained as part of the Babel monorepo but is considered legacy; Babel 7+ discourages use of this plugin and instead recommends using Object.setPrototypeOf or Object.create for performance reasons. The plugin does a shallow clone of all enumerable own properties from the target prototype. It works only with assignments like `obj.__proto__ = proto`, not `Object.setPrototypeOf`. It modifies Object.prototype, which can cause side effects. It is a single-purpose transform, not part of any preset-env. Alternative: use `@babel/plugin-transform-proto-to-assign` for Babel 7. The plugin is deprecated in favor of modern Object.setPrototypeOf.

Common errors

error Cannot find module 'babel-plugin-transform-proto-to-assign' ↓

cause Plugin not installed or not in node_modules.

fix

Run 'npm install --save-dev babel-plugin-transform-proto-to-assign' and ensure node_modules is present.

error Error: Plugin .__proto__ is not a function ↓

cause Attempting to use the plugin with Babel 7 where the package name has changed.

fix

Use '@babel/plugin-transform-proto-to-assign' instead, or downgrade to Babel 6.

error TypeError: Cannot set property __proto__ of undefined ↓

cause Assigning to __proto__ on a non-object value (null/undefined).

fix

Ensure the left-hand side of __proto__ assignment is an object.

Warnings

deprecated This plugin is part of Babel 6 and has no active development. For Babel 7+, use '@babel/plugin-transform-proto-to-assign' instead. ↓

fix Upgrade to Babel 7+ and install @babel/plugin-transform-proto-to-assign.

gotcha The transform does a shallow copy of properties at the moment of assignment, not a live prototype chain. Changing the prototype later does not affect already-assigned properties. ↓

fix Use Object.setPrototypeOf or Object.create for proper prototype chain if needed.

gotcha Modifying Object.prototype can cause performance issues and side effects in other code. Avoid if possible. ↓

fix Use explicit property assignment or Object.assign instead of __proto__ manipulation.

breaking Only transforms assignments like `obj.__proto__ = value`. Does not handle Object.setPrototypeOf or Reflect.setPrototypeOf. ↓

fix Add a separate plugin or polyfill if supporting Object.setPrototypeOf is required.

Install

npm install babel-plugin-transform-proto-to-assign

yarn add babel-plugin-transform-proto-to-assign

pnpm add babel-plugin-transform-proto-to-assign

Imports

default (plugin)
wrong
```
import p from 'babel-plugin-transform-proto-to-assign';
```
correct
```
module.exports = require('babel-plugin-transform-proto-to-assign');
```
This plugin is designed for Babel 6's CommonJS module system. In Babel 6, plugins are loaded via require; ES import syntax may not work directly. For Babel 7, use @babel/plugin-transform-proto-to-assign.

babel-core transform with plugin

wrong

require('babel-core').transform(code, { plugins: [require('babel-plugin-transform-proto-to-assign')] }); // may work but string name is preferred

correct

require('babel-core').transform(code, { plugins: ['transform-proto-to-assign'] });

In Babel 6, plugins can be specified by package name string. Passing the plugin object directly also works but string is standard.

.babelrc configuration
wrong
```
{
  "plugins": ["babel-plugin-transform-proto-to-assign"]
} // also valid, but shorter name without prefix is typical
```
correct
```
{
  "plugins": ["transform-proto-to-assign"]
}
```
Babel automatically resolves the 'babel-plugin-' prefix, so either 'transform-proto-to-assign' or 'babel-plugin-transform-proto-to-assign' works.

Quickstart

Shows installation, configuration, source code with __proto__ assignment, and compilation output.

// Step 1: Install the plugin
// npm install --save-dev babel-plugin-transform-proto-to-assign

// Step 2: Create .babelrc with the plugin
// {
//   "plugins": ["transform-proto-to-assign"]
// }

// Step 3: Write source with __proto__ assignment
var foo = { a: 1 };
var bar = { b: 2 };
bar.__proto__ = foo;

// Step 4: Compile with Babel
// npx babel script.js --out-file compiled.js

// compiled.js output (simplified):
// var _defaults = ...;
// _defaults(bar, foo);

// Note: Does not support dynamic prototype chain; only shallow copy of properties at assignment time.