babel-plugin-rewire-exports

Babel plugin for stubbing (ES6, ES2015) module exports. It allows to rewire the the exported values in all the importing modules. Unlike babel-plugin-rewire it doesn't modify the module internals (e.g. imports and top-level variables and functions). See How it works section for implementation details.

Exports

Plugin transforms module exports in such a way that they can be stubbed (or "rewired") via the following API:

• default export - plugin exports additional rewire(stub) function that allows to replace the original
• named exports - for each export (e.g. export var foo) an additional function rewire$foo(stub) is exported
• restore() function allows to restore the exports to their original values
• if there are existing rewire or restore top-level identifiers, the generated exports will be named rewire$default and restore$rewire respectively

Example

Named export:

//------ text.js ------
export let message = 'Hello world!'

//------ logger.js ------
import {message} from './text.js'

export default function () {
  console.log(message)
}

//------ main.js ------
import {rewire$message, restore} from './text.js'
import logger from './logger.js'

logger() // 'Hello world!'
rewire$message('I am now rewired')
logger() // 'I am now rewired'
restore()
logger() // 'Hello world!'

Default export:

//------ fetch.js ------
export default function (url) {
  // perform some expensive remote call
}

//------ adapter.js ------
import fetch from './fetch.js'

export function fetchItems() {
  return fetch('/items')
}

//------ test.js ------
import {rewire, restore} from './fetch.js'
import {fetchItems} from './adapter.js'

describe('adapter', function () {
  beforeEach(function () {
    rewire(this.spy = jasmine.createSpy('fetch'))
  })
  afterAll(restore)
  
  it('should call fetch', function () {
    fetchItems()
    expect(this.spy).toHaveBeenCalledWith('/items')
  })
})

Compatibility

• ❤️ Should work in modern browsers and Node once they start implementing ES6 modules natively - for example tested in latest Edge with experimental flags
• ✨ SystemJS
• 🌟 Webpack 2
• 💥 Rollup - requires v0.38+
• 📦 Seems to work with CommonJS and AMD (including bundling with webpack)

How it works

In ES6, imports are live read-only views on exported values:

//------ lib.js ------
export let counter = 3;
export function incCounter() {
    counter++;
}

//------ main1.js ------
import { counter, incCounter } from './lib';

// The imported value `counter` is live
console.log(counter); // 3
incCounter();
console.log(counter); // 4

// The imported value can’t be changed
counter++; // TypeError

This allows for any exports to be overwritten from within the module - and imports will be automatically updated via their bindings.

Transformations

Here's how various kinds of export declarations are transformed:

• Literals (export default 'foo') - the original value is copied to a variable to be stubbed and restored later: export {_default as default}
• Variables:
  • named exports (export var foo, export let bar or export {baz}) are left intact, but their initial values are similarly copied to temp variables.
  • default export (export default foo) is converted to a named export to enable live binding: export {foo as default}
• Constants (export const foo = 'bar') are ignored
• Functions (export default function () {…} or export function foo() {…}) are split into a function declaration and exported variable by the same name. The variable is hoisted to the very top of the module to preserve existing behavior.
• Classes (export default class {…} or export class foo {…}) are handled similar to functions except the variables are not hoisted (again to preserve the existing behavior).
• Re-exports (export * from './foo.js' or export {bar} from 'baz') are ignored

Installation

$ npm install babel-plugin-rewire-exports

Usage

Via .babelrc (Recommended)

.babelrc

{
  "plugins": ["rewire-exports"]
}

Via CLI

$ babel --plugins rewire-exports script.js

Via Node API

require("babel-core").transform("code", {
  plugins: ["rewire-exports"]
});