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JS bundler ESM/CJS interop

This repo contains tests for some JS bundler edge cases that I'm trying to get to work in esbuild relating to the default/__esModule interop mess. In the results below, ✅ indicates the result that I think should happen and 🚫 indicates otherwise. The high-level target behavior is this:

  • ESM that has been converted to CJS via Babel sets the __esModule flag. When this CJS is imported as ESM, it should behave like the original ESM. In particular, the default export should be equal to module.exports.default instead of module.exports.

  • The above rule is adjusted when the file ends in .mjs or .mts or package.json contains "type": "module". In that case node's behavior should be followed instead since it's likely that this code was intended to run using node's ESM support. In particular, the default export should be equal to module.exports instead of module.exports.default even if __esModule is present.

  • The __esModule marker should not be observable from ESM code. It should only be observable when ESM code is imported into CJS code.

Each test has a "direct" version and an "indirect" version. The indirect version uses Math.random() calls to verify the lack of special-casing regarding how properties are initialized and/or accessed. Certain bundlers do pattern-matching on the AST so for example exports.x = y might behave differently than exports[z] = y when z === 'x' even though those two expressions are equivalent in JavaScript.

All tests have been run through the JS bundlers Webpack, Rollup, Parcel, and esbuild. Tests have additionally been run through node for comparison, although please keep in mind that node is not a JS bundler and shouldn't be expected to implement bundler-specific features such as require() of ESM code.

Results

Testesbuildnodewebpackparcelrollup
Direct:
entry.js:
  import * as entry from './entry.js'
  input.works = entry.__esModule === void 0
Indirect:
entry.js:
  import * as entry from './entry.js'
  input.works =
    entry[Math.random() < 1 && '__esModule'] === void 0
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
🚫		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import './foo.js'
foo.js:
  import * as foo from './foo.js'
  input.works = foo.__esModule === void 0
Indirect:
entry.js:
  import './foo.js'
foo.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && '__esModule'] === void 0
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.default === '123'
foo.js:
  module.exports = '123'
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'] === '123'
foo.js:
  module[Math.random() < 1 && 'exports'] = '123'
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo.__esModule === void 0 && foo.bar === 123
foo.js:
  export let bar = 123
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && '__esModule'] === void 0 &&
    foo.bar === 123
foo.js:
  export let bar = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo.__esModule === false && foo.default.bar === 123
foo.js:
  export let __esModule = false
  export default { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && '__esModule'] === false &&
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  export let __esModule = false
  export default { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo.default.default.bar === 123
foo.js:
  exports.__esModule = false
  exports.default = { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].default.bar === 123
foo.js:
  exports[Math.random() < 1 && '__esModule'] = false
  exports[Math.random() < 1 && 'default'] = { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
✅
Direct:
entry.js:
  const foo = require('./foo.js')
  import * as foo2 from './foo.js'
  input.works = import('./foo.js').then(foo3 =>
    foo.bar === 123 && foo.__esModule === true &&
    foo2.bar === 123 && foo2.__esModule === void 0 &&
    foo3.bar === 123 && foo3.__esModule === void 0)
foo.js:
  export let bar = 123
Indirect:
entry.js:
  const foo = require('./foo.js')
  import * as foo2 from './foo.js'
  input.works = import('./foo.js').then(foo3 =>
    foo.bar === 123 &&
    foo2.bar === 123 &&
    foo3.bar === 123 &&
    foo[Math.random() < 1 && '__esModule'] === true &&
    foo2[Math.random() < 1 && '__esModule'] === void 0 &&
    foo3[Math.random() < 1 && '__esModule'] === void 0)
foo.js:
  export let bar = 123
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  const entry = require('./entry.js')
  input.works = entry.__esModule === void 0
  exports.foo = 123
Indirect:
entry.js:
  const entry = require('./entry.js')
  input.works =
    entry[Math.random() < 1 && '__esModule'] === void 0
  exports.foo = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  const entry = require('./entry.js')
  input.works = entry.__esModule === true
  export {}
Indirect:
entry.js:
  const entry = require('./entry.js')
  input.works =
    entry[Math.random() < 1 && '__esModule'] === true
  export {}
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  const entry = require('./entry.js')
  input.works = entry.__esModule === true
  export default 123
Indirect:
entry.js:
  const entry = require('./entry.js')
  input.works =
    entry[Math.random() < 1 && '__esModule'] === true
  export default 123
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.bar === 123 &&
    foo.__esModule === true
foo.js:
  export let bar = 123
Indirect:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.bar === 123 &&
    foo[Math.random() < 1 && '__esModule'] === true
foo.js:
  export let bar = 123
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.default === 123 &&
    foo.__esModule === true
foo.js:
  export default 123
Indirect:
entry.js:
  const foo = require('./foo.js')
  input.works =
    foo[Math.random() < 1 && 'default'] === 123 &&
    foo[Math.random() < 1 && '__esModule'] === true
foo.js:
  export default 123
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.baz === 123 &&
    foo.__esModule === true
foo.js:
  export * from './bar.js'
bar.js:
  export let baz = 123
Indirect:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.baz === 123 &&
    foo[Math.random() < 1 && '__esModule'] === true
foo.js:
  export * from './bar.js'
bar.js:
  export let baz = 123
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  import foo from './foo.js'
  input.works = foo.default.bar === 123 &&
    foo.bar === void 0
foo.js:
  module.exports = { default: { bar: 123 } }
Indirect:
entry.js:
  import foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123 &&
    foo.bar === void 0
foo.js:
  module[Math.random() < 1 && 'exports'] =
    { default: { bar: 123 } }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  import foo from './foo.js'
  input.works = foo === 123
foo.js:
  module.exports = 123
Indirect:
entry.js:
  import foo from './foo.js'
  input.works = foo === 123
foo.js:
  module[Math.random() < 1 && 'exports'] = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.default.bar === 123
foo.js:
  exports.__esModule = true
  exports.default = { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  exports[Math.random() < 1 && '__esModule'] = true
  exports[Math.random() < 1 && 'default'] = { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Direct:
entry.js:
  input.works = import('./foo.js')
    .then(foo => foo.default === 123 &&
      foo.__esModule === void 0)
foo.js:
  export default 123
Indirect:
entry.js:
  input.works = import('./foo.js')
    .then(foo =>
      foo[Math.random() < 1 && 'default'] === 123 &&
      foo[Math.random() < 1 && '__esModule'] === void 0)
foo.js:
  export default 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = typeof foo === 'object'
foo.js:
  module.exports = '123'
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works = typeof foo === 'object'
foo.js:
  module[Math.random() < 1 && 'exports'] = '123'
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo !== '123'
foo.js:
  module.exports = '123'
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works = foo !== '123'
foo.js:
  module[Math.random() < 1 && 'exports'] = '123'
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.default === void 0 &&
    foo.bar === 123
foo.js:
  exports.__esModule = true
  exports.bar = 123
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'] === void 0 &&
    foo.bar === 123
foo.js:
  exports.__esModule = true
  exports.bar = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.__esModule === true &&
    foo.default.bar === 123
foo.js:
  export let __esModule = true
  export default { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && '__esModule'] === true &&
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  export let __esModule = true
  export default { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.default.bar === 123
foo.js:
  export let __esModule = true
  export default { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  export let __esModule = true
  export default { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  import * as foo from './foo.js'
  input.works = foo.default.bar === 123
foo.js:
  export let __esModule = false
  export default { bar: 123 }
Indirect:
entry.js:
  import * as foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  export let __esModule = false
  export default { bar: 123 }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
✅

rollup
✅
Direct:
entry.js:
  const foo = require('./foo.js')
  input.works = foo.__esModule === true
foo.js:
  export let __esModule = 0
Indirect:
entry.js:
  const foo = require('./foo.js')
  input.works =
    foo[Math.random() < 1 && '__esModule'] === true
foo.js:
  export let __esModule = 0
esbuild
✅

esbuild
✅		 node
🚫

node
🚫		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import foo from './foo.js'
  input.works = foo === void 0
foo.js:
  module.exports = { bar: 123, __esModule: true }
Indirect:
entry.js:
  import foo from './foo.js'
  input.works = foo === void 0
foo.js:
  module[Math.random() < 1 && 'exports'] =
    { bar: 123, __esModule: true }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import foo from './foo.cjs'
  input.works = foo.default.bar === 123
foo.cjs:
  module.exports = {
    default: { bar: 123 }, __esModule: true }
package.json:
  { "type": "module" }
Indirect:
entry.js:
  import foo from './foo.cjs'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.cjs:
  module[Math.random() < 1 && 'exports'] =
    { default: { bar: 123 }, __esModule: true }
package.json:
  { "type": "module" }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.mjs:
  import foo from './foo.js'
  input.works = foo.default.bar === 123
foo.js:
  module.exports = {
    default: { bar: 123 }, __esModule: true }
Indirect:
entry.mjs:
  import foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  module[Math.random() < 1 && 'exports'] =
    { default: { bar: 123 }, __esModule: true }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.mts:
  import foo from './foo.js'
  input.works = foo.default.bar === 123
foo.js:
  module.exports = {
    default: { bar: 123 }, __esModule: true }
Indirect:
entry.mts:
  import foo from './foo.js'
  input.works =
    foo[Math.random() < 1 && 'default'].bar === 123
foo.js:
  module[Math.random() < 1 && 'exports'] =
    { default: { bar: 123 }, __esModule: true }
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
🚫

webpack
🚫		 parcel
🚫

parcel
🚫		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works = ns.foo === 123 &&
    keys.includes('foo') && !keys.includes('default')
foo.js:
  exports.__esModule = true
  exports.foo = 123
Indirect:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works = ns.foo === 123 &&
    keys.includes('foo') && !keys.includes('default')
foo.js:
  exports[Math.random() < 1 && '__esModule'] = true
  exports[Math.random() < 1 && 'foo'] = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as ns from './foo.js'
  input.works = ns.foo === 123 &&
    {}.hasOwnProperty.call(ns, 'foo') &&
    !{}.hasOwnProperty.call(ns, 'default')
foo.js:
  exports.__esModule = true
  exports.foo = 123
Indirect:
entry.js:
  import * as ns from './foo.js'
  input.works = ns.foo === 123 &&
    {}.hasOwnProperty.call(ns, 'foo') &&
    !{}.hasOwnProperty.call(ns, 'default')
foo.js:
  exports[Math.random() < 1 && '__esModule'] = true
  exports[Math.random() < 1 && 'foo'] = 123
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works =
    ns.default === 123 && !keys.includes('default')
foo.js:
  exports.__esModule = true
  Object.defineProperty(exports,
    'default', { value: 123 })
Indirect:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works =
    ns.default === 123 && !keys.includes('default')
foo.js:
  exports[Math.random() < 1 && '__esModule'] = true
  Object.defineProperty(exports,
    Math.random() < 1 && 'default', { value: 123 })
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
🚫

rollup
🚫
Direct:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works =
    ns.default === 123 && keys.includes('default')
foo.js:
  exports.__esModule = true
  Object.defineProperty(exports, 'default',
    { value: 123, enumerable: true })
Indirect:
entry.js:
  import * as ns from './foo.js'
  let keys = Object.keys(ns)
  input.works =
    ns.default === 123 && keys.includes('default')
foo.js:
  exports[Math.random() < 1 && '__esModule'] = true
  Object.defineProperty(exports, Math.random() < 1 && 'default',
    { value: 123, enumerable: true })
esbuild
✅

esbuild
✅		 node
✅

node
✅		 webpack
✅

webpack
✅		 parcel
✅

parcel
✅		 rollup
✅

rollup
✅
Percent handled: 100.0% 78.1% 71.9% 53.1% 46.9%

Visual summary

esbuild: ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅
node:    ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 ✅✅ 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅
webpack: ✅🚫 🚫🚫 ✅🚫 🚫🚫 ✅🚫 ✅🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 🚫🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅
parcel:  ✅✅ ✅✅ 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 🚫🚫 🚫🚫 ✅✅ 🚫🚫 🚫🚫 🚫🚫 🚫🚫 ✅✅ 🚫🚫 🚫🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅
rollup:  🚫🚫 🚫🚫 ✅✅ 🚫✅ ✅✅ 🚫✅ 🚫🚫 🚫🚫 🚫🚫 🚫🚫 ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ ✅✅ 🚫🚫 ✅✅ ✅✅ 🚫🚫 ✅✅ ✅✅ ✅✅ 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 🚫🚫 ✅✅

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