fibble

Fibble is a small library to insert test doubles into your test. It is heavily inspired by quibble but targeted at es modules only. It was also made to support pnpm which creates a slightly different layout in the node_modules folder than other package managers and is thus built independent of resolution algorithm.

api

There are two functions exported from fibble:

• replace(path, replacement) - Replaces the module at path with the replacement.

  • path (string) - A path to the module to replace. The path should be relative from the file you are calling replace in.
  • replacement (object) - The object to replace the replacement to do. The object should contain key that you import in the file you want to test. If you want to stub the default export, name a key default.
  • returns - Promise. The replacement is async so you should await its' completion.

• reset() - Removes all registered replacements. All imports made after this call will use their native dependencies.

usage

setup

There are two modes available depending on the version of node you run. This is because node has changed their API for loaders.

modern mode

Use this with node version 18.19.0 and above.

node --experimental-import-meta-resolve --import fibble/register [my app]

legacy mode

Use this with node version 18.18.x and below.

node --experimental-import-meta-resolve --loader fibble/loader [my app]

non supported versons

Fibble does not support node version 20.0.0 - 20.5.x.

mocha example

This example uses mocha/chai but feel free to use in your favorite test runner.

import { describe, beforeEach, it } from 'mocha'
import { expect } from 'chai'
import { replace, reset } from 'fibble'

describe('my module', function(){
  let subject

  beforeEach(async () => {
    await replace ('./path/to/module/to/replace.js', { someProperty: 'some replacement value' })

    subject = await import('./module-to-test.js').default // or the property you want to test
  })

  afterEach(reset) // clear all replacements

  it('contains the stubbed stuff', function() {
    expect(subject(), ...) // to work with the replacemed module
  })
})

Setup mocha like this:

// .mocharc.cjs
module.exports = {
  'node-option': ['experimental-import-meta-resolve', 'import=fibble/register'],
}