Spy and mock test utilities

Usage no npm install needed!

<script type="module">
  import testu from 'https://cdn.skypack.dev/testu';



Spy and mock test utilities

npm install testu

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Testu provides function spies and object mocks. The value prop is a small API with minimal magic.


import assert from 'assert'
import { createSpy, createMock } from 'testu'

const calls = []
const testFn = createSpy(calls, () => 4)

const value = testFn()

assert(value === 4)
assert(calls.length === 1)
assert(calls[0].returned.value === 4)

const object = {
  method: () => 1
const { calls: mockCalls, mock } = createMock(object, { method: () => 4 })
const value = mock.method()

assert(value === 4)
assert(mockCalls.length === 1)
assert(mockCalls[0].returned.value === 4)



createSpy(calls: Array<Call>, func: Function[, methodName: String]): Function

Create a spy which will call func with its context (this) and arguments. Each function call will append a Call to the calls array. The optional methodName can be used to label a set of calls from a particular function.


createMock(object[, implementations]): { calls: Array<Call>, mock }

Create a mock object with the properties of object. If implementations exist for a particular property, the mock will use that. Unimplemented methods will return undefined. Unimplemented non-methods will be copied over to the mock. Methods are spied automatically and recorded in calls.


Objects representing function calls.

interface Call {
  methodName: string?,
  context: any,
  args: Array<any>,
  arguments: Arguments,
  didThrow: boolean,
  threw?: {
    error: any
  didReturn: boolean,
  returned?: {
    value: any

Name origin

Testing for Gangsters, and Chuck Testu