Nano Spy

A tiny Node.js library to spy and mock methods in tests with great TypeScript support.

It will take only 6 KB in your node_modules and have 0 dependencies.

import { spyOn, restoreAll } from 'nanospy'

test.after.each(() => {
  restoreAll()
})

test('calls increase', () => {
  const spy = spyOn(counter, 'increase')
  counter.increase(5)
  assert.equal(spy.callCount, 1)
  assert.equal(spy.calls, [[5]])
})

Usage

Method Spy

Spy tracks method calls, but do not change method’s behavior.

import { spyOn, restoreAll } from 'nanospy'

test.after.each(() => {
  restoreAll()
})

test('calls increase', () => {
  const spy = spyOn(counter, 'increase')
  counter.increase(5)
  assert.is(spy.called, true)
  assert.equal(spy.callCount, 1)
  assert.equal(spy.calls, [[5]])
  assert.equal(spy.results, [true])
})

Mock

Mock change the method’s behavior.

const spy = spyOn(global, 'fetch', async () => {
  return {
    json: () => ({ posts })
  }
})

Or change next function call:

spy.nextResult({ ok: false })

spy.nextError(error)

Functions

spy can be used to track callback calls.

import { spy } from 'nanospy'

const fn = spy()
fn('a', 10)
fn.callCount //=> 1
fn.calls //=> [['a', 10]]

You can pass spy’s callback:

let fn = spy((name: string) => {
  console.log(`Hello, ${name}!`)
})
fn('Ivan') //=> Hello, Ivan!

Or change next function call:

fn.nextResult({ ok: false })

fn.nextError(error)

Promises

You can use helpers to test promises:

import { spy } from 'nanospy'

const fn = spy()

let resolve = fn.nextResolve()
fn().then(arg => {
  console.log('Resolved ' + arg)
})

await resolve(1) // => Resolved 1

For testing errors, you can use fn.nextReject().

Remocking

You can reassign mocked function with onCall method:

const obj = {
  mark: str => str + '!',
}
const spy = spyOn(obj, 'mark')

obj.mark('a')
assert.equal(spy.results, ['a!'])

spy.onCall(str => str + '?')
obj.mark('a')
assert.equal(spy.results, ['a!', 'a?'])