A mock clock for tests involving timing.

Don't just use setTimeout() and hope that the timings work out. This makes tests take forever and be non-deterministic and flaky.

Instead, mock the clock, and explicitly advance it so you can test timing issues precisely and deterministically.

import { Clock } from 'clock-mock'
// this also works:
// const { Clock } = require('clock-mock')

import t from 'tap' // or whatever you use

// the module you wrote that does timing stuff
import { myModuleThatDoesStuffWithTime } from '../index.js'

t.test('test timeouts precisely', t => {
  const c = new Clock()
  c.enter()
  myModuleThatDoesStuffWithTime.scheduleThing('foo', 100)
  c.advance(99)
  t.equal(myModuleThatDoesStuffWithTime.thingRan('foo'), false)
  c.advance(1) // the timeout fired!
  t.equal(myModuleThatDoesStuffWithTime.thingRan('foo'), true)

  c.exit()
  t.end()
})

Patches:

• Date class
• setTimeout
• setInterval
• setImmediate
• clearTimeout
• clearInterval
• performance.now()
• process.hrtime
• process.hrtime.bigint

• const c = new Clock()

  Returns a new Clock instance

• c.advance(n)

  Advance the clock by n ms. Use floats for smaller increments of time.

• c.flow(n, step = 5) => Promise<void>

  Advance the clock in steps, awaiting a Promise at each step, so that actual asynchronous events can occur, as well as timers.

• c.travel(time)

  Set the clock to a specific time. Will fire timers that you zoom past.

• c.enter()

  Mocks all the things in the global space.

  Returns exit function, for ease of doing t.teardown(c.enter()).

• c.exit()

  Puts all the mocked globals back to their prior state.

• c.setTimeout(fn, n = 1)

  Schedule a function to be run when the clock has advanced n ms beyond the current point.

  Only ms granularity.

• c.setInterval(fn, n = 1)

  Schedule a function to be run when the clock advances each multiple of n past the current point.

  If multiple steps are advanced at once, for example doing c.setInterval(fn, 1) ; c.advance(1000), then it will only call the function once. This allows you to simulate clock jitter.

  Only ms granularity.

• c.setImmediate(fn)

  Schedule a function to be run the next time the clock advances by any amount.

• c.clearTimeout(timer)

  Clear a timeout created by the clock.

• c.clearInterval(interval)

  Clear an interval created by the clock.

• c.now()

  Returns the current ms time on the clock.

• c.hrtime()

  Mock of process.hrtime(), returning [seconds, nanoseconds] on the clock.

• c.hrtimeBigint()

  Mock of process.hrtime.bigint(), returning BigInt representation of current nanosecond time.