clock

clock

a JavaScript idiomatic clock interface for inversion-of-control

installation

> npm install clock

example usage

in JavaScript

function announceTime(clock) {
  clock = clock || Date
  return 'It is now ' + clock.now()
}

announceTime()
// It is now 1499904588386

announceTime({now: function () { return 1234 }})
// It is now 1234

in TypeScript

import { Clock } from 'clock'

function announceTime(clock: Clock): string {
  clock = clock || Date
  return 'It is now ' + clock.now()
}

clock is an interface package

A Clock is any object with a method now() which returns a timestamp in milliseconds since the Unix epoch:

interface Clock {
  now (): number
}

The Date object satisfies the Clock interface.

This package will always be on the 1.x.x version range. Depend on this package so that other people will know that your module expects a Clock interface.

what problem this solves

Often in JavaScript, when we have code that needs to get the current time, we'll do Date.now() or new Date(). This is easy and works great!

function announceTime() {
  return 'It is now ' + Date.now()
}

However, this makes your code hard to test and difficult to predict, because your return values depend on side-effects other than your arguments. In functional programming terms, we no longer have referential transparency, since multiple calls to announceTime() will have different outputs for the same arguments.

What if we could abstract over the concept of time in our code?

With inversion of control, we can have our function ask someone else to provide a way to get the time, and just advertise that we expect something that lets us call .now() - either the default Date object, or anything else with a method now(). And to make life easier, we can fall back to the Date object by default.

function announceTime(clock) {
  clock = clock || Date
  return 'It is now ' + clock.now()
}

Now that clock is a parameter, when we call announceTime, we can decide what time it is:

const constantTime = {now: () => 0 }

constantTime is a clock that will always return 0, the Unix epoch.

If we use that, multiple calls to announceTime will always return the same value:

announceTime(constantTime)
// It is now 0
announceTime(constantTime)
// It is now 0
announceTime(constantTime)
// It is now 0

This means we can write a test:

const assert = require('assert')

assert.equal('It is now 0', announceTime(constantTime))

Abstracting clocks is something common in other platforms, such as Java. See the Java 8 Clock class for an example. Other uses for abstract clocks include skew-corrected clocks, simulating slowed down or sped up passage of time, and other fun tricks.

If you have a good example, analogy, or clearer way to express any of this information, please open a GitHub issue or PR!

api

This package includes some helpers:

systemClock : Clock (default export)

A facade for Date.now() JavaScript:

const systemClock = require('clock')
systemClock.now()
// 1499907736846

TypeScript:

import { systemClock } from 'clock'
systemClock.now()
// 1499907736846

constantClock: (time: number = 0) => Clock

Create a clock that always returns the same time. Useful for tests. The time argument defaults to 0, the Unix epoch. JavaScript:

const constantClock = require('clock').constantClock
const epochClock = constantClock()
epochClock.now()
// 0
epochClock.now()
// 0

TypeScript:

import { constantClock } from 'clock'
const epochClock = constantClock()
epochClock.now()
// 0
epochClock.now()
// 0

contributing

This package is developed in TypeScript but usable in all JavaScript projects, on Node.js or in browsers. The npm package includes compiled ES5 JavaScript, as well as TypeScript .d.ts type definition files.

Wanted: clarifications, example usage, translation of the readme into other human languages

special thank you

to @medikoo for the package name. Repo for clock<1.0.0 can be found at https://github.com/medikoo/clock. Thanks again!

license

ISC

cheers!