@nvnh/clock is a small library without dependencies for mocking time in Javascript. It's inspired by this clock package for Go.

The goal is to resemble Javascript's Date & Time interface as closely as possible, so you can easily replace it in your existing code.

export interface Clock {
  now(): number;
  newDate(): Date;
  setTimeout(callback: () => void, ms: number): TimeoutID;
  clearTimeout(id: TimeoutID): void;
  setInterval(callback: () => void, ms: number): IntervalID;
  clearInterval(id: IntervalID): void;
}

@nvnh/clock gives you more control over time:

• Use Clock in your application for default behavior.
• Use MockClock in tests to have complete control over time.
• Replace Clock by WarpClock in your application to see how your application would behave at a different point in time. You can time travel and speed up or slow down time. Useful for debugging time-related problems, eg. using your application around midnight, year changes, leap years or use of setTimeout() that would let you wait a long time.

Getting started

Installation

npm install @nvnh/clock

Get the current date

// Import default clock instance
import clock from "@nvnh/clock";

// use it just like Javascript's Date
console.log(clock.newDate().toISOString()); // eg. 2023-07-12T15:15:21.376Z
// will have the same output as
console.log(new Date().toISOString()); // eg. 2023-07-12T15:15:21.376Z

Use MockClock

import { createMockClock } from "@nvnh/clock/mock";

// Create a mock clock instance
const mockClock = createMockClock("2030-11-24T08:30:25Z");
// newDate() returns the initial date
console.log(mockClock.newDate().toISOString()) // 2030-11-24T08:30:25.000Z
// move time ahead by 2 seconds
mockClock.add(2000);
// newDate() returns the updated date & time
console.log(mockClock.newDate().toISOString()) // 2030-11-24T08:30:27.000Z
// move time ahead to the specified time
mockClock.goTo("2030-11-25T07:30:00Z");
// newDate() returns the new date & time
console.log(mockClock.newDate().toISOString()) // 2030-11-25T07:30:00.000Z

Example

Last week

The last week example implements a function with signature:

lastWeek(): { from: Date, to: Date }

Where:

• from is a Javascript date representing monday 00:00:00 of the previous week.
• to is a Javascript date representing sunday 23:59:59 of the previous week.

The code is documented and shows how you can use @nvnh/clock in your own code.