scaled-time

Speed up - or slowwww down - time in JavaScript with zero dependencies.

Installation

npm install scaled-time --save-dev

Environment Support

So far, scaled-time has only been tested in Node via Jest. It probably works in other places, and probably doesn't work in a bunch of places, too.

Usage

It's strongly discouraged to use this in production code. The use case is for speeding up tests that are dependent on new Date() or Date.now() and setTimeout().

It also doesn't try to be super precise, but it should be useful in its primary objective.

import { resetTime, scaleTime } from "../src";

describe("some tests", () => {
  // HIGHLY RECOMMENDED: return stuff to native JS after each test
  afterEach(() => resetTime());

  it("should scale time via Date", () => {
    const start = new Date().getTime();

    // begin speeding up time by 2x from this point forward
    scaleTime(2);

    // ... one second later in real-life
    const oneSecondLater = new Date().getTime();
    // oneSecondLater - start === ~2000

    // ... another half second later in real-life
    const oneAndAHalfSecondsAfterStart = Date.now();
    // oneAndAHalfSecondsAfterStart - start === ~3000

    // returns to normal after we reset everything
    resetTime();
    const backToReality = Date.now();
    // backToReality - start === ~1500
  });

  // we also expect setTimeout to run faster by roughly our scaling factor
  it("should scale time via setTimeout", (done) => {
    const start = Date.now();

    scaleTime(10);

    const simulatedMilliseconds = 1000;
    const realLifeMilliseconds = 100;

    setTimeout(() => {
      resetTime();

      const finish = Date.now();
      const diff = finish - start;

      // the precision is far from perfect, but it's close enough to help in many cases
      // you can see we're expecting an actual real-world difference of 100ms, despite running setTimeout for 1,000ms
      expect(diff).toBeGreaterThan(realLifeMilliseconds * 0.9);
      expect(diff).toBeLessThan(realLifeMilliseconds * 1.1);

      done();
    }, simulatedMilliseconds);
  });
});

Contributions

Contributions are welcome!

Future Plans

• Other JS native timer functions (eg. setInterval)
• Tests for slowing time down (theoretically should work, but haven't tested, yet)
• Tests for all variations of timer function method signatures (for now have only tested basic setTimeout signature)