Jest unit test for a debounce function

Question

I am trying to write a unit test for a debounce function. I'm having a hard time thinking about it.

This is the code:

function debouncer(func, wait, immediate) {
  let timeout;

  return (...args) => {
    clearTimeout(timeout);

    timeout = setTimeout(() => {
      timeout = null;
      if (!immediate) 
        func.apply(this, args);
    }, wait);

    if (immediate && !timeout) 
      func.apply(this, args);
  };
}

How should I start?

Answer 1

You will probably want to check the logic in your debouncer function:

• timeout will always be set by that last if() statement
• this will always be undefined since arrow functions use "the this value of the enclosing lexical context" and debouncer() is designed to be used as a stand-alone function.

Having said that, it sounds like your real question is about testing debounced functions.

Testing debounced functions

You can test that a function is debounced by using a mock to track function calls and fake timers to simulate the passage of time.

Here is a simple example using a Jest Mock Function and Sinon fake timers of a function debounced using debounce() from Lodash:

const _ = require('lodash');
import * as sinon from 'sinon';

let clock;

beforeEach(() => {
  clock = sinon.useFakeTimers();
});

afterEach(() => {
  clock.restore();
});

test('debounce', () => {
  const func = jest.fn();
  const debouncedFunc = _.debounce(func, 1000);

  // Call it immediately
  debouncedFunc();
  expect(func).toHaveBeenCalledTimes(0); // func not called

  // Call it several times with 500ms between each call
  for(let i = 0; i < 10; i++) {
    clock.tick(500);
    debouncedFunc();
  }
  expect(func).toHaveBeenCalledTimes(0); // func not called

  // wait 1000ms
  clock.tick(1000);
  expect(func).toHaveBeenCalledTimes(1);  // func called
});