Timer - JavaScript Challenges

You can find all the code in this post at the repo Github.

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Async programming timer related challenges

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Cache with time limit

class TimeLimitedCache {
  constructor() {
    this._cache = new Map();
  }

  set(key, value, duration) {
    const found = this._cache.has(key);

    if (found) {
      clearTimeout(this._cache.get(key).ref);
    }

    this._cache.set(key, {
      value,
      ref: setTimeout(() => {
        this._cache.delete(key);
      }, duration),
    });

    return found;
  }

  get(key) {
    if (this._cache.has(key)) {
      return this._cache.get(key);
    } else {
      return -1;
    }
  }

  count() {
    return this._cache.size;
  }
}

// Usage example
const timeLimitedCache = new TimeLimitedCache();
console.log(timeLimitedCache.set(1, 'first', 1000)); // => false
console.log(timeLimitedCache.get(1).value); // => 'first'
console.log(timeLimitedCache.count()); // => 1 
setTimeout(() => {
  console.log(timeLimitedCache.count()); // => 0
  console.log(timeLimitedCache.get(1)); // => -1
}, 2000);

---

Cancel interval

/**
 * @param {Function} callback
 * @param {number} delay
 * @param {...any} args
 * @returns {Function}
 */
function setCancellableInterval(callbackFn, delay, ...args) {
  const timerId = setInterval(callbackFn, delay, ...args);

  return () => {
    clearInterval(timerId);
  };
}

// Usage example
let i = 0;
// t = 0:
const cancel = setCancellableInterval(() => {
  i++;
}, 100);
// t = 50:
cancel();
// t = 100: i is still 0 because cancel() was called.

---

Cancel timeout

/**
 * @param {Function} callbackFn
 * @param {number} delay
 * @param {...any} args
 * @returns {Function}
 */
function setCancellableTimeout(callbackFn, delay, ...args) {
  const timerId = setTimeout(callbackFn, delay, ...args);

  return () => {
    clearTimeout(timerId);
  };
}

// Usage example
let i = 0;
// t = 0:
const cancel = setCancellableTimeout(() => {
  i++;
}, 100);
// t = 50:
cancel();
// t = 100: i is still 0 because cancel() was called.

---

Clear all timeout timers

/**
 * cancel all timer from window.setTimeout
 */

const timerQueue = [];

const originalSetTimeout = window.setTimeout;

window.setTimeout = function (callbackFn, delay, ...args) {
  const timerId = originalSetTimeout(callbackFn, delay, ...args);
  timerQueue.push(timerId);

  return timerId;
}


function clearAllTimeout() {
  while (timerQueue.length) {
    clearTimeout(timerQueue.pop());
  }
}


// Usage example
setTimeout(func1, 10000)
setTimeout(func2, 10000)
setTimeout(func3, 10000)
// all 3 functions are scheduled 10 seconds later
clearAllTimeout()
// all scheduled tasks are cancelled.

---

Debounce

/**
 * @param {Function} fn 
 * @param {number} wait 
 * @return {Function}
 */

function debounce(fn, wait = 0) {
  let timerId = null;

  return function (...args) {
    const context = this;
    clearTimeout(timerId);

    timerId = setTimeout(() => {
      timerId = null;
      fn.call(context, ...args);
    }, wait);
  }
}


// Usage example
let i = 0;
function increment() {
  i += 1;
}
const debouncedIncrement = debounce(increment, 100);

// t = 0: Call debouncedIncrement().
debouncedIncrement(); // i = 0

// t = 50: i is still 0 because 100ms have not passed.
//  Call debouncedIncrement() again.
debouncedIncrement(); // i = 0

// t = 100: i is still 0 because it has only
//  been 50ms since the last debouncedIncrement() at t = 50.

// t = 150: Because 100ms have passed since
//  the last debouncedIncrement() at t = 50,
//  increment was invoked and i is now 1 .

---

Throttle

/**
 * @param {Function} fn 
 * @param {number} wait 
 * @return {Function}
 */

function throttle(fn, wait = 0) {
  let shouldThrottle = false;

  return function (...args) {
    if (shouldThrottle) {
      return;
    }

    shouldThrottle = true;
    setTimeout(() => {
      shouldThrottle = false;
    }, wait);

    fn.call(this, ...args);
  }
}

// Usage example
let i = 0;
function increment() {
  i++;
}
const throttledIncrement = throttle(increment, 100);

// t = 0: Call throttledIncrement(). i is now 1.
throttledIncrement(); // i = 1

// t = 50: Call throttledIncrement() again.
//  i is still 1 because 100ms have not passed.
throttledIncrement(); // i = 1

// t = 101: Call throttledIncrement() again. i is now 2.
//  i can be incremented because it has been more than 100ms
//  since the last throttledIncrement() call at t = 0.
throttledIncrement(); // i = 2

---

Repeat interval

const URL = 'https://fastly.picsum.photos/id/0/5000/3333.jpg?hmac=_j6ghY5fCfSD6tvtcV74zXivkJSPIfR9B8w34XeQmvU';

function fetchData(url) {
  return fetch(url)
    .then((response) => {
      if (!response.ok) {
        throw new Error(`Error: ${response.status}`);
      }

      return response.blob();
    })
    .then((data) => {
      console.log(data);
    })
    .catch((err) => {
      console.log(`Error: ${err}`);
    });
}

function repeat(callbackFn, delay, count) {
  let currentCount = 0;

  const timerId = setInterval(() => {
    if (currentCount < count) {
      callbackFn();
      currentCount += 1;
    } else {
      clearInterval(timerId);
    }
  }, delay);

  return {
    clear: () => clearInterval(timerId),
  }
}

// Usage example
const cancel = repeat(() => fetchData(URL), 2000, 5);
setTimeout(() => {
  cancel.clear();
}, 11000);

---

Resumable interval

/**
 * @param {Function} callbackFn
 * @param {number} delay
 * @param {...any} args
 * @returns {{start: Function, pause: Function, stop: Function}}
 */

function createResumableInterval(callbackFn, delay, ...args) {
  let timerId = null;
  let stopped = false;

  function clearTimer() {
    clearInterval(timerId);
    timerId = null;
  }

  function start() {
    if (stopped || timerId) {
      return;
    }

    callbackFn(...args);
    timerId = setInterval(callbackFn, delay, ...args);
  }

  function pause() {
    if (stopped) {
      return;
    }

    clearTimer();
  }

  function stop() {
    stopped = true;
    clearTimer();
  }

  return {
    start,
    pause,
    stop,
  };
}

// Usage example
let i = 0;
// t = 0:
const interval = createResumableInterval(() => {
  i++;
}, 10);
// t = 10:
interval.start(); // i is now 1.
// t = 20: callback executes and i is now 2.
// t = 25:
interval.pause();
// t = 30: i remains at 2 because interval.pause() was called.
// t = 35:
interval.start(); // i is now 3.
// t = 45: callback executes and i is now 4.
// t = 50:
interval.stop(); // i remains at 4.

---

Implement setInterval()

/**
 * @param {Function} callbackFn
 * @param {number} delay
 * @return {object}
 */

// use `requestAnimationFrame`
function mySetInterval(callbackFn, delay) {
  let timerId = null;
  let start = Date.now();

  // loop
  function loop() {
    const current = Date.now();

    if (current - start >= delay) {
      callbackFn();
      start = current;
    }

    timerId = requestAnimationFrame(loop);
  }

  // run loop
  loop();

  return {
    clear: () => cancelAnimationFrame(timerId),
  }
}


const interval = mySetInterval(() => {
  console.log('Interval tick');
}, 1000);

// cancel
setTimeout(() => {
  interval.clear();
}, 5000);



// use `setTimeout`
function mySetInterval(callbackFn, delay) {
  let timerId = null;
  let start = Date.now();
  let count = 0;

  // loop
  function loop() {
    const drift = Date.now() - start - count * delay;
    count += 1;

    timerId = setTimeout(() => {
      callbackFn();
      loop();
    }, delay - drift);
  }

  // run loop
  loop();

  return {
    clear: () => clearTimeout(timerId),
  }
}

const interval = mySetInterval(() => {
  console.log('Interval tick');
}, 1000);

// cancel
setTimeout(() => {
  interval.clear();
}, 5000);

---

Implement setTimeout()

function setTimeout(callbackFn, delay) {
  let elapsedTime = 0;
  const interval = 100;

  const intervalId = setInterval(() => {
    elapsedTime += interval;

    if (elapsedTime >= delay) {
      clearInterval(intervalId);
      callbackFn();
    }
  }, interval);
}

// Usage example
mySetTimeout(() => {
  console.log('This message is displayed after 2 seconds.');
}, 2000);

---

Reference

• Window: setTimeout() method - MDN
• Window: setInterval() method - MDN
• Window: clearInterval() method - MDN
• Window: clearTimeout() method - MDN
• 2715. Timeout Cancellation - LeetCode
• 2725. Interval Cancellation - LeetCode
• 2622. Cache With Time Limit - LeetCode
• 2627. Debounce - LeetCode
• 2676. Throttle - LeetCode
• Rate limiting - Wikipedia.org
• 28. implement clearAllTimeout() - BFE.dev
• 4. implement basic throttle() - BFE.dev
• 5. implement throttle() with leading & trailing option - BFE.dev
• 6. implement basic debounce() - BFE.dev
• 7. implement debounce() with leading & trailing option- BFE.dev
• JavaScript Asynchronous: Exercises, Practice, Solutions - w3resource
• GreatFrontEnd

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