JavaScript Timer Details

1. Brief Introduction

In JavaScript , there are two timers: setInterval() and setTimeout() , and there are methods to cancel the timer.
These are all window objects, and window can be omitted when calling. These two methods are not in the JavaScript specification.

There are four timer method related methods:

Note: setTimeout() is executed only once, while setInterval() is executed periodically at a given interval.

2. setInterval

2.1 Description

setInterval() method can repeatedly call a function or execute a code segment according to a specified period. The period unit is milliseconds.
If setInterval() method is not closed by clearInterval() method or the page is closed, it will continue to be called.
There are multiple parameters for setInterval.
First, if the first parameter is a code segment, then the setInterval() method is optional.
Second, if the first parameter is a function, the setInterval() method can have multiple parameters.

let timerId = setInterval(func|code, delay, arg1, arg2, ...)

2.2 Parameters

Note: The parameter func|code is usually passed in as a function. For historical reasons, passing in a code string is supported, but is not recommended.

2.3 Return Value

The return value timeoutID is a positive integer, indicating the number of the timer. This value can be passed to clearTimeout() to cancel the timer.

2.4 Usage

This is an example of clicking a button and incrementing a number every second;

<p id="showNum"></p>
<button onclick="timer()">Click me to increase the number by one every second</button>

<script>
  const showNum = document.getElementById("showNum");

  let timerId; // Timer ID
  let num = 0;

  function timer() {
    timerId = setInterval(addNum, 1000);
  }

  function addNum() {
    showNum.innerText = `${num++}`;
  }

  // No code to stop the timer is written</script>

3. setTimeout

3.1 Description

setTimeout() returns an integer representing the timer number, which can be used to cancel the timer later.
setTimeout() allows us to defer the execution of a function until a certain interval has passed.

let timerId = setTimeout(func|code, delay, arg1, arg2, ...)

3.2 Parameters

The parameters of setTimeout() are the same as those of setInterval() .

Note: The parameter func|code is usually passed in as a function. For historical reasons, passing in a code string is supported, but is not recommended.

3.3 Usage

The usage of setTimeout() is the same as setInterval() .

However, unlike setTimeout() which is executed only once, setInterval() is executed periodically according to the specified time.

<p id="showNum"></p>
<button onclick="timer()">After clicking, wait for one second and the number increases by one</button>

<script>
  const showNum = document.getElementById("showNum");

  let timerId;
  let num = 0;
  addNum();

  function timer() {
    timerId = setTimeout(addNum, 1000);
  }

  function addNum() {
    showNum.innerText = `${num++}`;
  }

</script>

4. Cancel the timer

clearInterval() method cancels timer set by setInterval() .
clearTimeout() method cancels timer set by setTimeout() .

The usage is very simple, with only one parameter, which is timeoutID , the identifier of the timer you want to cancel.
This ID is returned by the corresponding setTimeout() or clearTimeout() call.

clearInterval(intervalID);
clearTimeout(timeoutID);

Note: Note that setTimeout() and setInterval() share the same number pool. Technically, clearTimeout() and clearInterval() are interchangeable. However, to avoid confusion, do not mix the cancel timing functions.

Usage is simple:

function timer() {
  timerId = setTimeout(addNum, 1000);
}

clearTimeout(timerId); // When the code runs to this line, the timer set by timer will be canceled.

5. Use timers in the console

You can also use timers in the browser console

console.time(timerName)

Create a timer named name and start it.

Note: Each timer must have a unique name, and a maximum of 10,000 timers can run simultaneously on a page.

console.timeEnd(timerName)

Calling console.timeEnd(name ) stops the timer and prints the elapsed time in milliseconds.

console.time(timerName);
console.timeEnd(timerName);

5.1 Usage

For loop 99999 times how much time example:

console.time(name);

let num;
for (let index = 0; index < 99999; index++) {
  num++;
}

console.timeEnd(name);

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