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Sometimes in setTimeout functions you want to make sure it doesn't already exist before executing an action.

Upon execution of a setTimeout, is there any value in clearing/re-assigning the variable containing the timeout ID? What about setInterval?

You'll notice in my example timer1 executes 2 logs, whereas timer2 does not.

function renderUtility(name, value) {
  var el = document.createElement('p');
  el.textContent = name + ": " + value
  document.body.appendChild(el);
}

var timer1 = setTimeout(function() {
  clearTimeout(timer1);
  renderUtility('timer1', timer1);
  if (timer1) renderUtility('timer1', "I exist");
}, 1000);

var timer2 = setTimeout(function() {
  clearTimeout(timer2);
  timer2 = null;
  renderUtility('timer2', timer2);
  if (timer2) renderUtility('timer2', "I exist");
}, 1000);
Kyll
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Denim Demon
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  • You mean "assigned", not "defined". – Bergi Feb 01 '16 at 01:52
  • Considering that all that is returned by `setTimeout` is a an integer "handle" (see: http://stackoverflow.com/questions/10068981/what-does-settimeout-return) setting it to null after calling clearTimeout really doesn't accomplish much. That being said I'm voting to close due to this being primarily opinion based. – Dexygen Feb 01 '16 at 01:56
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    Note that setTimeout, unlike setInterval, only fires once. There is no need (or point) in calling clearTimeout after it has fired. Set it to zero (or other falsy value) when the timer has been cleared or fired if you wish to use it to test if the timer is running. – traktor Feb 01 '16 at 02:01

2 Answers2

1

When clearing a setTimeout/ setInterval, should the variable containing this function also be set to null?

No. There's just no reason to do that. The variable contains an integer id for the timer/interval, not a function; it doesn't hold onto anything.

And of course, there's no more reason to call clearTimeout from the timeout callback itself, as the timeout has already expired by then.

You'll notice in my example timer1 executes 2 logs, whereas timer2 does not.

Just remove the if test for the id and both will work the same. When you're clearing timer/interval, you usually know what you've done.

If, and only if, you use the variable that contains the id for something else, like storing some state (is a timer active?), then it can make sense to set it to some different value when it has run or been cleared.

Bergi
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0

The variables timer1, timer2 only contains the reference id of the timer, when you clearTimeout, the timeout function is cleared but the id is still exist in the variable. you can either do the following

var timer2 = setTimeout(function() {
  renderUtility('timer2', timer2);
  timer2 = null;    
  if (timer2) renderUtility('timer2', "I exist");
 }, 1000);

This will only print the timer2 once. Note that clearTimeout is not needed as setTimeout only executes once. Although to check if the function has already executed we can keep another counter variable outside the timer function. Like following

<button onclick="startCount()">Start count!</button>
<input type="text" id="txt">
<button onclick="stopCount()">Stop count!</button>

<script>
var c = 0;
var t;
var timer_is_on = 0;

function timedCount() {
    document.getElementById("txt").value = c;
    c = c + 1;
    t = setTimeout(function(){timedCount()}, 1000);
}

function startCount() {
    if (!timer_is_on) {
        timer_is_on = 1;
        timedCount();
    }
}

function stopCount() {
    clearTimeout(t);
    timer_is_on = 0;
}
</script>

For more details checkout http://www.w3schools.com/jsref/met_win_cleartimeout.asp

Nadeem Manzoor
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