setTimeout and setInterval

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setTimeout and setInterval

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This is Implementation Specific

setTimeout and setInterval

Since JavaScript is asynchronous, it is possible to schedule the execution of a function by using the setTimeout and setInterval functions.

Note: Timeouts are not part of the ECMAScript Standard, they are implemented as part of the DOM.

function foo() {}
var id = setTimeout(foo, 1000); // returns a Number > 0

When setTimeout gets called, it will return the ID of the timeout and schedule foo to run in approximately one thousand milliseconds in the future. foo will then get executed exactly once.

Depending on the timer resolution of the JavaScript engine that is running the code, as well as the fact that JavaScript is single threaded and other code that gets executed might block the thread, it is by no means a safe bet that one will get the exact delay that was specified in the setTimeout call.

The function that was passed as the first parameter will get called by the global object, that means, that this inside the called function refers to that very object.

function Foo() {
   this.value = 42;
   this.method = function() {
       // this refers to the global object
       console.log(this.value); // will log undefined
   setTimeout(this.method, 500);
new Foo();

Note: As setTimeout takes a function object as its first parameter, an often made mistake is to use setTimeout(foo(), 1000), which will use the return value of the call foo and not foo. This is, most of the time, a silent error, since when the function returns undefined setTimeout will not raise any error.

Stacking calls with setInterval

While setTimeout only runs the function once, setInterval - as the name suggests - will execute the function every X milliseconds. But its use is discouraged.

When code that is being executed blocks the timeout call, setInterval will still issue more calls to the specified function. This can, especially with small intervals, result in function calls stacking up.

function foo(){
   // something that blocks for 1 second
setInterval(foo, 100);

In the above code foo will get called once and will then block for one second.

While foo blocks the code setInterval will still schedule further calls to it. Now, when foo has finished, there will already be ten further calls to it waiting for execution.

Dealing with possible blocking code

The easiest as well as most controllable solution, is to use setTimeout within the function itself.

function foo(){
   // something that blocks for 1 second
   setTimeout(foo, 100);

Not only does this encapsulate the setTimeout call, but it also prevents the stacking of calls and it gives additional itself can now decide whether it wants to run again or not.

Manually clearing timeouts

Clearing timeouts and intervals works by passing the respective ID to clearTimeout or clearInterval, depending which set function was used in the first place.

var id = setTimeout(foo, 1000);

Clearing all timeouts

As there is no built-in method for clearing all timeouts and/or intervals, it is necessary to use brute force in order to achieve this functionality.

// clear "all" timeouts
for(var i = 1; i < 1000; i++) {

There might still be timeouts that are unaffected by this arbitrary number; therefore, is is instead recommended to keep track of all the timeout IDs, so they can be cleared specifically.

Hidden use of eval

setTimeout and setInterval can also take a string as their first parameter. This feature should never be used, since it internally makes use of eval.

Note: Since the timeout functions are not specified by the ECMAScript standard, the exact workings when a string is passed to them might differ in various JavaScript implementations. As a fact, Microsoft's JScript makes use of the Function constructor in place of eval.

function foo() {
   // will get called
function bar() {
   function foo() {
       // never gets called
   setTimeout('foo()', 1000);

Since eval is not getting called directly in this case, the string passed to setTimeout will get executed in the global scope; thus, it will not use the local variable foofrom the scope of bar.

It is further recommended to not use a string for passing arguments to the function that will get called by either of the timeout functions.

function foo(a, b, c) {}
// NEVER use this
setTimeout('foo(1,2, 3)', 1000)
// Instead use an anonymous function
setTimeout(function() {
   foo(a, b, c);
}, 1000)

Note: While it is also possible to use the syntax setTimeout(foo, 1000, a, b, c), it is not recommended, as its use may lead to subtle errors when used withmethods.

In conclusion

Never should a string be used as the parameter of setTimeout or setInterval. It is a clear sign of really bad code, when arguments need to be supplied to the function that gets called. An anonymous function should be passed that then takes care of the actual call.

Further, the use of setInterval should be avoided since its scheduler is not blocked by executing JavaScript.

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