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Javascript's setInterval() seems to pay no attention to whether or not the code it calls throws exceptions. For instance, this will not terminate the program but call the function over and over again:

setInterval(function() { throw "error" }, 1000);

What is the reason for this behaviour? Is it documented anywhere?

Javascript's setInterval() seems to pay no attention to whether or not the code it calls throws exceptions. For instance, this will not terminate the program but call the function over and over again:

setInterval(function() { throw "error" }, 1000);

What is the reason for this behaviour? Is it documented anywhere?

• javascript
• function
• exception
• setinterval

Share Improve this question Follow asked Aug 6, 2014 at 23:02 Eugene YarmashEugene Yarmash 150k4444 gold badges343343 silver badges388388 bronze badges 18

• "will not terminate the program" --- how do you see that? As the whole browser tab crash? – zerkms Commented Aug 6, 2014 at 23:03
• What could possibly handle the exception sanely? – Ignacio Vazquez-Abrams Commented Aug 6, 2014 at 23:07
• Async callbacks and try...catch don't mix. – elclanrs Commented Aug 6, 2014 at 23:07
• 1 @Vivin Paliath: setInterval callback does terminate. – zerkms Commented Aug 6, 2014 at 23:13
• 1 @Vivin Paliath: well, a particular callback execution does terminate. Not sure why the whole iterative process should. setInterval just schedules a function invocation. – zerkms Commented Aug 6, 2014 at 23:16

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3 Answers 3

Sorted by: Reset to default 6

The MDN docs for throw say, for a thrown object:

If no catch block exists among caller functions, the program will terminate.

This isn't strictly accurate. ECMAScript spec section 10.4

A thrown exception may also exit one or more execution contexts.

This is more accurate.

What is an execution context? Well, there's one for your original program. When you make a function call, a nested context is created while that function is running. A call to eval will make one. 10.4 covers this.

In general, when an exception is thrown, it will pass up through all of the nested execution contexts until it is caught, or it reaches the outer execution context. If it is not caught within an execution context, that context will be terminated. Thus, if your original program is the outermost execution context, it will terminate on an uncaught exception?

When your handler function is called, due to your setTimeout interval expiring, it is the outermost execution context. Thus, the uncaught exception terminates it, but it is not nested within your original program. So nothing else gets terminated.

The thread that's handling the timer expires is part of the JS engine, so you're not going to terminate that either. Thus, once per interval, a new execution context is created when your handler function is called and so it repeats.

If you don't want the function to be called over and over again, catch the exception and clear the timer. This example counts down from five, printing to the console, and throws an exception when the value is less than zero:

var counter = 5;
var timer;

var decrement = function() {
    try {
        if ( counter < 0 ) {
            clearInterval(timer);
            throw new Error("Invalid count");
        }
        console.log(counter);
        counter -= 1;
    }
    catch ( error ) {
        console.error(error);
    }
}

timer = setInterval(decrement, 1000);

It is up to you to decide if the function should or should not be called again. An exception stops execution of the function but there is no "program" to terminate. The web browser is the "program". If it runs a function that throws an exception, it still continues to service other events. That is the expected behavior.

Why doesn't it cancel the timer on an exception? You may want that when trying to load a resource that may fail and clear the timer on success.

When you call setInterval, the runtime doesn't know if the callback will throw.

var shouldThrow = false;
setInterval(function () {
    if (shouldThrow) throw 'error';
    console.log('program has not stopped');
}, 5000);

// nothing is wrong at this point.
// thus, the program continues
shouldThrow = confirm('Throw error?');

You'll find that if the callback throws, it does terminate "the program," that being the callback itself. In this example, if you click the OK button, the callback will not proceed to the console.log call. It does continue to call the callback repeatedly though. If you later change shouldThrow again, you'll see more messages in the console.

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