Preventing a function from running multiple times in JavaScript

Question

I'm working with some code that does this:

var _init = false;

this.init = function(){

    if(_init)
        return;
    else
        _init = true;

    // Do a bunch of stuff here
}

It seems to me that there is a tiny race condition there that I'd like to eliminate. It's possible for a second instance of the init function to start running before the first instance has gotten around to setting _init to true. Unlikely, but non-zero, yes?

Given that, is there a straightforward way to eliminate this race condition short of something like a Singleton pattern?

Since javascript is single threaded, there is only one thread of execution at any given time so nothing else can call your function while you're testing the `_init` variable. — jfriend00, Oct 05 '11 at 19:52

score 4 · Accepted Answer · edited Oct 05 '11 at 19:50

4

javascript is single threaded (ignoring web-workers for the moment) so you should be ok -- there should be no race conditions.

I think the "standard" way of doing this, however, would be to use a self-invoking function

(function(){
    // init stuff here, but you don't need to have any of the _init stuff
})() // <-- this causes your function to be invoked immediately

edited Oct 05 '11 at 19:50

jfriend00

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answered Oct 05 '11 at 19:48

hvgotcodes

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score 0 · Answer 2 · answered Oct 05 '11 at 19:59

One easy way to ensure the function runs only once is simply to delete the method at the end:

this.init = function(){
    // Do a bunch of stuff here

    // now delete:
    delete this.init;
}

or you could reassign the property to a no-op, if you need to be able to call it again:

this.init = function(){
    // Do a bunch of stuff here

    this.init - function() {};
}

But this only ensures that the function runs once per instance - if you need it to only run once, ever, your flag-based approach is probably better, and as other posters have suggested, your worries about a race condition are unfounded with single-threaded code.

Preventing a function from running multiple times in JavaScript

2 Answers2