Question

this has interested me purely as research and personal development. i have a namespaced set of functions / variables.

within 1 function I need to call another through setTimeout but keeping the scope to ‘this’. i am struggling with this a little, can’t seem to bind it for when the setTimeout runs.

var foo = {
    ads: ["foo","bar"],
    timeDelay: 3,
    loadAds: function() {
        var al = this.ads.length;
            if (!al)
                return; // no ads

            for(var i = 0; i < al; i++) {
                setTimeout(function() {
                    this.scrollAd(this.ads[i]);
                }.apply(this), this.timeDelay * 1000);
            }
        },
        scrollAd: function(adBlock) {
            console.log(adBlock);

        }
    };
};

the .apply(this) DOES change the scope as the console.log outputs the right object back, but it runs the function immediately and then the exception/warning comes up as the callback remains empty:

useless setTimeout call (missing quotes around argument?)

is there an elegant way of doing this at all? i know i could do

var _this = this;

and reference _this in the anon callback. for example, in mootools i’d use .bind(this) instead…

and no, as this involves animating, i don’t want to use " " around the string as it will need to be eval’d and would impact performance…

Answer 1

for(var i = 0; i < al; i++) {
    setTimeout(function() {
        this.scrollAd(this.ads[i]);
    }.apply(this), this.timeDelay * 1000);
}

apply doesn’t bind a function, it calls it. So you execute the scroll straight away and then pass its return value (undefined) to setTimeout, which is ineffective.

You probably meant to use a closure like this over this and the loop variable (which must be closed or it will be the same, post-loop value for every timeout):

for(var i = 0; i < al; i++) {
    setTimeout(function(that, j) {
        return function() {
            that.scrollAd(that.ads[j]);
        };
    }(this, i), this.timeDelay * 1000);
}

However you may prefer to use the new ECMAScript Fifth Edition function binding feature, which has a much more compact syntax:

for (var i= 0; i<al; i++)
    setTimeout(this.scrollAd.bind(this, this.ads[i]), this.timeDelay*1000);

(There’s an implementation of function.bind for browsers that don’t have have it natively at the bottom of this answer.)