javascript return this after html element method runs setTimeout

Ok, so i have this code, adding a method (from what i understand) to the HTML element prototype:

HTMLElement.prototype.fadeIn = function( seconds ) {

var self = this;
var miliseconds = seconds * 1000;
var hold = this.style.transition;
this.style.transition = "opacity " + seconds + "s ease";
this.style.opacity = 1;

setTimeout(
    function () {
        self.style.transition = hold;
        return self;
    },
miliseconds);

};

my intention is that, at the end of the timeout, the function will return the instance of the HTMLElement so i can chain the fadeIn function with another one.

However it does not, and doing a console.log to what it returns, it says it's undefined. Can anyone give some advice as to how can i do this?