Question

I have a pretty good understanding of Javascript, except that I can’t figure out a nice way to set the ‘this’ variable. Consider:

var myFunction = function(){     alert(this.foo_variable); }  var someObj = document.body; //using body as example object someObj.foo_variable = 'hi'; //set foo_variable so it alerts  var old_fn = someObj.fn;   //store old value someObj.fn = myFunction;   //bind to someObj so 'this' keyword works someObj.fn();               someObj.fn = old_fn;       //restore old value 

Is there a way to do this without the last 4 lines? It’s rather annoying… I’ve tried binding an anonymous function, which I thought was beautiful and clever, but to no avail:

var myFunction = function(){     alert(this.foo_variable); }  var someObj = document.body;        //using body as example object someObj.foo_variable = 'hi';        //set foo_variable so it alerts someObj.(function(){ fn(); })();    //fail. 

Obviously, passing the variable into myFunction is an option… but that’s not the point of this question.

Thanks.

Answer 1

There are two methods defined for all functions in JavaScript, call(), and apply(). The function syntax looks like:

call( /* object */, /* arguments... */ ); apply(/* object */, /* arguments[] */); 

What these functions do is call the function they were invoked on, assigning the value of the object parameter to this.

var myFunction = function(){     alert(this.foo_variable); } myFunction.call( document.body );