How do I pass the this context to a function?

Javascripts .call() and .apply() methods allow you to set the context for a function.

var myfunc = function(){
    alert(this.name);
};

var obj_a = {
    name:  "FOO"
};

var obj_b = {
    name:  "BAR!!"
};

Now you can call:

myfunc.call(obj_a);

Which would alert FOO. The other way around, passing obj_b would alert BAR!!. The difference between .call() and .apply() is that .call() takes a comma separated list if you're passing arguments to your function and .apply() needs an array.

myfunc.call(obj_a, 1, 2, 3);
myfunc.apply(obj_a, [1, 2, 3]);

Therefore, you can easily write a function hook by using the apply() method. For instance, we want to add a feature to jQuerys .css() method. We can store the original function reference, overwrite the function with custom code and call the stored function.

var _css = $.fn.css;
$.fn.css = function(){
   alert('hooked!');
   _css.apply(this, arguments);
};

Since the magic arguments object is an array like object, we can just pass it to apply(). That way we guarantee, that all parameters are passed through to the original function.

Use function.call:

var f = function () { console.log(this); }
f.call(that, arg1, arg2, etc);

Where that is the object which you want this in the function to be.