http://www.mennovanslooten.nl/blog/post/65
I’ve been meaning to write this post for a while, now, mostly as a reminder to myself. The subject is the two methods available for all JavaScript functions: apply() and call(). Confusingly, they both do the same thing in very, very similar ways: executing the function in the scope of a specified object. I’ve always had trouble remembering their major difference, however, so I decided to finally write this post and hope it sticks.
I can make this a very technical post about the ins and outs of these methods or I can just summarize by writing the following: apply() and call() allow you to specify whatthiswill refer to inside that function, even when that function is a method on another object. An example:
var someObject = {
myProperty : 'Foo',
myMethod : function() {
alert(this.myProperty);
}
};
someObject.myMethod(); // alerts 'Foo'
var someOtherObject = {
myProperty : 'Bar'
};
someObject.myMethod.call(someOtherObject); // alerts 'Bar'
someObject.myMethod.apply(someOtherObject); // alerts 'Bar'
As you can see, even thought it is someObject’s method that is executed, call() and apply() allows me to specify that the keyword this refers to someOtherObject.
The difference between these methods is in the way you can pass arguments to the original function. Let’s extend the example to include arguments:
var someObject = {
myProperty : 'Foo',
myMethod : function(prefix, postfix) {
alert(prefix + this.myProperty + postfix);
}
};
someObject.myMethod('<', '>'); // alerts '<Foo>'
var someOtherObject = {
myProperty : 'Bar'
};
someObject.myMethod.call(someOtherObject, '<', '>'); // alerts '<Bar>'
someObject.myMethod.apply(someOtherObject, ['<', '>']); // alerts '<Bar>'
Obviously, the difference is this: If you want to pass arguments to the function apply() needs an array and call() needs a list of arguments. That’s it! Ridiculously hard to remember, I know.