Question

With the following:

function Person() {
    this.name = "john";
    this.getName = function() {
        return this.name;
    };
}

var me = new Person();

Why would a proceeding:

setTimeout(function(){alert(me.getName())}, 3000);

returns john while a proceeding:

setTimeout(me.getName(), 3000);

create a

Uncaught TypeError: Object [object DOMWindow] has no method 'getName' 

Answer 1

The problem is that setTimeout takes a function as the first argument.

In your code snippet:

setTimeout(function(){alert(me.getName())}, 3000);

You are passing in a function as the first argument which is then evaluated to do the alert.

In the second snippet you’re passing in the result of invoking the function so this:

setTimeout(me.getName(), 3000);

Becomes this:

setTimeout(‘john’, 3000);

Because of the way setTimeout works it will allow you to pass in a string as the first argument that the runtime will attempt to invoke as a function resulting in an error.