Question

Methods are simply properties that hold function values. This is a simple method:

var rabbit = {};
rabbit.speak = function(line) {
  console.log("The rabbit says '" + line + "'");
};

rabbit.speak("I'm alive.");
// → The rabbit says 'I'm alive.'

Usually a method needs to do something with the object it was called on. When a function is called as a method—looked up as a property and immediately called, as in object.method() —the special variable this in its body will point to the object that it was called on.

function speak(line) {
  console.log("The " + this.type + " rabbit says '" +
              line + "'");
}
var whiteRabbit = {type: "white", speak: speak};
var fatRabbit = {type: "fat", speak: speak};

whiteRabbit.speak("Oh my ears and whiskers, " +
                  "how late it's getting!");
// → The white rabbit says 'Oh my ears and whiskers, how
//   late it's getting!'
fatRabbit.speak("I could sure use a carrot right now.");
// → The fat rabbit says 'I could sure use a carrot
//   right now.'

The code uses the this keyword to output the type of rabbit that is speaking. Recall that the apply and bind methods both take a first argument that can be used to simulate method calls. This first argument is in fact used to give a value to this .

There is a method similar to apply , called call . It also calls the function it is a method of but takes its arguments normally, rather than as an array. Like apply and bind , call can be passed a specific this value.

speak.apply(fatRabbit, ["Burp!"]);
// → The fat rabbit says 'Burp!'
speak.call({type: "old"}, "Oh my.");
// → The old rabbit says 'Oh my.'

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.