Functions - Test-Driven JavaScript Development - page 89

Listing 5.25 Calling a function as a method on an object
function addToArray() {
var targetArr = arguments[0];
arguments.slice = Array.prototype.slice;
var add = arguments.slice(1);
return targetArr.concat(add);
}
Calling the addToArray function will work exactly as the one presented in
Listing 5.8. The ECMAScript specification specifically calls for many built-in meth-
ods to be generic, allowing them to be used with other objects that exhibit the right
qualities. For instance, the arguments object has both a length property and
numeric indexes, which satisfies Array.prototype.slice 's requirements.
5.4.2 Explicitly Setting this
When all we want is to control the value of this for a specific method call, it
is much better to explicitly do so using the function's call or apply methods.
The Function.prototype.call method calls a function with the first ar-
gument as this . Additional arguments are passed to the function when calling
it. The first example of addToArray in Listing 5.8 used this method call Ar-
ray.prototype.slice with arguments as this . Another example can be
found in our previous circle example, as Listing 5.26 shows.
Listing 5.26 Using call
assertEquals(10, circle.diameter.call({ radius: 5 }));
Here we pass an object literal that defines a radius property as the this
when calling circle.diameter .
5.4.3 Using Primitives As this
The first argument to call can be any object, even null . When passing null ,
the global object will be used as the this value. As we will see in Chapter 8,
ECMAScript 5th Edition, this is about to change—in ECMAScript5 strict mode,
passing null as the this value causes this to be null , not the global object.
When passing primitive types, such as a string or boolean, as the this value,
the value is wrapped in an object. This can be troublesome, e.g., when calling