Question

Why does the following work:

function sum(a,b) { return a + b; }
var result = sum.call(null,3,4);     // 7

Why is result defined? I am invoking sum as a method of null. But null is not an object and cannot have properties!

What is going on?

Answer 1

The first argument for Function.prototype.call is the context, which defines the this value for the execution context of the invoked function, nothing else.

So basically, you’re saying that this is referring to null (at least, in ES5 strict mode), but since you don’t access this anyway, it makes no difference.

In non-strict mode, this cannot be null, so it’s replaced with the global object instead.