javascript中的示例堆栈，需要澄清“ this”的用法

Question

I found this example and I'm unable to understand how it works:

 function Stack() { this.top = null; } Stack.prototype.push = function(val) { this.top = { data : val, next : this.top } } var S1 = new Stack(); S1.push(1); S1.push(2); console.log(S1); 

why is 'next : this.top' resolving into the previous push's 'this.top' object? and not just returning null?

Answer 1

When a function is called as a method of an object, its this is set to the object the method is called on.

you can see this documentation for more details

Answer 2

this in JavaScript is a game changer and it should be taken care of. The thing with this is that it references the context of the function you invoked.

In this case, when .push is invoked this will reference the Stack object instance.

Btw console.log prints as it is being resolved, but you can't expect to have ti resolved at that point in time.