The "prototype" property is widely used by the core of JavaScript itself. All built-in constructor functions use it.
First we’ll look at the details, and then how to use it for adding new capabilities to built-in objects.
Object.prototype
Let’s say we output an empty object:
let obj = {};
alert( obj ); // "[object Object]" ?Where’s the code that generates the string "[object Object]"? That’s a built-in toString method, but where is it? The obj is empty!
…But the short notation obj = {} is the same as obj = new Object(), where Object is a built-in object constructor function, with its own prototype referencing a huge object with toString and other methods.
Here’s what’s going on:
When new Object() is called (or a literal object {...} is created), the [[Prototype]] of it is set to Object.prototype according to the rule that we discussed in the previous chapter:
So then when obj.toString() is called the method is taken from Object.prototype.
We can check it like this: