What is `this` inside Number/ String Prototype?

Question

So I'm extending some native JS objects and noticed some oddities when it comes to this. When I log this on strings, I notice it's an array-like object which also has a property called [[PrimitiveValue]] which contains the value the value I'd expect, but cannot be retrieved using a obj['[[PrimitiveValue]]'] syntax.

Take this example:

String.prototype.method = function() {
  return this;
};

Number.prototype.method = function() {
  return this;
};

var str = "12";
var num = 12;

console.log( str.method() );
console.log( num.method() );

Wanting to get the actual value, I tried something like this however it gets logged as undefined:

String.prototype.method = function() {
  return this['[[PrimitiveValue]]'];
};

Number.prototype.method = function() {
  return this['[[PrimitiveValue]]'];
};

I noticed it was possible to get the raw value using some type conversion, however this feels really hackish. (this + 0 for Number, this + '' for String.)

So what type of object am I dealing with here? How do I effectively interact with this inside a native JS object's prototype?

Answer 1

Those are just the native objects; you use this just like in a normal prototype function.

Note that you are indeed working with objects: instances built with the "String" or "Number" or some other native constructor. Things like "hello world" and 12.54 are not objects, they're primitives. They don't have prototypes.

When you use a primitive object as if it were an object, the runtime automatically promotes it to an object instance. That happens when you use a primitive value with the . or [] operators.

From such objects (instances of String, Number, or Boolean), the primitive value can be fetched with this.valueOf().

Answer 2

In the String prototype, this refers to String object type. It has the length property, but it is not a real array, it an is array-like object. You can access the this value at subscripts like this[0] an so forth, but to convert it an array so you can use methods such as forEach and all the other prototype methods of the Array, you may do var args = Array.prototype.call(this); For example let's illustrate this:

String.prototype.hello = function() {
  console.log(this);
  console.log(typeof this);
  console.log(Object.getOwnPropertyNames(this));    
}
"adfadfda".hello();

To get the full string as called on the hello method, you can use this.toString(); which would return in this case "adfadfda"

To actually see the type of object that the this value represents we can do:

Object.prototype.toString.call(this);

Which would return:

[object String]