'this' keyword in built in functions

Question

I don't quite understand how does the 'this' keyword in built-in functions like String, Number, etc point to the primitive value directly instead of the object like the 'this' keyword is supposed to. Is it just directly set by javascript engine?

I have this example:

String.prototype.repeatify = function(times) {
    console.log(this);   // Output - String {"hello"}
    var str = "";
    for(var i=0; i<times; i++) {
        str += this;     //how does 'this' gets set to 'hello' and not the object- String{  }??
    }
    return str;
}
var string = new String('hello');
console.log(string.repeatify(3));   //Output - prints hellohellohello

I was expecting the output in the chrome devtools to be [object Object][object Object][object Object]

`this` is the current scope of the function, as you run it on a `String` type, `this` is the string itself — Icepickle, Nov 21 '18 at 08:01
`this` is not `Object { }` or `String { }` in built in functions ?? — roan shaz, Nov 21 '18 at 08:04
@roanshaz why do you think that `this` should be that `[object Object]` because that is what you expect. — KarelG, Nov 21 '18 at 08:12
@KarelG I thought that the 'this' always binds itself to the object that creates it ( meaning 'this' points to `String { }` object — roan shaz, Nov 21 '18 at 08:20
This has little to do with `this`, and all with how primitives types are implicitly (un)boxed to/from objects and how the `+` operator implicitly gets the primitive value of an object. — deceze, Nov 21 '18 at 08:20
@deceze can you please direct me in the direction of this topic — roan shaz, Nov 21 '18 at 08:23
@Karan yes, i did that .. i guess im a little confused.. i need some time off and then i will see the question deceze pointed to — roan shaz, Nov 21 '18 at 08:38
@deceze when you say boxed .. do you mean [[ PrimitiveValue ]] ?? — roan shaz, Nov 21 '18 at 11:14

score 1 · Answer 1 · answered Nov 21 '18 at 08:10

There's a lot to this, but it's essentially the target of the function invocation.

Some examples:

String.prototype.whatever = function() {
  return this + ' ugh.. whatever.'
  // `this` is the string this method is called against
}

Array.prototype.slip = function() {
  return ['Hellooo', ...this];
  // `this` is the array in which the function is invoked on
}

function doSomething() {
  console.log(this + ' something');
  // `this` is whatever the function is bound to
}

console.log('Excuse me'.whatever()) // Excuse me ugh.. whatever.
console.log([1, 2, 3].slip()) // ["Hellooo", 1, 2, 3]

// In this case, the string 'I will do' becomes `this` in the doSomething function above as it's explicitly bound
doSomething.bind('I will do')(); // I will do something

so when inside the `String.prototype.whatever = function() { return this + ' ugh.. whatever.' // 'this' is the string this method is called against }` how would one get the length of this string ??? — marvinIsSacul, Nov 21 '18 at 08:27

score -1 · Answer 2 · answered Nov 21 '18 at 08:09

-1

var string = 'hello'; // ------------
// this refers to the               |
// ----------v--string itself => 'hello'
console.log(string.repeatify(3));

You're defining repeatify on String prototype so this is just a type of String. Thus, it will log hellohellohello.

answered Nov 21 '18 at 08:09

Bhojendra Rauniyar

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Now this looks like it could have been a comment (and the duplicate is more explanatory than this answer) – Icepickle Nov 21 '18 at 08:16

'this' keyword in built in functions

2 Answers2