Why can I not change the prototype of a string?

Question

Let's suppose we have the following code, where for some reason I want stubbornly to change the prototype of a, b and c to the prototype of Obj.

Question: Why will the object and the array accept the change of prototype, but the string won't?

var Obj = function() {}

Obj.prototype = {
  log: function(s) {
    console.log(s);
  }
}

var
  a = {1110: "1110"},
  b = [1110],
  c = "1110";

Object.setPrototypeOf(a, Obj.prototype);
Object.setPrototypeOf(b, Obj.prototype);
Object.setPrototypeOf(c, Obj.prototype);

a.log("shuh");
b.log("shuh");
c.log("shuh");

zerkms · Accepted Answer · 2016-09-19 00:32:27Z

2

That's because "1110" is not an object, but of a primitive string type. So it does not have a notion of prototypes.

answered Sep 19, 2016 at 0:32

zerkms

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2 Comments

Angel Politis Over a year ago

So, we can only change the prototype of objects; not of everything under the sun, right? Thanks @zerkms.

zerkms Over a year ago

@AngelPolitis yep, in JS only objects have prototypes.

Oriol · Accepted Answer · 2016-09-19 01:32:13Z

1

You can't change the prototype of a string primitive, because only objects have a [[SetPrototypeOf]] internal method. This makes sense because the prototype determines property inheritance, but primitive values don't have properties, neither own nor inherited.

However, you can still change the prototype of a string object:

var Obj = function() {}

Obj.prototype = {
  log: function(s) {
    console.log(s);
  }
};

var a = {1110: "1110"},
    b = [1110],
    c = new String("1110");

Object.setPrototypeOf(a, Obj.prototype);
Object.setPrototypeOf(b, Obj.prototype);
Object.setPrototypeOf(c, Obj.prototype);

a.log("shuh");
b.log("shuh");
c.log("shuh");

edited Sep 19, 2016 at 1:32

answered Sep 19, 2016 at 1:26

Oriol

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Comments

Vlad Bezden · Accepted Answer · 2016-09-19 01:39:51Z

1

You can add log method to Object.prototype and all object will have it. This way you don't have to create separate log prototype for your objects, numbers and string, ... Here is an example of doing it

'use strict'
console.clear()

Object.prototype.log = function(s) {
    console.log(s);
}

var
  a = {1110: "1110"},
  b = [1110],
  c = "1110";

a.log("shuh");
b.log("shuh");
c.log("shuh");

edited Sep 19, 2016 at 1:39

answered Sep 19, 2016 at 1:08

Vlad Bezden

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2 Comments

zerkms Over a year ago

"Because all data types in JavaScript inherit from Object" --- that's not correct.

Oriol Over a year ago

Types themselves don't inherit, they are not values. Did you mean values from all types? But only objects do inherit, and not necessarily from Object.prototype.

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