What is the prototype of the String constructor function (String.proto)?

Question

var mString = new String('A');

console.log(typeof mString); // object

console.log(mString instanceof String); // true

console.log(mString instanceof Object); // true

console.log(mString.__proto__ === String.prototype); // true

console.log(mString.__proto__.__proto__ === Object.prototype); // true

Now, why

console.log(String.__proto__.__proto__ === Object.prototype); // true

and not

console.log(String.__proto__ === Object.prototype); // false

when walking up the prototype chain?

What is between String and Object prototypes?

Between a String instance and the Object prototype is the String prototype — Jonas Wilms
– Jonas Wilms, Commented Dec 4, 2017 at 21:37
Don't use the deprecated .__proto__ getter. Use Object.getPrototypeOf instead. — Bergi
– Bergi, Commented Dec 4, 2017 at 21:44

Michael Kurowski · Accepted Answer · 2017-12-04 21:40:43Z

2

Function.prototype === String.__proto__ //true

It's a function's prototype, because String is a constructor function.

answered Dec 4, 2017 at 21:40

Michael Kurowski

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3 Comments

Joseph Ditton Over a year ago

Its also worth noting that this is true for all classes not just String. For example (class A {}).__proto__ === Function.prototype is true

Michael Kurowski Over a year ago

Because classes are just syntactic sugar over function constructors.

JSstruck Over a year ago

Awesome. Thanks, guys.

XCS · Accepted Answer · 2017-12-04 21:31:40Z

-1

When you use instanceof Object it means that the variable is either an Object or "extends" an Object.

As mentioned here "The instanceof operator tests whether the prototype property of a constructor appears anywhere in the prototype chain of an object."

So mString.__proto__ is not equal to Object.prototype, but it Object.prototype is indeed higher on the prototype chain, that's why they are not equal. The String prototype inherits the Object prototype.

answered Dec 4, 2017 at 21:31

XCS

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