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javascriptobjecttypeof

Why does Object.prototype not set the value of an object?

I was playing around with javascript in my web browser when I noticed something strange.
I defined a class TestClass like this:

class TestClass{};
var testObj=new TestClass();
Object.setPrototypeOf(testObj, null);

When I console.loged testObj, there were no properties.

Here is where I became confused.
typeof testObj returned "object". But when I ran the following:

Object.prototype.testProperty='testing';

testObj didn't have the property testProperty, even though I'd defined it for all objects and according to typeof, testObj was an object.

Why is this happening? Don't all values inherit properties from Object?

This also happens for [[Scopes]] objects. When, in chrome, I console.log a function and I save its [[Scopes]] property as a global variable, typeof returns "object" but the global variable does not have the property testProperty.

Solution

  • Don't all values inherit properties from Object?

    No. Most objects inherit from Object.prototype, but some don't - like the Object.prototype object itself. Also objects whose prototype was explicitly set to null, like the testObj one you created in your code or a Object.create(null). Other cases are objects from different realms, which inherit from a different Object.prototype object.