写入__proto__对象

Question

Say I have an object like so:

const obj1 = {foo:'bar'};

and then I create a "copy", ( obj1 is now the proto of obj2 ):

const obj2 = Object.create(obj1);

if I call

obj2.foo = 3;

or

obj2.x = {};

my question is - will obj1 ever get written to, or will it never be written to? As far as I know, only the top level object will get written to - the prototype object is never modified.

In other words, are there any circumstances where is I set properties obj2, obj1 will get modified?

Answer 1

obj1 can be written if it's accessed directly like Object.getPrototypeOf(obj2).foo = 3 but it's unlikely that this can be done not on purpose.

It's possible to write to obj1 not on purpose if it was designed without inheritance in mind, eg:

const obj1 = {
  get foo() { return obj1._foo },
  set foo(v) { obj1._foo = v}
}

const obj2 = Object.create(obj1);
obj2.foo = 3;

Referring an object directly instead of this inside object methods was a common way to maintain proper context in legacy ES3/ES5 code.

Answer 2

Are there any circumstances where is I set properties obj2 , obj1 will get modified?

Only when obj1 has defined a setter property that is inherited by obj2 (and what exactly happens then depends on the implementation of the setter). Otherwise, a property assignment on obj2 will always create/change properties on obj2 itself.