First, here is an example that works as expected:
let a = { foo: 10 }
let b = { bar: 20 }
a.__proto__ = b
// returns value from prototype just fine
a.bar; // 20
And here is an example that is under the question, that does not work as expected. Why?
// "a" has no prototype when created this way
let a = Object.create(null);
// "b" has prototype and it is a JS native Object with a whole slew of its native props
let b = {};
// assign "a" a prototype
a.__proto__ = b;
// let's try
a.toString; // undefined
// but...
a.__proto__ .toString; // function toString() { [native code] }
Why a.toString returns undefined although a prototype with that property was assigned to it?