Assigning prototype chain throws TypeError in Javascript

Question

I'm studying inheritance in JavaScript using prototype chaining.

Here's the code: https://jsfiddle.net/pnyf1ska/6/

// INHERITANCE USING JS

// SUPER CLASS CONSTRUCTOR
function bank () {
    this.cash = 1000;
}

// SUB CLASS CONSTRUCTOR
function thief () {
    // CALL BANK CONSTRUCTOR
    bank.call(this);
}

// ADD A METHOD TO BANK.PROTOTYPE
bank.prototype.steal = function () {
    console.log('a thief stole '+this.cash+' dollars!');
}

// THIEF EXTENDS BANK AND INHERITS STEAL METHOD.
//thief.prototype = bank.prototype;
// thief.prototype.__proto__ = bank.prototype // also works!
thief.prototype.__proto__.__proto__ = bank.prototype;  // why does this break?

let t = new thief();
t.steal();  // OUTPUT 1000.

Here thief.prototype.__proto__.__proto__ = bank.prototype breaks and outputs error:

"<a class='gotoLine' href='#47:37'>47:37</a> Uncaught TypeError: Immutable prototype object '#&lt;Object&gt;' cannot have their prototype set"

Since thief.prototype.__proto__ = bank.prototype works I thought thief.prototype.__proto__.__proto__ = bank.prototype would work as well.

Can you explain the error and how to fix it? Thanks.

Why not just thief.prototype = bank.prototype;? jsfiddle.net/f71dcbt4 — Unmitigated
– Unmitigated, Commented Jul 14, 2020 at 1:01
@hev1 so it goes up prototype chain of thief multiple times to find steal() method. just for demonstration. — Logan Lee
– Logan Lee, Commented Jul 14, 2020 at 1:02
Using it makes no sense; thief.prototype.__proto__ just returns Object.prototype. And the __proto__ of that is just null. — Unmitigated
– Unmitigated, Commented Jul 14, 2020 at 1:05

Unmitigated · Accepted Answer · 2020-07-15 13:02:31Z

2

To make thief inherit all methods and fields from bank, just set thief.prototype = bank.prototype. What you are currently using makes no sense; thief.prototype.__proto__ just returns Object.prototype and the __proto__ of that is just null (and is not a writable property).

// INHERITANCE USING JS

// SUPER CLASS CONSTRUCTOR
function bank () {
    this.cash = 1000;
}

// SUB CLASS CONSTRUCTOR
function thief () {
    // CALL BANK CONSTRUCTOR
    bank.call(this);
}

// ADD A METHOD TO BANK.PROTOTYPE
bank.prototype.steal = function () {
    console.log('a thief stole '+this.cash+' dollars!');
}

thief.prototype = bank.prototype;

let t = new thief();
t.steal();  // OUTPUT 1000.

edited Jul 15, 2020 at 13:02

answered Jul 14, 2020 at 1:11

Unmitigated

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