Prototypes

JavaScript · Reference cheat sheet

📋 Overview

JavaScript inheritance is prototype-based. Every object has an internal [[Prototype]] link (exposed as __proto__ / Object.getPrototypeOf). Property lookup walks the chain until found or null. Classes are syntax sugar over constructor functions and prototypes.

🔧 Core concepts

Prototype chain: obj → Object.getPrototypeOf(obj) → … → null.
Own vs inherited: hasOwnProperty / Object.hasOwn vs chain lookup.
Constructor: Fn.prototype is assigned as [[Prototype]] of new Fn() instances.
Object.create(proto): make an object with a chosen prototype.
Class: class C extends B wires C.prototype → B.prototype.
new.target: detects construct calls.

const proto = { greet() { return "hi"; } };
const obj = Object.create(proto);
obj.greet(); // "hi"

💡 Examples

function Animal(name) {
  this.name = name;
}
Animal.prototype.speak = function () {
  return `${this.name} makes a noise`;
};

function Dog(name) {
  Animal.call(this, name);
}
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;
Dog.prototype.speak = function () {
  return `${this.name} barks`;
};

const d = new Dog("Rex");
d.speak(); // "Rex barks"
d instanceof Dog; // true
d instanceof Animal; // true

// Modern equivalent
class Cat extends Animal {
  speak() {
    return `${this.name} meows`;
  }
}

// Inspect
Object.getPrototypeOf(d) === Dog.prototype;
Object.hasOwn(d, "name"); // true
Object.hasOwn(d, "speak"); // false

// Null-prototype map (no inherited keys)
const dict = Object.create(null);
dict.admin = true;
"toString" in dict; // false

// Static vs instance
class Mathy {
  static dual(n) {
    return n * 2;
  }
  half() {
    return this.value / 2;
  }
}

// Shadowing
const a = Object.create({ x: 1 });
a.x = 2; // own property shadows prototype
delete a.x; // reveals prototype x again

⚠️ Pitfalls

Mutating built-in prototypes (Array.prototype) breaks libraries — avoid in app code.
for...in enumerates enumerable inherited keys — prefer Object.keys / hasOwn.
Setting __proto__ is legacy; use Object.setPrototypeOf sparingly (engine deopts).
instanceof can fail across realms/iframes with different globals.

oop.md — class syntax
objects.md — property model
new.md — construction
this_keyword.md — method receivers
proxy_reflect.md — intercepting lookup

Prototypes

Prototypes

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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