Understanding the JavaScript Prototype Chain
Sep 20, 2025 am 04:58 AM
Prototype chain is the core mechanism for JavaScript to implement inheritance. Each object has a __proto__ attribute that points to its constructor's prototype, and it is searched upwards along this chain when accessing the attribute. For example, after Dog.prototype is set with Object.create(), the instance myDog can call the eat method. To correctly set up the prototype chain, you need to: 1. Use Object.create(SuperClass.prototype) to create a subclass prototype; 2. Add a subclass method; 3. Reset the constructor manually. Common problems include: wrong assignment of prototypes, not calling constructors with new, and improper order of prototype modification. The prototype chain finally points to Object.prototype, and then goes up to null. By mastering these core concepts and steps, you can clarify the logic of the prototype chain.
JavaScript's prototype chain is the key to understanding the object inheritance mechanism. Many people find it complicated when they first come into contact. In fact, just grasp a few core concepts and you can clarify the logic.
What exactly is a prototype?
Each function has a prototype property, which is an object. By default, this object has a constructor property pointing to the function itself. When you create an instance with a constructor, there will be a pointer inside the instance (usually called __proto__ ) pointing to the constructor's prototype .
For example:
function Person() {}
const p1 = new Person();
console.log(p1.__proto__ === Person.prototype); // true This indicates that the instance's __proto__ points to the prototype object of the constructor.
How are prototype chains formed?
When accessing an object's properties or methods, the JavaScript engine will first look up on the object itself. If it is not found, you will search upwards along __proto__ until it is found, or find the top of the prototype chain - that is, Object.prototype .
for example:
function Animal() {}
Animal.prototype.eat = function () {
console.log('Eating...');
};
function Dog() {}
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.bark = function () {
console.log('Woof!');
};
const myDog = new Dog();
myDog.eat(); // Eating... Here, the prototype of Dog inherits from the prototype of Animal , so myDog instance can access eat method. This is what the prototype chain does: implement inheritance.
How to set up the prototype chain correctly?
There are a few points to pay attention to when setting up the prototype chain:
- Use
Object.create()to set up prototypes more clearly and safely - Do not directly assign the constructor's prototype as an instance of another constructor (although feasible but error-prone)
- Remember to reset the constructor property, otherwise it may mislead subsequent use
Common practices are as follows:
- When creating a prototype object of a subclass, use
Object.create(SuperClass.prototype) - Add your own method on the subclass prototype
- If necessary, manually specify the constructor to point to the subclass
Frequently Asked Questions in Prototype Chain
Some developers will encounter strange problems after modifying the prototype, such as:
- Forgot to call the constructor with
newcauses this pointing error - The prototype chain was not correctly set during multi-layer inheritance, resulting in the method not being found
- The order of modifying the prototype is wrong, covering the existing definition
In addition, the prototype chain is not infinitely long. After finding out that the last prototype is null , it's over. For example:
console.log(Object.prototype.__proto__); // null
Basically that's it. The prototype chain looks a bit tangled, but as long as you understand the mechanism of "objects find attributes through __proto__ " and practice them in combination with actual code, you will be able to master them soon.
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