The meaning of javascript prototype chain

JavaScript is a very flexible language and a prototype-based language, which means that all objects have a prototype object, which can contain properties and methods. In JavaScript, all objects inherit properties and methods from their prototype.

The prototype chain is a very important and basic concept in JavaScript. Understanding the prototype chain is the key to understanding JavaScript object-oriented programming.

The prototype chain is based on prototypal inheritance, which means that one object can inherit its properties and methods from another object. In JavaScript, every object has an internal property [[Prototype]] that points to its prototype object. When trying to access a property or method that doesn't exist on an object, the JavaScript engine looks on the object's prototype object.

If the property or method is not found on the prototype object, continue searching on the prototype object's prototype object until the top-level prototype object is found. The chain structure formed in this way is the prototype chain.

The top-level object of the prototype chain is Object.prototype, and all objects inherit from it. The Object.prototype object contains common properties and methods of JavaScript objects, such as toString() and hasOwnProperty().

In JavaScript, we can explicitly define the prototype of an object by using the Object.create() method. For example, we can create a new object and set its prototype to the prototype of the Person object:

var Person = function(name){
  this.name = name;
};

Person.prototype.sayHello = function(){
  console.log("Hello, my name is " + this.name);
};

var john = new Person("John");

var jane = Object.create(Person.prototype);

jane.name = "Jane";

jane.sayHello(); // 输出： Hello, my name is Jane

In the above example, we created a Person object and added a sayHello method to its prototype object, Then a new object jane is created through the Object.create() method, and its prototype is set to the prototype of the Person object. In this way, jane can access the properties and methods on the Person object through the prototype chain, including the sayHello method.

At the same time, we can view the prototype of an object through the Object.getPrototypeOf() method:

console.log(Object.getPrototypeOf(jane) === Person.prototype); // 输出： true

When we want to access the properties or methods of an object, the JavaScript engine will first search for the object If the properties and methods are not found, they will be searched in the prototype object. If the property or method does not exist on the prototype object, it will continue to search on the prototype object's prototype until it finds an object with the property or method, or reaches the top level of the prototype chain (Object.prototype).

If the property or method is not found in the entire prototype chain, undefined will be returned. If we write code that tries to access a property or method with an undefined value, a TypeError exception will be thrown. Therefore, when using the prototype chain, we should be very careful to ensure that both the object and its prototype object contain the properties and methods we need.

In general, the prototype chain is a very important concept in JavaScript. Understanding the meaning and mechanics of the prototype chain is key to mastering object-oriented programming in JavaScript. Understanding the role and use of the prototype chain can help us write better and more efficient JavaScript code.

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