JavaScript does not have the concept of classes in languages such as JAVA in the strict sense before ES6. ES6 added class, but in fact this class is just syntactic sugar for constructors and prototypes before ES6. To really understand the most complex parts of JavaScript, you have to start with the initial constructors and prototypes. Prototype is one of the characteristics of JavaScript. Although the object-oriented concepts such as classes and instances are similar to languages such as Java, they are not the same in essence. In JavaScript, each object instantiated by the constructor generally only contains its own properties, but when we call a method or property that is not on the object, the instantiated object will look up its own prototype. . If the prototype has not been found, and the prototype object has its own prototype, it will go back to the top prototype along the prototype chain.