JavaScript中调用person实例的say()提示“不是函数”问题求助

Hey there! Let's break down why you're getting that say is not a function error when calling your Person instance's method—this is super common, and usually boils down to a few easy-to-fix mistakes:

• 构造函数未正确挂载方法到实例/原型
  A frequent slip-up is defining the say method in a way that doesn't attach it to your Person instances. For example, this wrong approach keeps say private to the constructor scope:

  function Person(name) {
    this.name = name;
    // This creates a local variable, not an instance method
    var say = function() {
      console.log(`Hi, I'm ${this.name}`);
    }
  }
  

  Fix it by either attaching the method directly to this in the constructor, or (better for memory efficiency) adding it to the constructor's prototype:

  // Prototype approach (recommended)
  function Person(name) {
    this.name = name;
  }
  Person.prototype.say = function() {
    console.log(`Hi, I'm ${this.name}`);
  };
  
  // Constructor instance method approach
  function Person(name) {
    this.name = name;
    this.say = function() {
      console.log(`Hi, I'm ${this.name}`);
    }
  }
  

• 实例化时遗漏了new关键字
  If you call your constructor without new, this won't point to a new Person instance—instead, it binds to the global object (in non-strict mode), and the function returns undefined by default. No instance means no say method!

  // Wrong: no new keyword
  const person = Person("Alice"); 
  // Correct: uses new to create an instance
  const person = new Person("Alice");
  

  To guard against this mistake, add a check inside your constructor:

  function Person(name) {
    if (!(this instanceof Person)) {
      return new Person(name);
    }
    this.name = name;
  }
  Person.prototype.say = function() {
    console.log(`Hi, I'm ${this.name}`);
  };
  

• 意外覆盖了原型对象
  If you reassign Person.prototype to a brand new object after adding the say method, you'll wipe out all previously attached prototype methods. Like this:

  Person.prototype.say = function() { /* ... */ };
  // This replaces the entire prototype, losing the say method
  Person.prototype = {
    walk: function() { console.log("Walking..."); }
  };
  

  Fix this by using Object.assign to merge new methods into the existing prototype instead of replacing it:

  Object.assign(Person.prototype, {
    walk: function() { console.log("Walking..."); }
  });
  

• ES6类定义中的语法错误
  If you're using ES6 classes, a tiny syntax mistake can prevent the method from being recognized. For example, mixing up arrow function syntax or forgetting parentheses:

  // Wrong: invalid method syntax
  class Person {
    constructor(name) {
      this.name = name;
    }
    say => { // This is not how class methods are defined
      console.log(`Hi, I'm ${this.name}`);
    }
  }
  // Correct: standard class method syntax
  class Person {
    constructor(name) {
      this.name = name;
    }
    say() {
      console.log(`Hi, I'm ${this.name}`);
    }
  }
  

内容的提问来源于stack exchange，提问作者J C