函数表现

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function F1() {
  this.name = "f1";
}

function F2() {
  this.name = "F2";
  return {};
}

console.log("1: ", new F1().name);
console.log("2: ", F1().name);
console.log("3: ", new F2().name);
console.log("4: ", F2().name);

运行结果 👇

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1: f1

2: error: Uncaught TypeError: Cannot read property 'name' of undefined

3: undefined

4: undefined

引用 MDN 的一段话关于 new 操作符

The new operator lets developers create an instance of a user-defined object type or of one of the built-in object types that has a constructor function. The new keyword does the following things:

1. Creates a blank, plain JavaScript object;
2. Links (sets the constructor of) this object to another object;
3. Passes the newly created object from Step 1 as the this context;
4. Returns this if the function doesn’t return its own object.

new做了这 4 件事

1. 创建一个空的简单 JavaScript 对象（即{}）
2. 链接该对象（即设置该对象的构造函数）到另一个对象
3. 将步骤 1 新创建的对象作为 this 的上下文
4. 如果该函数没有返回对象，则返回 this

函数执行默认返回 undefined
new构造函数默认返回this
指定返回值优先级最高