Detailed explanation of JavaScript constructor and 'new' operator

javascriptThe column introduces the "new" operator.

Constructors and operators "new"

The regular {...} syntax allows creation of an object. But we often need to create many similar objects, such as multiple users or menu items, etc.

This can be achieved using constructors and the "new" operator.

Constructor

Constructor is technically a regular function. But there are two conventions:

1. Their names start with a capital letter.
2. They can only be executed by the "new" operator.

For example:

function User(name) {  this.name = name;  this.isAdmin = false;
}let user = new User("Jack");

alert(user.name); // Jackalert(user.isAdmin); // false复制代码

When a function is executed using the new operator, it follows the following steps:

1. a A new empty object is created and assigned to this.
2. Function body execution. Usually it will modify this, adding new properties to it.
3. Returns the value of this.

In other words, new User(...) does something like:

function User(name) {  // this = {};（隐式创建）

  // 添加属性到 this
  this.name = name;  this.isAdmin = false;  // return this;（隐式返回）}复制代码

So new User("Jack" ) results in the same object:

let user = {  name: "Jack",  isAdmin: false};复制代码

Now, if we want to create other users, we can call new User("Ann"),new User( "Alice") etc. Much shorter and easier to read than using literal creation every time.

This is the main purpose of the constructor - to implement reusable object creation code.

Let us stress it again - technically any function can be used as a constructor. That is: any function can be run through new, which will execute the above algorithm. "Capitalization" is a common convention to make it clear that a function will be run using new.

new function() { ... }

If we have many lines of code that create a single complex object, we can encapsulate them in a constructor, like this:

let user = new function() {  this.name = "John";  this.isAdmin = false;  // ……用于用户创建的其他代码
  // 也许是复杂的逻辑和语句
  // 局部变量等};复制代码

The constructor cannot be called again because it is not saved anywhere, just created and called. Therefore, this tip is intended to encapsulate the code that builds a single object without needing to reuse it in the future.

Constructor pattern test: new.target

Advanced content:

The syntax content involved in this section is rarely used unless You want to understand everything, otherwise you can just skip the syntax.

Inside a function, we can use the new.target property to check whether it has been called using new.

For regular calls, it is empty. For calls using new, it is equal to the function:

function User() {
  alert(new.target);
}// 不带 "new"：User(); // undefined// 带 "new"：new User(); // function User { ... }复制代码

It can be used inside the function to determine the function Is it the "constructor mode" that is called through new, or the "regular mode" that is not called through new.

We can also make the new call do the same job as a regular call, like this:

function User(name) {  if (!new.target) { // 如果你没有通过 new 运行我
    return new User(name); // ……我会给你添加 new
  }  this.name = name;
}let john = User("John"); // 将调用重定向到新用户alert(john.name); // John复制代码

This approach is sometimes used in libraries to make the syntax more flexible . In this way, when people call the function, the program will work regardless of whether new is used.

It's not a good thing to use it everywhere, though, because omitting new makes it difficult to observe what's going on in the code. And through new we can all know that this creates a new object.

Constructor's return

Normally, a constructor does not have a return statement. Their job is to write all the necessary stuff into this and automatically convert it into the result.

However, if there is a return statement, then the rule is simple:

• If return returns an object, Then return this object instead of this.
• If return returns a primitive type, it is ignored.

In other words, return with an object returns that object, in all other cases this is returned.

For example, here return overrides this by returning an object:

function BigUser() {  this.name = "John";  return { name: "Godzilla" };  // <-- 返回这个对象}

alert( new BigUser().name );  // Godzilla，得到了那个对象复制代码

Here is an example where return is empty (Or we could put a primitive type after it, no effect):

function SmallUser() {  this.name = "John";  return; // <-- 返回 this}

alert( new SmallUser().name );  // John复制代码

Normally constructors don't have a return statement. Here we mention the special behavior of returned objects mainly for completeness.

Omit brackets

By the way, if there are no parameters, we can omit the brackets after new:

let user = new User; // <-- 没有参数// 等同于let user = new User();复制代码

Omitting brackets here is not considered A "good style", but the syntax is allowed by the specification.

Methods in constructors

Using constructors to create objects brings great flexibility. A constructor may have parameters that define how the object is constructed and what to put in it.

当然，我们不仅可以将属性添加到 this 中，还可以添加方法。

例如，下面的 new User(name) 用给定的 name 和方法 sayHi 创建了一个对象：

function User(name) {  this.name = name;  this.sayHi = function() {
    alert( "My name is: " + this.name );
  };
}let john = new User("John");

john.sayHi(); // My name is: John/*
john = {
   name: "John",
   sayHi: function() { ... }
}
*/复制代码

类 是用于创建复杂对象的一个更高级的语法，我们稍后会讲到。

总结

• 构造函数，或简称构造器，就是常规函数，但大家对于构造器有个共同的约定，就是其命名首字母要大写。
• 构造函数只能使用 new 来调用。这样的调用意味着在开始时创建了空的 this，并在最后返回填充了值的 this。

我们可以使用构造函数来创建多个类似的对象。

JavaScript 为许多内置的对象提供了构造函数：比如日期 Date、集合 Set 以及其他我们计划学习的内容。

对象，我们还会回来哒！

在本章中，我们只介绍了关于对象和构造器的基础知识。它们对于我们在下一章中，学习更多关于数据类型和函数的相关知识非常重要。

在我们学习了那些之后，我们将回到对象，在 info:prototypes 和 info:classes 章节中深入介绍它们。

作业题

先自己做题目再看答案。

1. 两个函数 — 一个对象

重要程度：⭐️⭐️

是否可以创建像 new A()==new B() 这样的函数 A 和 B？

function A() { ... }function B() { ... }let a = new A;let b = new B;

alert( a == b ); // true复制代码

如果可以，请提供一个它们的代码示例。

2. 创建 new Calculator

重要程度：⭐️⭐️⭐️⭐️⭐️

创建一个构造函数 Calculator，它创建的对象中有三个方法：

• read() 使用 prompt 请求两个值并把它们记录在对象的属性中。
• sum() 返回这些属性的总和。
• mul() 返回这些属性的乘积。

例如：

let calculator = new Calculator();
calculator.read();

alert( "Sum=" + calculator.sum() );
alert( "Mul=" + calculator.mul() );复制代码

3. 创建 new Accumulator

重要程度：⭐️⭐️⭐️⭐️⭐️

创建一个构造函数 Accumulator(startingValue)。

它创建的对象应该：

• 将“当前 value”存储在属性 value 中。起始值被设置到构造器 startingValue 的参数。
• read() 方法应该使用 prompt 来读取一个新的数字，并将其添加到 value 中。

换句话说，value 属性是所有用户输入值与初始值 startingValue 的总和。

下面是示例代码：

let accumulator = new Accumulator(1); // 初始值 1accumulator.read(); // 添加用户输入的 valueaccumulator.read(); // 添加用户输入的 valuealert(accumulator.value); // 显示这些值的总和复制代码

相关免费学习推荐：JavaScript（视频）

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