JavaScript new关键字疑问：somevar = new Person.doSomething()的实例创建逻辑

Great question — this is a common point of confusion because of operator precedence in JavaScript. Let’s break down exactly what’s happening here, and how it differs from the two-step approach you mentioned.

首先，new Person.doSomething() 到底在做什么？ #

In JavaScript, the dot operator (.) has higher precedence than the new keyword (when new is used without parentheses). That means the expression is actually parsed as:

new (Person.doSomething)()

In plain terms:

• You’re first accessing the doSomething property of the Person class/object (this is usually a static method, not an instance method).
• Then you’re using new to invoke that doSomething function as a constructor.
• The result stored in somevar is a new instance created by the Person.doSomething constructor function.

和 new Person() 再调用 doSomething() 的区别 #

These two approaches are completely different—they don’t do the same thing at all:

1. new Person.doSomething() #

• Target: The static Person.doSomething function (attached directly to the Person class, not to its instances).
• Outcome: Creates a new instance of whatever Person.doSomething defines as its constructor.
• somevar holds this new instance.

2. Two-step approach: #

const person = new Person();
const somevar = person.doSomething();

• Step 1: Creates a new instance of the Person class, stored in person.
• Step 2: Calls the instance method doSomething() on that Person instance.
• somevar holds the return value of person.doSomething() (if the method doesn’t return anything, this will be undefined).

代码示例对比 #

Let’s use concrete code to see the difference:

class Person {
  constructor() {
    this.instanceName = "Alice";
  }

  // Instance method (only exists on Person instances)
  doSomething() {
    console.log("Running instance method");
    return "Instance method result";
  }

  // Static method (attached to Person class, can be used as a constructor)
  static doSomething() {
    this.staticProperty = 30;
    console.log("Running static method as constructor");
  }
}

// Case 1: new Person.doSomething()
const instanceFromStatic = new Person.doSomething();
console.log(instanceFromStatic.staticProperty); // Output: 30
console.log(instanceFromStatic instanceof Person.doSomething); // Output: true

// Case 2: Two-step approach
const personInstance = new Person();
const methodResult = personInstance.doSomething(); // Logs: "Running instance method"
console.log(methodResult); // Output: "Instance method result"
console.log(personInstance.instanceName); // Output: Alice

关键总结 #

• new Person.doSomething() ≠ new Person() + doSomething()
• The first syntax treats Person.doSomething as a constructor, while the second uses Person as the constructor and calls an instance method afterward.

内容的提问来源于stack exchange，提问作者flag81