Javascript 新关键字

Question

以下方式创建实例是什么意思？

somevar = new Person.doSomething()

这是创建实例和调用方法的快捷方式吗，比如

person = new Person ()
person.doSomething()

还是其他原因？

提前致谢。

Answer 1

不，这不会创建 Person 的新实例，然后对其调用方法。它创建一个 Person.doSomething() 是什么的实例。所以，实际上，你这两个是等价的：

const Person = {
  doSomething: function(foo, bar){
    this.foo = foo;
    this.bar = bar;
  }
}

//create a new instance of `Person.doSomething`
const p1 = new Person.doSomething(1, "one");

//take a reference of `Person.doSomething`
const temp = Person.doSomething;
//create an instance of it
const p2 = new temp(2, "two");

console.log("p1:", p1);
console.log("p1 instanceof Person.doSomething:", p1 instanceof Person.doSomething);

console.log("p2:", p2);
console.log("p2 instanceof Person.doSomething:", p2 instanceof Person.doSomething);

您只能使用 constructable 函数获取实例。这些是普通函数（使用 function 关键字声明）和 class 构造函数：

function PlainConstructable(a, b) {
  this.foo = a;
  this.bar = b;
}

const plainInstance = new PlainConstructable("plain", "instance");

class ClassConstructable {
  constructor(a, b) {
    this.foo = a;
    this.bar = b;
  }
}

const classInstance = new ClassConstructable("class", "instance");


console.log(`plainInstance:
  instanceof PlainConstructable: ${plainInstance instanceof PlainConstructable}
  what it holds: ${JSON.stringify(plainInstance)}`);
  
console.log(`classInstance:
  instanceof ClassConstructable: ${classInstance instanceof ClassConstructable}
  what it holds: ${JSON.stringify(classInstance)}`);

不可建造的有：

arrow functions
shorthand methods 在对象中或类
大多数内置函数（例外情况在文档中明确提及）

const arrowFunction = () => {};

const plainObject = {
  shorthandMethod() {}
}

try {
  new arrowFunction();
  console.log("Constructing arrow function successful.");
} catch(e) {
  console.log(
    `Cannot construct arrow function
     ${e}`
   )
}

try {
  new plainObject.shorthandMethod();
  console.log("Constructing shorthand method successful.");
} catch(e) {
  console.log(
    `Cannot construct shorthand method
     ${e}`
   )
}

try {
  new parseInt();
  console.log("Constructing built-in successful.");
} catch(e) {
  console.log(
    `Cannot construct built-in
     ${e}`
   )
}

Javascript 新关键字

Javascript new keyword

javascript

new-operator