js 语法 new function与function

最新推荐文章于 2024-05-07 17:50:46 发布

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最新推荐文章于 2024-05-07 17:50:46 发布

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本文详细介绍了在JavaScript中使用new操作符调用构造函数的行为。解释了当使用new关键字创建对象时，JavaScript如何设置构造函数、this指向及返回值的规则。并通过示例展示了不同返回值类型对构造函数的影响。

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By the Javascript spec, when a function is invoked with new, Javascript creates a new object, then sets the "constructor" property of that object to the function invoked, and finally assigns that object to the name this. You then have access to the this object for the body of the function.

Once the function body is executed, Javascript will return:

ANY object if the function manually returns one:

function Car(){
  this.num_wheels = 4;
  return {num_wheels:37};
}

var car = new Car();
alert(car.num_wheels); // 37!
The this object if the function has no return statement OR if the function returns a value of a type other than object

function Car() {
  this.num_wheels = 4;
  return 'VROOM';
}

var car = new Car();
alert(car.num_wheels) // 4
alert(Car()); // No 'new', so this alerts 'VROOM'