使用带有和不带有“ new”的构造函数创建对象

Question

I have noticed I can create the same object using a constructor in two different ways.

var myObj = Object()

var myObj = new Object()

I can add properties to both using these methods. myObj.age = 1 and myObj['age'] = 1 . The properties of both can be accesed the same way.

So what is the actual difference between these two ways I created myObj ? Also is one of these the better way to create an object?

Answer 1

The difference is that the first one simply calls Object() as a function, within the scope of the window object.

The second one actually instantiates a new object. It's the one you want to use to create an object.

The difference may not be obvious with the Object() function, but let's say you create your own type, like so:

function User(name) {
    this.name = name;
}
var u1 = User("John");
var u2 = new User("Jane");

console.log(u1); // *undefined* because `User()` doesn't return anything.
console.log(this.name); // John
console.log(window.name); // John
console.log(u2.name); // "Jane"

The Object function itself is a special case--it does create a new Object. But since most functions don't work that way, it's good to get in the habit of using the new keyword when instantiating things. If you're just creating a plain old Object , on the other hand, most people prefer the more concise syntax:

var myObj = {};

Answer 2

The first statement is a function call, meaning that myObj will get whatever is returned in the Object() function. As it happens, the function Object() will provide you with a reference to an Object object, whereas 'normal' constructors will not.

See fe the following:

function O(){
  this.bla= "bla";
  return this;
}

Calling O() here will yield a reference to window , not to an instance of O .