内置Javascript中的构造函数

Question

When I do:

var person = new Object();  
person.name = "alex";
console.log(person)

output is:

Object { name="alex"}

However, say I drop the "new" word and do:

var person = Object();  
person.name = "alex";
console.log(person)

Output is also:

Object { name="alex"}

Why?

Answer 1

Because some built-in functions are just defined to act this way. For example see ES5 15.2.1.1 for Object :

15.2.1.1 Object ( [ value ] )

When the Object function is called with no arguments or with one argument value , the following steps are taken:

1. If value is null , undefined or not supplied, create and return a new Object object exactly as if the standard built-in Object constructor had been called with the same arguments ( 15.2.2.1 ).
2. Return ToObject(value) .

They test whether they have been called with new or not and if not act like they'd have been called with new .

Not all constructors work like this. For example Date will return a string when called without new .

---

You can implement this yourself:

function Foo() {
    if(!(this instanceof Foo)) {
        return new Foo();
    }
    // do other init stuff
}

Foo() and new Foo() will act the same way (it gets more tricky with variable arguments though).

Answer 2

Since your example is an Object type of built-in function, as it is answered above it is the same for this type, it does not work the same way for most of the other built-in functions such as Number(). You should be very careful when invoking them with the 'new' keyword or not. Because by default the 'new' keyword with a function constructor returns an object, not a primitive type directly. So you can not, for example, check strict equality on two variables that one of them is declared and assigned using new Number() , and the other is with Number()

An example would be:

var num1 = Number(26); 
var num2 = new Number(26);

num1 == num2; // returns true

num1 === num2; // returns false

You may checkout the difference at the console log:

console.log(num1);
> 26

console.log(num2);
> Number {26}
>     __proto__: Number
>     [[PrimitiveValue]]: 26