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What is the difference between var num=30 and var num=new Number(30) in JavaScript?

The first creates a primitive. The other an object.

In theory there is a difference but in practice none. The JavaScript engine automagicly boxes a primitive to an object when it needs to be an object.

var myPrimitiveNumber = 42;
// calling .toFixed will 'box' the primitive into
a number object,
// run the method and then 'unbox' it back to a
console.log( myPrimitiveNumber.toFixed(2) );

The only usage I've found is if you want to return a primitive from a constructor function.

function Foo() {
    return 42;

var foo = new Foo();
console.log(foo); // foo is instance of Foo

function Bar() {
    return new Number(42);

var bar = new Bar();
console.log(bar); // bar is instance of Number

Categories : Javascript

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