Why do people declare something as null in JavaScript? [duplicate]

Question

Possible Duplicate:
Why is there a null value in JavaScript?

I don't understand what null is for. 'Undefined' as well.

Answer 1

It helps to appreciate the difference between a value and a variable.

A variable is a storage location that contains a value.

null means that the storage location does not contain anything (but null itself is just a special kind of value). Read that last sentence carefully. There is a difference between what null is and what null means. 99% of the time you only care about what null means.

Undefined means that the variable (as opposed to the value) does not exist.

Answer 2

null is an object you can use when creating variables when you don't have a value yet:

var myVal = null;

When you do this, you can also use it to check to see if it's defined:

// null is a falsy value
if(!myVal) {
    myVal = 'some value';
}

I wouldn't use it this way, normally. It's simple enough to use 'undefined', instead:

var myVal; // this is undefined

And it still works as a falsy value.

Creating Objects

When you create an object in javascript, I like to declare all my object properties at the top of my object function:

function myObject() {
    // declare public object properties
    this.myProp = null;
    this.myProp2 = null;

    this.init = function() {
        // ... instantiate all object properties
    };
    this.init();
}

I do this because it's easier to see what the object properties are right off the bat.