Why is null an object and what's the difference between null and undefined?

Question

Why is null considered an object in JavaScript?

Is checking

if ( object == null )
      Do something

the

Answer 1

One way to make sense of null and undefined is to understand where each occurs.

Expect a null return value in the following situations:

• Methods that query the DOM

  console.log(window.document.getElementById("nonExistentElement"));
  //Prints: null
  

• JSON responses received from an Ajax request

    {
      name: "Bob",
      address: null
    }

• RegEx.exec.

• New functionality that is in a state of flux. The following returns null:

        var proto = Object.getPrototypeOf(Object.getPrototypeOf({}));

       // But this returns undefined:

        Object.getOwnPropertyDescriptor({}, "a");

All other cases of non-existence are denoted by undefined (as noted by @Axel). Each of the following prints "undefined":

    var uninitalised;
    console.log(uninitalised);

    var obj = {};
    console.log(obj.nonExistent);

    function missingParam(missing){
        console.log(missing);
    }

    missingParam();

    var arr = [];
    console.log(arr.pop());        

Of course if you decide to write var unitialised = null; or return null from a method yourself then you have null occurring in other situations. But that should be pretty obvious.

A third case is when you want to access a variable but you don't even know if it has been declared. For that case use typeof to avoid a reference error:

if(typeof unknown !== "undefined"){
    //use unknown
}

In summary check for null when you are manipulating the DOM, dealing with Ajax, or using certain ECMAScript 5 features. For all other cases it is safe to check for undefined with strict equality:

if(value === undefined){
  // stuff
}