Difference between Array.length = 0 and Array =[]?

Can some one explain the conceptual difference between both of them. Read somewhere that the second one creates a new array by destroying all references to the existing array and the .length=0 just empties the array. But it didn't work in my case

//Declaration 
var arr = new Array();

The below one is the looping code that executes again and again.

$("#dummy").load("something.php",function(){
   arr.length =0;// expected to empty the array
   $("div").each(function(){
       arr = arr + $(this).html();
   });
});

But if I replace the code with arr =[] in place of arr.length=0 it works fine. Can anyone explain what's happening here.

foo = [] creates a new array and assigns a reference to it to a variable. Any other references are unaffected and still point to the original array.

foo.length = 0 modifies the array itself. If you access it via a different variable, then you still get the modified array.

> Read somewhere that the second one creates a new array by destroying all references to the existing array

That is backwards. It creates a new array and doesn't destroy other references.

var foo = [1,2,3];
var bar = [1,2,3];
var foo2 = foo;
var bar2 = bar;
foo = [];
bar.length = 0;
console.log(foo, bar, foo2, bar2);

gives:

[] [] [1, 2, 3] []

---

> arr.length =0;// expected to empty the array

and it does empty the array, at least the first time. After the first time you do this:

> arr = arr + $(this).html();

… which overwrites the array with a string.

The length property of a string is read-only, so assigning 0 to it has no effect.

The difference here is best demonstrated in the following example:

var arrayA = [1,2,3,4,5];

function clearUsingLength (ar) {
    ar.length = 0;
}

function clearByOverwriting(ar) {
    ar = [];
}

alert("Original Length: " + arrayA.length);
clearByOverwriting(arrayA);
alert("After Overwriting: " + arrayA.length);
clearUsingLength(arrayA);
alert("After Using Length: " + arrayA.length);

Of which a live demo can be seen here: http://www.jsfiddle.net/8Yn7e/

When you set a variable that points to an existing array to point to a new array, all you are doing is breaking the link the variable has to that original array.

When you use array.length = 0 (and other methods like array.splice(0, array.length) for instance), you are actually emptying the original array.

Are you sure it really works?

I did a little experiment here, and trying to "add" an Array with a String resulted in a string.

function xyz(){
    var a = [];
    alert(typeof(a+$("#first").html()));
    // shows "string"
}

http://www.jsfiddle.net/4nKCF/

(tested in Opera 11)