jQuery each array problem

Question

I'm looping through an array using the jquery each function. I assign a temp variable for it to loop through instead of the actual array itself as I am modifying the original array using splice. However, it looks like temp is getting modified even when I splice array.

function example (Data, index, array) {
            var temp = array;
            $.each(temp, function(i, v) {    
                if(Data["b"+v].length > index) {
                    //do stuff
                } else {
                    array.splice(i,1);
                }
            });
            if(array.length > 0) {
                example(Data, index+1, array);
            }
}
array = [1,2,3,4]
Data = {"b1":[a,b,c,d],"b2":[e,f,g,h], "b3":[i,j], "b4":[k,l,m,n]};
example(Data, 0, array);

On the third call of example, on the 4th iteration of temp, v becomes undefined and therefore the next line pumps out an error of "cannot read length of undefined". This happens right after array.splice(3,1) is called which seems like temp is pointing to the same place as array instead of being a copy of it.
Can anyone help?

Felix Kling · Accepted Answer · 2011-09-18 11:32:07Z

3

Arrays and objects are assigned by reference. temp and array reference the same array. You can create a shallow copy using .slice() ^[MDN]:

var temp = array.slice();

Instead of creating a copy, you could iterate over the array in reverse order:

for(var i = array.length; i--; ) {
    if(Data["b"+array[i]].length > index) {
        //do stuff
    } else {
        array.splice(i,1);
    }
}

answered Sep 18, 2011 at 11:32

Felix Kling

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1 Comment

Michael Over a year ago

Going through array in reverse order seems much cleaner, thank you.

broofa · Accepted Answer · 2011-09-18 11:31:41Z

2

temp is just a reference to the same array, so temp and array are the same thing. You want to make a copy, like so:

temp = array.slice(0);

answered Sep 18, 2011 at 11:31

broofa

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JaffaTheCake · Accepted Answer · 2011-09-18 11:35:01Z

2

Assignments in JavaScript are by reference, it doesn't copy the object. Eg...

var obj1 = {};
var obj2 = obj1;
obj2.hello = "world";
console.log( obj1.hello ); // logs "world"

This is because obj1 and obj2 are pointing to the same object in memory.

If you want to make a copy of an array, the slice method can be used...

var arrayCopy = myArray.slice(0)

Now arrayCopy & myArray can be edited independently. However, be aware that although the arrays themselves are independent, they point to the same objects...

arrayCopy[0] === myArray[0]; // true
arrayCopy[0] = {my: "new object"};
arrayCopy[0] === myArray[0]; // now false

answered Sep 18, 2011 at 11:35

JaffaTheCake

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