JavaScript Variable Declaration

Question

This is a really stupid question, but I'm just drawing a blank here...

What type of variable declaration is this:

var s1 = [1,2,3,4]

Also, How can I construct a variable like this from multiple objects when the amount of those objects is unknown. This is what I came up with, which doesn't work.

var s1 = [];
for(x in data[i].uh) {
 s1 += data[i].uh[x];
}

• KJYe.Name: that's an array. developer.mozilla.org/en/JavaScript/Reference/Global_Objects/…

Answer 1

var s1 = [1,2,3,4]

is an array declaration of four integers using "Array Literal Notation"

You don't need a loop to copy the array, simply do this:

var s1 = data.slice(0);

or in your example you might want this:

var s1 = data[i].uh.slice(0);

Read more about copying arrays here: http://my.opera.com/GreyWyvern/blog/show.dml/1725165

"The slice(0) method means, return a slice of the array from element 0 to the end. In other words, the entire array. Voila, a copy of the array."

• The_asMan: Be careful with duplicating an array using slice as it is a reference copy and not a value copy. Meaning if you change one you change the other.
• Nick Brunt: @The_asMan No, it copies the array. Consider this code: var foo = [1,2,3,4,5]; alert(foo[2]); /* Returns 3 */ var bar = foo.slice(0); alert(bar[2]); /* Returns 3 */ bar[2] = 0; alert(bar[2]); /* Returns 0 */ alert(foo[2]); /* Returns 3 */ w3schools.com/jsref/jsref_slice_array.asp

Answer 2

That is called an Array, which can be declared with new Array() or by using the array literal [] as in your example. You can use the Array.push() method (see docs) to add a new value to it:

var s1 = [];
for(x in data[i].uh) {
 s1.push(data[i].uh[x]);
}

• Vitaliy Isikov: Oh wow. Okay, I thought array's had to be declared with new Array().

Answer 3

This

var s1 = [1,2,3,4]

is an array declaration.

To add an element to an array, use the push method:

var s1 = [];
for(x in data[i].uh) {
 s1.push(data[i].uh[x]);
}

Answer 4

s1 is an array, it's a proper Javascript object with functions.

var s1 = [];

is the recommend way to create an array. As opposed to:

var s1 = new Array();

(see: http://www.hunlock.com/blogs/Mastering_Javascript_Arrays)

To add items to an array use s1.push(item) so your code would be:

var s1 = [];
for(x in data[i].uh) {
 s1.push(data[i].uh[x]);
}

As a side note, I wouldn't recommend using for-in, at least not without checking hasOwnProperty.

Answer 5

It's declaring a local variable with an Array with 4 members.

If you want to append to an Array, use the push() method.

Answer 6

That is an array. To add to arrays you would use Array.push(). For example:

var s1 = [];
s1.push(1);
s1.push(2);