JavaScript Array

Question

According to a tutorial I am reading, if you want to create a table with 5 columns and 3 rows to represent data like this...

45 4 34 99 56
3 23 99 43 2
1 1 0 43 67

...it says that you can use the following array

var table = new Array(5)
table[0] = [45, 4, 34, 99, 56];
table[1] = [3, 23, 99, 43, 2];
table[2] = [1, 1, 0, 43, 67];

But I would have expected it to be like this

var table = new Array(3)
table[0] = [45, 4, 34, 99, 56];
table[1] = [3, 23, 99, 43, 2];
table[2] = [1, 1, 0 43, 67]

If, as in the tutorial, var table is initially declared as an array with 5 elements, and then the first (table[0]), second (table[1]) and third (table[2]) are filled with the data, what happens with the other two elements that were initially set in the array with new Array(5). Why do you need to use 5?

• JaredPar: Is this an online tutorial? If so can you post a link?
• Pointy: Get a better tutorial - you really never need to do new Array(3) in Javascript. Always use var table = []; - pre-allocating the size doesn't really do any good.
• mjmitche: @JaredPar it`s from a book called something like JavaScript cookbook
• Bob: Just a fun fact about JavaScript: it doesn't actually store X amount of variables and allocate all that space. The only thing it tracks is the length and any data actually stored in the array is tracked by the index. So table[120] would simply be undefined, unless you assign it a value, in which case the length of the array will increase to 121, and the "120" member of table will be assigned the value. All "new Array(3)" does is set the length to 3, nothing more.
• Zoidberg: @Pointy JSON notation is the way to go, this tutorial sounds like it is sadly out of date.

Answer 1

You are correct, it sounds like the example in the tutorial is wrong.

• Zoidberg: Yep sounds like a typo... plus I would use JSON notation to define such a table, so much easier to read.

Answer 2

That's most probably a typo on the tutorial.

new Array(5) basically creates an array object with 5 elements, initialized as undefined. JavaScript is smart enough to "resize" the array object when you had originally set it smaller than you needed. So, after you set var table = new Array(3), you can still add a tenth element to it e.g. table[9] = [1, 2, 3, 4, 5];

• Wayne: Just be aware that table.length is now 10 even though you've added only 6 elements.