Question

How can I subtract 1 (or any number) from the index without decrementing it (without it’s value changing in the variable) ?

Here’s a snippet of my code:

if (i > 0 & areaImages[i].id == areaImages[i-1].id)

And I get this error message from Firebug:

TypeError: areaImages[i - 1] is undefined

Answer 1

Nothing is decremented. The actual reason is you used the wrong operator.

if (i > 0 && areaImages[i].id == areaImages[i-1].id)
//        ^^

For booleans, both & and && will return same (up to ==) result. But the crucial difference is that && is short-circuiting, i.e. when the left-hand-side of && is false, then the right-hand-side will not be evaluated.

The problem of the original code is that & is not short-circuiting, so areaImages[i-1].id will be evaluated even if i <= 0. But i-1 is an invalid index, so areaImages[i-1] is undefined, and you cannot get a property from undefined, causing the TypeError.