javascript array index test

Question

if I have an array myArray which I am looping through:

for(var i=0;i<myArray.length;i++) {
   if(myArray[i] == myArray[i+1])
      //do something
}

Clearly, myArray[i+1] will fail on the last iteration. Short of testing if i == (myArray.length - 1), is there a clean way to not fail on that if statement and just have it evaluate to false (if myArray[i+1] is out of bounds)?

Oh, and I know I'm probably better off keeping a variable pointing to the previous value and in each iteration just testing if the current value is equal to that stored previous value. But I'd still like to know how to do it as shown in the question. — bba
– bba, Commented Sep 21, 2010 at 14:35
"Clearly, myArray[i+1] will fail on the last iteration." Well, yes and no, depends what you mean by "fail". It won't throw an exception, but it's almost certainly not what you want. It will compare myArray[i] with undefined. Regardless, best to stop earlier, as shown in KennyTM's answer. — T.J. Crowder
– T.J. Crowder, Commented Sep 21, 2010 at 14:36

kennytm · Accepted Answer · 2010-09-21 14:41:16Z

4

Why not just iterate up to the (N-2)th item?

for(var i=0;i<myArray.length-1;i++) {
  ...

if you must iterate till the end, the only way to work in general is to explicitly check if the index is valid. Either

for (var i = 0; i < myArray.length; ++ i)
  if (i+1 != myArray.length) {
    ...

or

for (var i = 0; i < myArray.length; ++ i)
  if (!(i+1 in myArray)) {
    ...

However, if you can ensure all items in the array cannot be undefined or null, then your original code already works because an index out-of-bound will return undefined, and undefined == x will be false unless x is also undefined or null.

edited Sep 21, 2010 at 14:41

answered Sep 21, 2010 at 14:35

kennytm

526k110 gold badges1.1k silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

bba Over a year ago

well I feel dumb now. yea that would definitely work. still is there a graceful way in javascript to have an out of bounds array pointer evaluate to false instead of throwing an error?

bba Over a year ago

...because actually the way my logic is setup (and I'm not saying it can't be refactored) I still need to get to that last iteration (N-1)th item.

RoToRa Over a year ago

Actually myArray[i+1] wouldn't fail with an error. It would simply return undefined, which compared to myArray[i] which would return false (unless of course myArray[i] is also undefined) and everything would be ok.

bba Over a year ago

interesting. so, since contractually the array should never have underfined or null, it wouldn't be poor code to leave it as is then, i guess..even though coming from a Java background it looks dangerous

Ryan Kinal Over a year ago

As T.J. Crowder said above, myArray[myArray.length] will evaluate to undefined, which is a falsy value. It shouldn't throw an error unless you're then trying to access a property of that array element.

hellvinz · Accepted Answer · 2010-09-21 14:38:37Z

0

Look at the jquery source for some inspiration: http://github.com/jquery/jquery/blob/3a0a35288304ab5289a1/src/core.js#L632

answered Sep 21, 2010 at 14:38

hellvinz

3,50022 silver badges16 bronze badges

Collectives™ on Stack Overflow

javascript array index test

2 Answers 2

5 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related