Javascript: Iterating over an array causes an infinite loop?

Question

I'm trying to iterate over some data in Javascript, using the following code:

for (var i = 0; i < fromdata.length; i++) {
 var mainid = fromdata[i].id;
 var sub = afcHelper_Submissions[mainid];
 /* do more stuff */

fromdata is an array of Objects that looks something like this:

[{ type="ffu", to=" Jon Corzine ", id=1, more...}, { type="ffu", to=" Jon Corzine ", id=2, more...}]

As you can see, I just want to get each object's id and store it to mainid and then do some more with it; however, I run into trouble: looping! Looping! Looping! The loop keeps running again and again. It never stops, and just manages to freeze up Firebug.

Update: Here's the "do more stuff", in all its pastebin glory: http://pastebin.com/Mfr90uq7. Note that I changed the variable name from sub to sub_m to avoid a potential conflict, but the problem persisted.

• Arun P Johny: try to print the value of i and fromdata.length in the console and see what is happening
• JJJ: Are you sure you're not adding more elements to fromdata (and thus increasing fromdata.length)? Do you change i at any point by mistake?
• luk2302: then pretty much someting in /* do more stuff */ is the problem. its good that you don´t post all the code you have, but you should neither post too little.
• Sudarshan Tanwar: what is afcHelper_Submissions[] here ?
• luk2302: are you sure that is in an infinite loop as stepping inside the loop body all the time OR is the program just not stopping / coming to an end which would indicate that the error is somewhere else. Are you getting output from this line console.log(sub_m.action) endlessly often?

Answer 1

A loop only can be infinite if the condition is always true. In your case it looks that it should reach a false , but provably formData is getting new elements each iteration of the loop or i is being modified and returned to previous values.

What I recomend is to create variables that will be used only for comparasion purposes:

var max = fromdata.length;
for (var count = 0; count < max; count++) {
 var i = count;
 // your stuff using formdata and i

Now max and count will not be modified by the code in the loop and the loop will reach an end.