Lesson Learned: JavaScript For-In loops

For-In loops in JavaScript are really nice... until they aren't.

Not knowing exactly how they work might bite you.  I'll start with an example:

var arr1 = ["a", "b", "c", "d"];
for(var i in arr1)
    console.log(i, arr1[i], i+1, arr1[i+1]);

Seems simple enough.  What will be printed?

0 a 01 undefined
1 b 11 undefined
2 c 21 undefined
3 d 31 undefined 

Whoops... I am an idiot!  That's not exactly what I wanted.  Note that i is a string, not a number.  I wasted an hour of my life trying to figure out why i+1 was not working properly... only to come to this horribly obvious conclusion.  It was totally worth a blog post... and a beer.

Sometimes the old-fashioned for loops are better for Arrays:

var arr1 = ["a", "b", "c", "d"];
for(var i = 0; i < arr1.length; i++)
    console.log(i, arr1[i], i+1, arr1[i+1]);