There are three ways to get a substring from a string in JavaScript:

1. str.substr(startIndex,length)
2. str.substring(startIndex,endIndexPlusOne)
3. str.slice(startIndex,endIndexPlusOne)

substr and substring

All browsers treat str.substring(-2) as str.substring(0).

substring(4,2) inputs are automatically reversed when the second param is less than the first, resulting in substring(2,4). So substring(12,-2) is treated as substring(0,12).

IE treats str.substr(-2) as str.substr(0). All others support neg index in substr.

All browsers support str.slice(-2).

slice is the only method that properly handles negative indexes across all browsers. Here's a demo, run in your current browser.

Note if you are viewing this in IE, str.substr(-12,5) results in "abcde", whereas in other browsers "opqrs".

So in conclusion, there are too many ways to skin the substring cat. substr has cross-browser compatibility issues and substring doesn't support negative indexes. slice is always safe, so just use slice.