substr shim

substr.js

// There is no String.substr on Salesforce. This uses String.substring instead.
// substring's arguments are different, so this just converts substr args to
// their substring equivalents.
function substrShim (str, start, len) {
    if (len <= 0)
        return '';

    if (start < 0)
        start = str.length + start;

    if (len != null)
        len = Math.min(str.length - 1, start + len);

    return str.substring(start, len);
}