Number formatter

gistfile1.js

function formatNumber(num) {
    var thousands = ',';
    var decimal = '.';
    
    var numStr = '' + num;
    var formatted = '';
    
    if (typeof num !== 'number') {
        throw new Error('Must pass a number, received "' + num + '" instead.');
    }
    
    // is there a decimal point?
    if ((0 | num) - num !== 0) {
        formatted = decimal + numStr.substring(numStr.indexOf('.')+1);
        numStr = numStr.substring(0, numStr.indexOf('.'));
    }
    
    // every 3 digits, add the seprator
    while (numStr.length > 3) {
        formatted = thousands + numStr.substr(-3) + formatted;
        numStr = numStr.substring(0, numStr.length - 3);
    }
    // add anything left
    formatted = numStr + formatted;
    
    return formatted;
}