DecimalFormat Currency Formatting

decimalFormat.js

Number.prototype.currency = function (format) {
    var amt = this, neg;
    
    // If no formatting string supplied
    // or amount is not a number, return as is
    if (!format || isNaN(amt)) return amt;
    
    // Extract placeholders from format string
    var formFig = format.match(/\#(.*)\#/g).pop();
    
    // Is number negative?
    if (amt < 0) {
        neg = true;
        amt = (amt * -1);
    }
    
    // Remove any decimals
    amt = amt.toString().replace(/[ ,.]/, '');
    
    // Split and flip the numbers and format
    var formArr = formFig.split('').reverse(),
        amtArr = amt.split('').reverse();
        
    // Add leading zeros to small amounts, if there's a separator in last 3 digits
    if (amtArr.length < 3 && format.slice(-3).match(/[ ,.]/)) {
        while (3 - amtArr.length) {
            amtArr.push('0');
        }
    }
    
    // Loop through the formatting, and look for separators
    // Only get separators that fit within the length of the amount
    for (var i = 0; i < amtArr.length; ++i) {
        // If we find a matching separator, splice it into the amount in the same place
        if (/[ ,.]/.test(formArr[i])) {
            amtArr.splice(i, 0, formArr[i]);
        }
    }
    
    // Flip the amount back in the right direction, and rejoin
    var amount = amtArr.reverse().join('');
  
    // Handle Negatives (minus or parentheses)
    if (!neg) format = format.replace(/[\-\(\)]/gi, '');
      
    // Merge the amount back with the currency symbols
    var money = format.replace(/\#(.*)\#/g, amount);
    
    return money;
};

To use:

var price = 123456;

price.currency('-£###,###,###.##'); // Returns: £1,234.56
price.currency('-###,###,###円'); // Returns: 123,456円