formatNumber.js

// console.log('US', formatNumber(" #,###.##", 1234.56)) //US
// console.log('US', formatNumber(" #,###.##", 12345.67)) //US
// console.log('US', formatNumber(" #,###.##", 123412345.67)) //US
// console.log('DE', formatNumber(" #.###,##", 1234.56)) //DE
// console.log('DE', formatNumber("#.###,##", 12345.67)) //DE
// console.log('DE', formatNumber("#.###,##", 1234512345.67)) //DE
// console.log('FR', formatNumber('# ###,##', 1234.56)) //FR
// console.log('FR', formatNumber('# ###,##', 12345.67)) //FR
// console.log('FR', formatNumber('# ###,##', 1234512345.67)) //FR
// console.log('CH', formatNumber("#'###.##", 1234.56)) //CH
// console.log('CH', formatNumber("#'###.##", 12345.67)) //CH
// console.log('CH', formatNumber("#'###.##", 1234512345.67)) //CH

// console.log('IN', formatNumber("#,##,###.##", 12345)) //foo
// console.log('IN', formatNumber("#,##,###.##", 345.67))//foo
// console.log('IN', formatNumber("#,##,###.##", 12345.67)) //foo
// console.log('IN', formatNumber("#,##,###.##", 12345.67)) //foo
// console.log('IN', formatNumber("#,##,###.##", 1234512345.67)) //foo


function formatNumber(pattern, number) {
    pattern = pattern.trim();
    let formattedNumber;
    const separators = pattern.match(/[^\#]/g);
    const decimal = separators[separators.length-1];
    const groupSeparator = separators[0];
    const groupLengths = pattern.split(decimal)[0].split(groupSeparator).slice(1).map(a => a.length);
    const groupLength = pattern.split(decimal)[0].split(groupSeparator).pop().length;
    if(groupLengths.length === 1) {
        const integerRegex = new RegExp(/^[+-]?\d+/)
        const groupingRegexPattern = `(\\d)(?=(\\d{${groupLength}})+$)`;
        const groupingRegex = new RegExp(groupingRegexPattern, 'g');
        formattedNumber =  number.toString().replace('.', decimal).replace(integerRegex, function(int) {
            return int.replace(groupingRegex, `$1${groupSeparator}`);
        });
    } else { //this is for multiple grouping lengths, like India #,##,###.##
        
        const lastGroupLength = groupLengths[groupLengths.length - 1];
        const firstGroupLength = groupLengths[0];
        const decimals =  number.toString().split('.')[1] || '';
        const formattedDecimal = decimals ? `${decimal}${decimals}` : '';
        let integers = number.toString().split('.')[0];
        let lastGroup = integers.substring(integers.length-lastGroupLength);
        const otherNumbers = integers.substring(0,integers.length-lastGroupLength);
        const regex = new RegExp(`\\B(?=(\\d{${firstGroupLength}})+(?!\\d))`, 'g');

        lastGroup = otherNumbers != '' ? `,${lastGroup}` : lastGroup;
        const formattedIntegers = otherNumbers.replace(regex, ',') + lastGroup;
        formattedNumber = `${formattedIntegers}${formattedDecimal}`;
        
    }
    
    return formattedNumber;
}