sprintf.js

function sprintf () {
    // http://kevin.vanzonneveld.net
    // +   original by: Ash Searle (http://hexmen.com/blog/)
    // + namespaced by: Michael White (http://getsprink.com)
    // +    tweaked by: Jack
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +      input by: Paulo Freitas
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +      input by: Brett Zamir (http://brett-zamir.me)
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // *     example 1: sprintf("%01.2f", 123.1);
    // *     returns 1: 123.10
    // *     example 2: sprintf("[%10s]", 'monkey');
    // *     returns 2: '[    monkey]'
    // *     example 3: sprintf("[%'#10s]", 'monkey');
    // *     returns 3: '[####monkey]'
    var regex = /%%|%(\d+\$)?([-+\'#0 ]*)(\*\d+\$|\*|\d+)?(\.(\*\d+\$|\*|\d+))?([scboxXuidfegEG])/g;
    var a = arguments,
        i = 0,
        format = a[i++];

    // pad()
    var pad = function (str, len, chr, leftJustify) {
        if (!chr) {
            chr = ' ';
        }
        var padding = (str.length >= len) ? '' : Array(1 + len - str.length >>> 0).join(chr);
        return leftJustify ? str + padding : padding + str;
    };

    // justify()
    var justify = function (value, prefix, leftJustify, minWidth, zeroPad, customPadChar) {
        var diff = minWidth - value.length;
        if (diff > 0) {
            if (leftJustify || !zeroPad) {
                value = pad(value, minWidth, customPadChar, leftJustify);
            } else {
                value = value.slice(0, prefix.length) + pad('', diff, '0', true) + value.slice(prefix.length);
            }
        }
        return value;
    };

    // formatBaseX()
    var formatBaseX = function (value, base, prefix, leftJustify, minWidth, precision, zeroPad) {
        // Note: casts negative numbers to positive ones
        var number = value >>> 0;
        prefix = prefix && number && {
            '2': '0b',
            '8': '0',
            '16': '0x'
        }[base] || '';
        value = prefix + pad(number.toString(base), precision || 0, '0', false);
        return justify(value, prefix, leftJustify, minWidth, zeroPad);
    };

    // formatString()
    var formatString = function (value, leftJustify, minWidth, precision, zeroPad, customPadChar) {
        if (precision != null) {
            value = value.slice(0, precision);
        }
        return justify(value, '', leftJustify, minWidth, zeroPad, customPadChar);
    };

    // doFormat()
    var doFormat = function (substring, valueIndex, flags, minWidth, _, precision, type) {
        var number;
        var prefix;
        var method;
        var textTransform;
        var value;

        if (substring == '%%') {
            return '%';
        }

        // parse flags
        var leftJustify = false,
            positivePrefix = '',
            zeroPad = false,
            prefixBaseX = false,
            customPadChar = ' ';
        var flagsl = flags.length;
        for (var j = 0; flags && j < flagsl; j++) {
            switch (flags.charAt(j)) {
            case ' ':
                positivePrefix = ' ';
                break;
            case '+':
                positivePrefix = '+';
                break;
            case '-':
                leftJustify = true;
                break;
            case "'":
                customPadChar = flags.charAt(j + 1);
                break;
            case '0':
                zeroPad = true;
                break;
            case '#':
                prefixBaseX = true;
                break;
            }
        }

        // parameters may be null, undefined, empty-string or real valued
        // we want to ignore null, undefined and empty-string values
        if (!minWidth) {
            minWidth = 0;
        } else if (minWidth == '*') {
            minWidth = +a[i++];
        } else if (minWidth.charAt(0) == '*') {
            minWidth = +a[minWidth.slice(1, -1)];
        } else {
            minWidth = +minWidth;
        }

        // Note: undocumented perl feature:
        if (minWidth < 0) {
            minWidth = -minWidth;
            leftJustify = true;
        }

        if (!isFinite(minWidth)) {
            throw new Error('sprintf: (minimum-)width must be finite');
        }

        if (!precision) {
            precision = 'fFeE'.indexOf(type) > -1 ? 6 : (type == 'd') ? 0 : undefined;
        } else if (precision == '*') {
            precision = +a[i++];
        } else if (precision.charAt(0) == '*') {
            precision = +a[precision.slice(1, -1)];
        } else {
            precision = +precision;
        }

        // grab value using valueIndex if required?
        value = valueIndex ? a[valueIndex.slice(0, -1)] : a[i++];

        switch (type) {
        case 's':
            return formatString(String(value), leftJustify, minWidth, precision, zeroPad, customPadChar);
        case 'c':
            return formatString(String.fromCharCode(+value), leftJustify, minWidth, precision, zeroPad);
        case 'b':
            return formatBaseX(value, 2, prefixBaseX, leftJustify, minWidth, precision, zeroPad);
        case 'o':
            return formatBaseX(value, 8, prefixBaseX, leftJustify, minWidth, precision, zeroPad);
        case 'x':
            return formatBaseX(value, 16, prefixBaseX, leftJustify, minWidth, precision, zeroPad);
        case 'X':
            return formatBaseX(value, 16, prefixBaseX, leftJustify, minWidth, precision, zeroPad).toUpperCase();
        case 'u':
            return formatBaseX(value, 10, prefixBaseX, leftJustify, minWidth, precision, zeroPad);
        case 'i':
        case 'd':
            number = (+value) | 0;
            prefix = number < 0 ? '-' : positivePrefix;
            value = prefix + pad(String(Math.abs(number)), precision, '0', false);
            return justify(value, prefix, leftJustify, minWidth, zeroPad);
        case 'e':
        case 'E':
        case 'f':
        case 'F':
        case 'g':
        case 'G':
            number = +value;
            prefix = number < 0 ? '-' : positivePrefix;
            method = ['toExponential', 'toFixed', 'toPrecision']['efg'.indexOf(type.toLowerCase())];
            textTransform = ['toString', 'toUpperCase']['eEfFgG'.indexOf(type) % 2];
            value = prefix + Math.abs(number)[method](precision);
            return justify(value, prefix, leftJustify, minWidth, zeroPad)[textTransform]();
        default:
            return substring;
        }
    };

    return format.replace(regex, doFormat);
}