String.format

StringFormat.js

String.format = function(pattern/*, args ... */) {

    var result = "";

    function parse() {
        // %[argument_index$][flags][width][.precision][conversion]
        // $0 - the entire matched part
        // $1 - argument_index '$'
        // $2 - flags
        // $3 - width
        // $4 - '.' precision
        // $5 - 't' 'T' prefix for conversion
        // $6 - conversion char
        var specifier = /%(\d+\$)?([-#+ 0,(<]*)?(\d+)?(\.\d+)?([tT])?([a-zA-Z%])/;
        var s = pattern || "";
        var fsa = [];
        var sa;
        while (sa = specifier.exec(s)) {
            if (sa.index > 0) {
                fsa.push(new FormatString(s.substring(0, sa.index)));
            }
            fsa.push(new FormatString(sa));
            s = s.substring(sa.index + sa[0].length);
        }
        if (s) {
            fsa.push(new FormatString(s));
        }
        return fsa;
    }

    function FormatString(sa) {
        this.index = 0; // ordinal index
        var fixedText;
        if (typeof sa == "string") { // fixed string
            this.index = -2; // fixed string
            fixedText = sa; // should check Text
        } else {
            if (sa[1]) { // %1$
                this.index = parseInt(sa[1].substring(0, sa[1].length - 1));
            } else if ((sa[2] || "").indexOf('<') != -1) {
                this.index = -1; // relative index
            } else if (sa[6] == '%') {
                this.index = -2;
                fixedText = "%"
            }
        }

        this.print = function(arg) {
            if (fixedText) {
                result += fixedText;
            } else if (arg != null) {
                // todo: specifier format argument
                result += arg;
            }
        }
    }

    var args = arguments;
    var ordinalIndex = 0;
    var lastIndex = -1;
    parse().forEach(function (fs){
        switch (fs.index) {
        case -2:  // fixed string, "%n", or "%%"
            fs.print(null);
            break;
        case 0:  // ordinary index
            lastIndex = ++ordinalIndex;
            fs.print(args[lastIndex]);
            break;
        case -1:  // relative index
            fs.print(args[lastIndex]);
            break;
        default:  // explicit index
            lastIndex = fs.index;
            fs.print(args[lastIndex]);
            break;
        }
    });

    return result;
};

(function() {
    function assertEquals(a,b) {if (a != b) console.log(a + " != " + b)}
    assertEquals(String.format("%s,%s,%s",         1,2,3) , "1,2,3");
    assertEquals(String.format("%d,%d,%d",         1,2,3) , "1,2,3");
    assertEquals(String.format("%3$s,%2$s,%1$s",   1,2,3) , "3,2,1");
    assertEquals(String.format("%3$d,%2$d,%1$d",   1,2,3) , "3,2,1");
    assertEquals(String.format("%s,%s,%%",         1,2,3) , "1,2,%");
    assertEquals(String.format("%d xxx %d z%% %s", 1,2)   , "1 xxx 2 z% ");
    console.log("test finished");
})();