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sprintf is actually the best javascript implementation of sprintf (string.format). But wait, shouldn’t it be some more .NET-like stuff in SharePoint environment? Indeed there is! (Well, not only in SP, but ASP.NET)

String.format(“Hello {0}”, “world”) does exactly the same thing as on server side. Wow, it opens for many opportunities, e.g. in jQuery tmpl:

String.format validates arguments, and if all is OK, it invokes another function String._toFormattedString:

function String$_toFormattedString(useLocale, args) {
    var result = '';
    var format = args[0];
    for (var i=0;;) {
        var open = format.indexOf('{', i);
        var close = format.indexOf('}', i);
        if ((open < 0) && (close < 0)) {             
            result += format.slice(i);
            break;
        }
        if ((close > 0) && ((close < open) || (open < 0))) {
            if (format.charAt(close + 1) !== '}') {
                throw Error.argument('format', Sys.Res.stringFormatBraceMismatch);
            }
            result += format.slice(i, close + 1);
            i = close + 2;
            continue;
        }
        result += format.slice(i, open);
        i = open + 1;
        if (format.charAt(i) === '{') {
            result += '{';
            i++;
            continue;
        }
        if (close < 0) throw Error.argument('format', Sys.Res.stringFormatBraceMismatch);
        var brace = format.substring(i, close);
        var colonIndex = brace.indexOf(':');
        var argNumber = parseInt((colonIndex < 0)? brace : brace.substring(0, colonIndex), 10) + 1;
        if (isNaN(argNumber)) throw Error.argument('format', Sys.Res.stringFormatInvalid);
        var argFormat = (colonIndex < 0)? '' : brace.substring(colonIndex + 1);
        var arg = args[argNumber];
        if (typeof(arg) === "undefined" || arg === null) {
            arg = '';
        }
        if (arg.toFormattedString) {
            result += arg.toFormattedString(argFormat);
        }
        else if (useLocale && arg.localeFormat) {
            result += arg.localeFormat(argFormat);
        }
        else if (arg.format) {
            result += arg.format(argFormat);
        }
        else
            result += arg.toString();
        i = close + 1;
    }
    return result;
}

By the way, take a look at summary part of String.format:

/// <summary locid="M:J#String.format" />
/// <param name="format" type="String"></param>
/// <param name="args" parameterArray="true" mayBeNull="true"></param>
/// <returns type="String"></returns>

Great summary, how could we get intellisense on that in Visual Studio? Anyone know?

Another aspect of String.format I have discovered is that unfortunately you cannot format digitals like {0:00} like in C# (e.g. 8 gets 08). Then I recommend you zlib licensed javascript library called StringFormat from Daniel Mester Pirttijärvi.