JavaScript equivalent to printf/String.Format

Question

I\'m looking for a good JavaScript equivalent of the C/PHP printf() or for C#/Java programmers, String.Format() (IFormatProvider for .

Answer 1

It's funny because Stack Overflow actually has their own formatting function for the String prototype called formatUnicorn. Try it! Go into the console and type something like:

"Hello, {name}, are you feeling {adjective}?".formatUnicorn({name:"Gabriel", adjective: "OK"});

You get this output:

Hello, Gabriel, are you feeling OK?

You can use objects, arrays, and strings as arguments! I got its code and reworked it to produce a new version of String.prototype.format:

String.prototype.formatUnicorn = String.prototype.formatUnicorn ||
function () {
    "use strict";
    var str = this.toString();
    if (arguments.length) {
        var t = typeof arguments[0];
        var key;
        var args = ("string" === t || "number" === t) ?
            Array.prototype.slice.call(arguments)
            : arguments[0];

        for (key in args) {
            str = str.replace(new RegExp("\\{" + key + "\\}", "gi"), args[key]);
        }
    }

    return str;
};

Note the clever Array.prototype.slice.call(arguments) call -- that means if you throw in arguments that are strings or numbers, not a single JSON-style object, you get C#'s String.Format behavior almost exactly.

"a{0}bcd{1}ef".formatUnicorn("foo", "bar"); // yields "aFOObcdBARef"

That's because Array's slice will force whatever's in arguments into an Array, whether it was originally or not, and the key will be the index (0, 1, 2...) of each array element coerced into a string (eg, "0", so "\\{0\\}" for your first regexp pattern).

Neat.