JavaScript equivalent to printf/String.Format


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

My basic requirement is a thousand separator format for numbers for now, but something that handles lots of combinations (including dates) would be good.

I realize Microsoft's Ajax library provides a version of String.Format(), but we don't want the entire overhead of that framework.

7/31/2018 8:09:28 PM

Accepted Answer

From ES6 on you could use template strings:

let soMany = 10;
console.log(`This is ${soMany} times easier!`);
// "This is 10 times easier!

See Kim's answer below for details.


Try sprintf() for JavaScript.

If you really want to do a simple format method on your own, don’t do the replacements successively but do them simultaneously.

Because most of the other proposals that are mentioned fail when a replace string of previous replacement does also contain a format sequence like this:

"{0}{1}".format("{1}", "{0}")

Normally you would expect the output to be {1}{0} but the actual output is {1}{1}. So do a simultaneously replacement instead like in fearphage’s suggestion.

1/31/2019 7:07:10 PM

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) ?
            : arguments[0];

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

    return str;

Note the clever 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).



Number Formatting in JavaScript

I got to this question page hoping to find how to format numbers in JavaScript, without introducing yet another library. Here's what I've found:

Rounding floating-point numbers

The equivalent of sprintf("%.2f", num) in JavaScript seems to be num.toFixed(2), which formats num to 2 decimal places, with rounding (but see @ars265's comment about Math.round below).

(12.345).toFixed(2); // returns "12.35" (rounding!)
(12.3).toFixed(2); // returns "12.30" (zero padding)

Exponential form

The equivalent of sprintf("%.2e", num) is num.toExponential(2).

(33333).toExponential(2); // "3.33e+4"

Hexadecimal and other bases

To print numbers in base B, try num.toString(B). JavaScript supports automatic conversion to and from bases 2 through 36 (in addition, some browsers have limited support for base64 encoding).

(3735928559).toString(16); // to base 16: "deadbeef"
parseInt("deadbeef", 16); // from base 16: 3735928559

Reference Pages

Quick tutorial on JS number formatting

Mozilla reference page for toFixed() (with links to toPrecision(), toExponential(), toLocaleString(), ...)


From ES6 on you could use template strings:

let soMany = 10;
console.log(`This is ${soMany} times easier!`);
// "This is 10 times easier!

Be aware that template strings are surrounded by backticks ` instead of (single) quotes.

For further information:

Note: Check the mozilla-site to find a list of supported browsers.


jsxt, Zippo

This option fits better.

String.prototype.format = function() {
    var formatted = this;
    for (var i = 0; i < arguments.length; i++) {
        var regexp = new RegExp('\\{'+i+'\\}', 'gi');
        formatted = formatted.replace(regexp, arguments[i]);
    return formatted;

With this option I can replace strings like these:

'The {0} is dead. Don\'t code {0}. Code {1} that is open source!'.format('ASP', 'PHP');

With your code the second {0} wouldn't be replaced. ;)


I use this simple function:

String.prototype.format = function() {
    var formatted = this;
    for( var arg in arguments ) {
        formatted = formatted.replace("{" + arg + "}", arguments[arg]);
    return formatted;

That's very similar to string.format:

"{0} is dead, but {1} is alive!".format("ASP", "ASP.NET")

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