Is there a C# String.Format() equivalent in JavaScript? [duplicate]

Try sprintf() for javascript.

Or

// First, checks if it isn't implemented yet.
if (!String.prototype.format) {
  String.prototype.format = function() {
    var args = arguments;
    return this.replace(/{(\d+)}/g, function(match, number) { 
      return typeof args[number] != 'undefined'
        ? args[number]
        : match
      ;
    });
  };
}

"{0} is dead, but {1} is alive! {0} {2}".format("ASP", "ASP.NET")

Both answers pulled from JavaScript equivalent to printf/string.format

I am using:

String.prototype.format = function() {
    var s = this,
        i = arguments.length;

    while (i--) {
        s = s.replace(new RegExp('\\{' + i + '\\}', 'gm'), arguments[i]);
    }
    return s;
};

usage: "Hello {0}".format("World");

I found it at Equivalent of String.format in JQuery

UPDATED:

In ES6/ES2015 you can use string templating for instance

'use strict';

let firstName = 'John',
    lastName = 'Smith';

console.log(`Full Name is ${firstName} ${lastName}`); 
// or
console.log(`Full Name is ${firstName + ' ' + lastName}');

Based on @Vlad Bezden answer I use this slightly modified code because I prefer named placeholders:

String.prototype.format = function(placeholders) {
    var s = this;
    for(var propertyName in placeholders) {
        var re = new RegExp('{' + propertyName + '}', 'gm');
        s = s.replace(re, placeholders[propertyName]);
    }    
    return s;
};

usage:

"{greeting} {who}!".format({greeting: "Hello", who: "world"})

String.prototype.format = function(placeholders) {
    var s = this;
    for(var propertyName in placeholders) {
        var re = new RegExp('{' + propertyName + '}', 'gm');
        s = s.replace(re, placeholders[propertyName]);
    }    
    return s;
};

$("#result").text("{greeting} {who}!".format({greeting: "Hello", who: "world"}));

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="result"></div>

I created it a long time ago, related question

String.Format = function (b) {
    var a = arguments;
    return b.replace(/(\{\{\d\}\}|\{\d\})/g, function (b) {
        if (b.substring(0, 2) == "{{") return b;
        var c = parseInt(b.match(/\d/)[0]);
        return a[c + 1]
    })
};