support Format a JavaScript string using placeholders and an object of substitutions?

javascript template example (8)

The requirements of the original question clearly couldn't benefit from string interpolation, as it seems like it's a runtime processing of arbitrary replacement keys.

However, if you just had to do string interpolation, you can use:

const str = `My name is ${} and my age is ${replacements.age}.`

Note the backticks delimiting the string, they are required.

For an answer suiting the particular OP's requirement, you could use String.prototype.replace() for the replacements.

The following code will handle all matches and not touch ones without a replacement (so long as your replacement values are all strings, if not, see below).

var replacements = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"},
    str = 'My Name is %NAME% and my age is %AGE%.';

str = str.replace(/%\w+%/g, function(all) {
   return replacements[all] || all;


If some of your replacements are not strings, be sure they exists in the object first. If you have a format like the example, i.e. wrapped in percentage signs, you can use the in operator to achieve this.


However, if your format doesn't have a special format, i.e. any string, and your replacements object doesn't have a null prototype, use Object.prototype.hasOwnProperty(), unless you can guarantee that none of your potential replaced substrings will clash with property names on the prototype.


Otherwise, if your replacement string was 'hasOwnProperty', you would get a resultant messed up string.


As a side note, you should be called replacements an Object, not an Array.

I have a string with say: My Name is %NAME% and my age is %AGE%.

%XXX% are placeholders. We need to substitute values there from an object.

Object looks like: {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"}

I need to parse the object and replace the string with corresponding values. So that final output will be:

My Name is Mike and my age is 26.

The whole thing has to be done either using pure javascript or jquery.

Answer #2

You can use a custom replace function like this:

var str = "My Name is %NAME% and my age is %AGE%.";
var replaceData = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"};

function substitute(str, data) {
    var output = str.replace(/%[^%]+%/g, function(match) {
        if (match in data) {
        } else {

var output = substitute(str, replaceData);

You can see it work here:

Answer #3

How about using ES6 template literals?

var a = "cat";
var b = "fat";
console.log(`my ${a} is ${b}`); //notice back-ticked string

More about template literals...

Answer #4

If you want to do something closer to console.log like replacing %s placeholders like in

>console.log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright")
>Hello Loreto how are you today is everything allright?

I wrote this

function log() {
  var args =;
  var rep= args.slice(1, args.length);
  var i=0;
  var output = args[0].replace(/%s/g, function(match,idx) {
    var subst=rep.slice(i, ++i);
    return( subst );
res=log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright");
<span id="console"/>

you will get

>log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright")
>"Hello Loreto how are you today is everything allright?"


I have added a simple variant as String.prototype useful when dealing with string transformations, here is it:

String.prototype.log = function() {
    var args =;
    var rep= args.slice(0, args.length);
    var i=0;
    var output = this.replace(/%s|%d|%f|%@/g, function(match,idx) {
      var subst=rep.slice(i, ++i);
      return( subst );
    return output;

In that case you will do

"Hello %s how are you %s is everything %s?".log("Loreto", "today", "allright")
"Hello Loreto how are you today is everything allright?"

Try this version here

Answer #5

You can use JQuery(jquery.validate.js) to make it work easily.

$.validator.format("My name is {0}, I'm {1} years old",["Bob","23"]);

Or if you want to use just that feature you can define that function and just use it like

function format(source, params) {
    $.each(params,function (i, n) {
        source = source.replace(new RegExp("\\{" + i + "\\}", "g"), n);
    return source;
alert(format("{0} is a {1}", ["Michael", "Guy"]));

credit to jquery.validate.js team

Answer #6

Just use replace()

var values = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"};
var substitutedString = "My Name is %NAME% and my age is %AGE%.".replace("%NAME%", $values["%NAME%"]).replace("%AGE%", $values["%AGE%"]);

Answer #7

As with modern browser, placeholder is supported by new version of Chrome / Firefox, similar as the C style function printf().


  • %s String.
  • %d,%i Integer number.
  • %f Floating point number.
  • %o Object hyperlink.


console.log("generation 0:\t%f, %f, %f", a1a1, a1a2, a2a2);

BTW, to see the output:

  • In Chrome, use shortcut Ctrl + Shift + J or F12 to open developer tool.
  • In Firefox, use shortcut Ctrl + Shift + K to open developer tool. (Don't use F12 which will open Firebug which is not maintained anymore, and its console tab won't show message).

@Update - nodejs support

Seems nodejs don't support %f, instead, could use %d in nodejs. With %d number will be printed as floating number, not just integer.