How do I put variables inside javascript strings?

With Node.js v4 , you can use ES6's Template strings

var my_name = 'John';
var s = `hello ${my_name}, how are you doing`;
console.log(s); // prints hello John, how are you doing

You need to wrap string within ` (backtick) instead of ' (apostrophe)

if you are using ES6, the you should use the Template literals.

//you can do this
let sentence = `My name is ${ user.name }. Nice to meet you.`

read more here.

util.format does this.

It will be part of v0.5.3 and can be used like this:

var uri = util.format('http%s://%s%s', 
      (useSSL?'s':''), apiBase, path||'/');

As of node.js >4.0 it gets more compatible with ES6 standard, where string manipulation greatly improved.

The answer to the original question can be as simple as:

var s = `hello ${my_name}, how are you doing`;
// note: tilt ` instead of single quote '

Where the string can spread multiple lines, it makes templates or HTML/XML processes quite easy. More details and more capabilitie about it: Template literals are string literals at mozilla.org.

Do that:

s = 'hello ' + my_name + ', how are you doing'

Update

With ES6, you could also do this:

s = `hello ${my_name}, how are you doing`

I wrote a function which solves the problem precisely.

First argument is the string that wanted to be parameterized. You should put your variables in this string like this format "%s1, %s2, ... %s12".

Other arguments are the parameters respectively for that string.

/***
 * @example parameterizedString("my name is %s1 and surname is %s2", "John", "Doe");
 * @return "my name is John and surname is Doe"
 *
 * @firstArgument {String} like "my name is %s1 and surname is %s2"
 * @otherArguments {String | Number}
 * @returns {String}
 */
const parameterizedString = (...args) => {
  const str = args[0];
  const params = args.filter((arg, index) => index !== 0);
  if (!str) return "";
  return str.replace(/%s[0-9]+/g, matchedStr => {
    const variableIndex = matchedStr.replace("%s", "") - 1;
    return params[variableIndex];
  });
}

Examples

parameterizedString("my name is %s1 and surname is %s2", "John", "Doe");
// returns "my name is John and surname is Doe"

parameterizedString("this%s1 %s2 %s3", " method", "sooo", "goood");
// returns "this method sooo goood"

If variable position changes in that string, this function supports it too without changing the function parameters.

parameterizedString("i have %s2 %s1 and %s4 %s3.", "books", 5, "pencils", "6");
// returns "i have 5 books and 6 pencils."

A few ways to extend String.prototype, or use ES2015 template literals.

var result = document.querySelector('#result');
// -----------------------------------------------------------------------------------
// Classic
String.prototype.format = String.prototype.format ||
  function () {
    var args = Array.prototype.slice.call(arguments);
    var replacer = function (a){return args[a.substr(1)-1];};
    return this.replace(/(\$\d+)/gm, replacer)
};
result.textContent = 
  'hello $1, $2'.format('[world]', '[how are you?]');

// ES2015#1
'use strict'
String.prototype.format2 = String.prototype.format2 ||
  function(...merge) { return this.replace(/\$\d+/g, r => merge[r.slice(1)-1]); };
result.textContent += '\nHi there $1, $2'.format2('[sir]', '[I\'m fine, thnx]');

// ES2015#2: template literal
var merge = ['[good]', '[know]'];
result.textContent += `\nOk, ${merge[0]} to ${merge[1]}`;

<pre id="result"></pre>

var print = console.log;

function dformat(str, ...arr) {
  return str.replace(/%(\d+)/g, function(_,i) {
    return arr[--i];
  });
}

function wformat(str, dict) {
  return str.replace(/%(\w+)/g, function(_,k) {
    return dict[k];
  });
}

function sformat(str, dict) {
  return str.replace(/\$(\w+)/g, function(_,m) {
    return dict[m];
  });
}

function tformat(str, dict) {
  return str.replace(/\${(\w+)}/g, function(_,m) {
    return dict[m];
  });
}

print(1, dformat("uid:%1, name:%2", 120, "someone") )
print(2, wformat("color: %name", {name: "green"})   )
print(3, sformat("img: $url", {url: "#"})   )
print(4, tformat("${left} ${right}", {right:"1000", left: "7fff"}) )

Try sprintf in JS or you could use this gist

If you are using node.js, console.log() takes format string as a first parameter:

 console.log('count: %d', count);

var user = "your name";
var s = 'hello ' + user + ', how are you doing';

Here is a Multi-line String Literal example in Node.js.

> let name = 'Fred'
> tm = `Dear ${name},
... This is to inform you, ${name}, that you are
... IN VIOLATION of Penal Code 64.302-4.
... Surrender yourself IMMEDIATELY!
... THIS MEANS YOU, ${name}!!!
...
... `
'Dear Fred,\nThis is to inform you, Fred, that you are\nIN VIOLATION of Penal Code 64.302-4.\nSurrender yourself IMMEDIATELY!\nTHIS MEANS YOU, Fred!!!\n\n'
console.log(tm)
Dear Fred,
This is to inform you, Fred, that you are
IN VIOLATION of Penal Code 64.302-4.
Surrender yourself IMMEDIATELY!
THIS MEANS YOU, Fred!!!


undefined
>

You can implement your own format function that takes a string and key-value pairs.

the function:

function format(str, args) {
    return str.replace(/%(\w+)/g, (_, key) => args[key]);
}

const message = "Hello, %name! How are you doing?";
const args = {
    name: "Alice"
};

const formattedMessage= format(message, args);

I do it like this

String.prototype.format = function (...args) {
  return args.reduce((_str, val) => _str.replace(/%[0-9]\$s+/, val), this)
};

var format = "%1$s has %2$s messages".format("UserName", "50")

document.getElementById("demo").innerHTML = format;

<h2>Strin format</h2>

<p id="demo"></p>