How to interpolate variables in strings in JavaScript, without concatenation?

You can take advantage of Template Literals and use this syntax:

`String text ${expression}`

Template literals are enclosed by the back-tick (` `) (grave accent) instead of double or single quotes.

This feature has been introduced in ES2015 (ES6).

Example

var a = 5;
var b = 10;
console.log(`Fifteen is ${a + b}.`);
// "Fifteen is 15.

How neat is that?

Bonus:

It also allows for multi-line strings in javascript without escaping, which is great for templates:

return `
    <div class="${foo}">
         ...
    </div>
`;

Browser support:

As this syntax is not supported by older browsers (mostly Internet Explorer), you may want to use Babel/Webpack to transpile your code into ES5 to ensure it will run everywhere.

Side note:

Starting from IE8+ you can use basic string formatting inside console.log:

console.log('%s is %d.', 'Fifteen', 15);
// Fifteen is 15.

Prior to Firefox 34 / Chrome 41 / Safari 9 / Microsoft Edge, nope, that was not possible in javascript. You would have to resort to:

var hello = "foo";
var my_string = "I pity the " + hello;

Prior to Firefox 34 / Chrome 41 / Safari 9 / Microsoft Edge, no. Although you could try sprintf for JavaScript to get halfway there:

var hello = "foo";
var my_string = sprintf("I pity the %s", hello);

well you could do this, but it's not esp general

'I pity the $fool'.replace('$fool', 'fool')

You could easily write a function that does this intelligently if you really needed to