ECMAScript 6 (ES6) introduces a new type of literal, namely template literals. They have many features, variable interpolation among others, but most importantly for this question, they can be multiline.
A template literal is delimited by backticks :
var html = ` <div> <span>Some HTML here</span> </div> `;
(Note: I'm not advocating to use HTML in strings)
Original ES5 answer:
"foo \ bar"