For ES6, you can use String / Template Literals.
TIPS: you can use Babel (or use babel-loader with Webpack) for cross-browser support.
var html = ` <div> <span>Hello</span> </div>`
String concatenation way.
var text = 'I am happy.' + 'I am ' + 'sleeping now';
Slash concatenation (ES5).
var text = 'I \am \Home.';
Join array concatenantion.
var text = ['I ','am ','home.'].join('');