String concatenation is the easiest part, however, it’s not recommended approach since there will be an impact in the Speed and the performance factor. The best approach is to use a delimiter ‘\’ to create multiline strings.
var multiLineString = "This is an example of concatenating" + "two lines together";
var multiLineString = "This is an example of concatenating \ two lines together \ appending one more line here...";
ECMAScript6(ES6) introduces a new type of literal called as template literals. One of the features is multiline strings using the backticks.
var multiLineString = `his is an example of concatenating two lines together appending one more line here...`;