What is the best way to have long string literals in Javascript?

The syntax you are referring to is often known as here-document (or HEREDOC) and no, it is not available in Javascript.

Adding a backslash as you have been doing is the appropriate way to span strings over multiple lines in JavaScript.

Having html inline like that is bad practice, but if you really want to handle it cleaner, try this:

Place a hidden div on your page with the html you want, and replace the custom params with something like {title}. When calling update, pass yourdiv.innerHTML.replace(...