Jade conditional (if/else) to add class to div inline

If you don't want the class attribute to be added when there is no value, you can assign it undefined instead of an empty string. Here is the previous example, slightly modified:

div#demo.collapse(class=typeof fromEdit === "undefined" ? undefined : "in")

Update: Also, if you are using pug, you can now add as many class= declarations as you want with different conditions and they'll get concatenated in the resulting class attribute. e.g.:

#demo.collapse(class=cond1 && 'class1' class=cond2 && 'class2')

This works:

div#demo.collapse(class=typeof fromEdit === "undefined" ? "" : "in")

Try it out here.