From these answers I learned that dependencies are required to run the application while devDependencies are only required while developing (like unit tests). But how about this: My application depends on jQuery, but during a build step (with the help of my devDependencies ), everything is bundled into one file.

Are devDependencies included in bundle?

Your project have many installed dependencies in the node_modules folder that should not be included in your client-side JavaScript production bundle. Examples of such dependencies are the devDependencies that are used for testing and building.

What should be included in a devDependencies?

devDependencies should contain packages which are used during development or which are used to build your bundle, for example, mocha, jscs, grunt-contrib-watch, gulp-jade and etc.