Mastering JavaScript Functional Programming - Second Edition

Usual side effects

In programming, there are (too many!) things that are considered side effects. In JavaScript programming, including both frontend and backend coding, the more common ones you may find include the following:

• Changing global variables.
• Mutating objects received as arguments.
• Performing any kind of I/O, such as showing an alert message or logging some text.
• Working with, and changing, the filesystem.
• Updating a database.
• Calling a web service.

• Querying or modifying the DOM.
• Triggering any external process.
• Just calling another function that happens to produce a side effect of its own. You could say that impurity is contagious: a function that calls an impure function automatically becomes impure on its own!

With this definition, ...