Side Effects

Everything that happens inside function but is not returned by it.

• printf etc.
• audio
• video
• mutation of local static variables, non-local variables,
• mutable reference arguments or input/output streams.

A funciton without any side effects is called pure function.

Side effects in React ​

Basically anything that's not directly related to the rendering of the component:

• using the browser API
• accessing the external APIs
• setting timeouts, intervals etc.
• adding new elements to DOM