From the official React documentation:

One of the key concepts that makes React, React is purity. A pure component or hook is one that is:

• Idempotent – You always get the same result every time you run it with the same inputs – props, state, context for component inputs; and arguments for hook inputs.
• ...

An example of this key concept from the React docs

Components must always return the same output with respect to their inputs – props, state, and context. This is known as idempotency. Idempotency is a term popularized in functional programming. It refers to the idea that you always get the same result every time you run that piece of code with the same inputs.

This means that all code that runs during render must also be idempotent in order for this rule to hold. For example, this line of code is not idempotent (and therefore, neither is the component):

function Clock() {
  const time = new Date(); //