just been reading this: https://reactjs.org/docs/react-api.html#reactpurecomponent
and trying to work out when would a function not return something pure?
surely if you give a component the same props/state then it will always return the same result?
Imagine if you had a component that showed the weather and a background image of the sun if it's daytime and a background picture of the moon if it's night.
You would pass in the temperature, wind speed etc. through props, but you would display the correct image depending on the time of day.
So, if you pass the temperature of 15 degrees during the day and during the night, the component will look differently even though you have passed the same props.
If your function component has any side effects, then it would not be a pure component. For example, reading the current time via Date.now() is a side effect. If that date is passed via prop, then you're fine. But React/JavaScript cannot guarantee that a function component doesn't have side effects.
Consider the following function components.
function Impure() {
return <p>{Date.now()}</p>
}
function Pure(date) {
return <p>{date}</p>
}