tutorial: In the onDestroy example, explicitly show the memory leak

[fivemru]

In the onDestroy example, the current solution does not explicitly (visually) show the memory leak and the moment when the component was destroyed.
If I remove the onDestroy block in the utils.js file on the site, nothing visually changes. It will not be clear if the callback has been executed.
I decided to make a more illustrative example in my opinion, what do you think about it?

Demo repl

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Tests

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[benmccann]

This contains a superset of the code in #5513

[fivemru]

This contains a superset of the code in #5513

Sorry, forgot to switch to master, did a revert of commit 159cf86.

[tanhauhau]

The idea of visualising the memory leak is good.

probably a few suggestions over here to make it better:

• probably update the instructions as well, to explain what we are seeing (the timer still ticking even though the component is unmounted), and why is it wrong / bad
• actually another way of visualising the memory leak would be to have a console.log, but i think either way is fine for me.
• probably name the inner component with a better name, such as Timer instead of Inner. try make it more friendly and real.
• same thing goes for the counter=1, something more friendly, like the current message, such as "The component has been created since 5 seconds"

[fivemru]

Demo A
Demo B

[stale]

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