Question: Is there a function to keep draft before submitting a question?
If there is not, could you consider to implement it?

Answer: 
You can post your question and delete it right away. Then edit and undelete it later. 

Ultimately, it's best to simply use a Markdown editor on your computer and post the final result when you're done.
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> Is there a function to keep draft before submitting a question?
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Not yet, I am afraid. However as long as you do not delete it, there would be a temporary memory in our browser. One of the alternative solution is to use editor, eg. <https://stackedit.io/>

> 
> If there is not, could you consider to implement it?
> 
> 
> 

I hope the Stack overflow developer can consider this brilliant idea. I should think more to post the question when it is not ready. Presumably it will leads to down vote.

Keep inspiring!