New guy. How do I read a value from a text box and then output it in another text box?

New people help Ah! How to implement reading a value from a text box and then outputting it in another text box
For example, there are text boxes A and B, a button C, in the text box A to enter 1234, click on the button C, in the text box B display 1234, is the use of $_post to take the value? Then customize a function when clicking on the button C is to read the value in a and output in B? Ask the great God to help.
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JS will be able to achieve
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Do you have a local jquery.js file?




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getElementById, wait a few more functions.
Even jquery doesn't have to be used for simplicity.

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