What happens to the variable's value when the "then" block is exited in TypeScript and React?

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It depends on how the variable is declared and used within the "then" block. If the variable is declared within the "then" block using the "let" keyword, it will no longer be in scope and its value will be lost. If the variable is declared outside the "then" block or with the "var" keyword, its value will persist outside of the block.

However, if the variable is being used to hold a value returned from a Promise or async function, its value may not be immediately available outside of the "then" block due to the asynchronous nature of the code. In this case, it may be necessary to use a callback function, async/await or another asynchronous pattern to properly handle the return value.