TypeScript is a programming language developed as a superset of JavaScript.

It enriches the capabilities of JavaScript with the ability to optionally assign static types to variables. Objects defined in this way have predetermined features, which, with the support of the source code editor, gives the user additional opportunities to validate the code while writing it. This prevents many types of errors and significantly improves group work.

As TypeScript is a superset of JavaScript, any error-free script written in JS will also work properly under TS. In this case, the editor may highlight type errors, but it will not affect its correct operation. It also means that any application written in TS is automatically compiled into JS.

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--Wikipedia: https://en.wikipedia.org/wiki/TypeScript