TypeScript is a superset of JavaScript, so any JavaScript syntax is also legal TypeScript. It adds static typing, so the compiler checks types before the code runs.
Angular uses it because:
Type errors appear at compile time instead of runtime.
Editors use the types for autocomplete, refactoring and navigation.
Interfaces and access modifiers build on classes and inheritance for object-oriented code.
ES6 and later features compile down to older JavaScript.
In a large codebase, a changed type flags every line that breaks.
Error messages name the expected type, so debugging is faster.
Rewriting in plainer words…
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