Extracted from https://stackoverflow.com/a/50396312
As you are likely aware, TypeScript adds a static type system to JavaScript, and that type system gets erased when the code is transpiled. The syntax of TypeScript is such that some expressions and statements refer to values that exist at runtime, while other expressions and statements refer to types that exist only at design/compile time. Values have types, but they are not types themselves. Importantly, there are some places in the code where the compiler will expect a value and interpret the expression it finds as a value if possible, and other places where the compiler will expect a type and interpret the expression it finds as a type if possible.
The compiler does not care or get confused if it is possible for an expression to be interpreted as both a value and a type.