Typescript

TypeScript is a programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript, adding optional static typing to the language. It is designed for the development of large applications and transpiles to JavaScript.
It may be used to develop JavaScript applications for both client-side and server-side execution (as with Node.js or Deno). Multiple options are available for transpilation. The default TypeScript Compiler can be used, or the Babel compiler can be invoked to convert TypeScript to JavaScript.

TypeScript provides static typing through type annotations to enable type checking at compile time. This is optional and can be ignored to use the regular dynamic typing of JavaScript.

function add(left: number, right: number): number {
    return left + right;
}

The annotations for the primitive types are number, boolean and string. Typescript also supports data types with the following annotations: Array, Enums and void.

Additional data types are: Tuple, Union, never and any. An array with predefined data types at each index is Tuple type. A variable that holds more than one type of data is Union type. When you are sure that something is never going to occur you use never type. Weakly- or dynamically-typed structures are of any type.