Optional Primitive Type Annotation

Primivite types: boolean, string, number, object, function

Hello, folks! I'm not sure if this has been proposed like this before, so here I come.

What if we have an optional type annotation following the typescript colon syntax, that allow us to tell the runtime variables are constrained to a single primitive type.

  • The notation only works for variable definitions, not for object properties.
  • The runtime would only allow assignments to typed variables in case the types are equal, or explicitly casted (eg.: Number('345'))
  • Above also applies to calling a function with typed parameters.
  • Same also applies to function return types
  • The runtime would probably be able to apply some optimization to those variables.

What this proposal is NOT

Sample Code

let a: boolean = true;
let b: number;
b = 2;

let c: string;

c = String(3456);

function foobar(a: string, b): string {
    return a.toUpperCase().slice(b);
}

c = foobar(c, b);

Extra points yet to address:

  • how null and undefined fits in that model
  • callback type information
  • whether runtime should check available properties and methods for each type
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https://github.com/tc39/proposal-type-annotations

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