Typescript runtime mixins

runtime-mixin.ts

// Inspired by:
// https://github.com/tc39/proposal-pipeline-operator/blob/37119110d40226476f7af302a778bc981f606cee/README.md#object-decorators
//
// Once pipe operator is implemented in TS, use this example to check if it
// survives type checking.


// Describe mixed in functionality

interface Foo {
  foo(): number;
}

interface Bar {
  bar(): string;
}

// This is typechecked. Try changing e.g. the spelling of `foo`, or `3` into
// `"yes"`.
function mixinFoo<T>(x: T): T & Foo {
  return Object.assign(x, {foo: function() { return 3; }});
}

// This is typechecked, too. Note the factory is not generic, but the returned
// function is.
function mixinBarFactory(msg: string) {
  return function mixinBar<T>(x: T): T & Bar {
    return Object.assign(x, {bar: function() { return msg; }});
  };
}

// Random type to prove the point
class Something {}

function mixinTest(x: Something) {
  const y = mixinBarFactory("hello")(mixinFoo(x));
  y; // Check my type!
}