Speeding up the JavaScript ecosystem, one library at a time(marvinh.dev)

    let stringProcessor = (foo: string | number): string => {
      //do stuff with foo
      return "string" + foo
    }

    type MyFoo = string | number //somewhere in the codebase

    //you will get zero hints that this is a polymorphic function
    let formatFoos = (foos: MyFoo[]) => foos.map((foo) => `${foo} deoptimized`))
    

    interface Foo {
      foo: MyFoo  //polymorphic
      bar: string
      baz?: number //this is polymorphic too
    }

    let doStuff = (obj: Foo): string {
      //do stuff with obj
      return "string"
    }
    doStuff({bar: "abc"})
    doStuff({bar: "abc", baz: 123}) //we're now polymorphic

    interface Foo {
      bar: string //all basic types, so we're good?
      baz: number //no more optional, so we're fine right?
    }

    let doStuff = (obj: Foo): string {
      //do stuff with obj
      return "string"
    }
    doStuff({baz: 123, bar: "abc"})
    doStuff({bar: "abc", baz: 123}) //we changed order, so now we're now polymorphic