unions.ts

var str: "value" = "test" // error
var str2: "value" = "test" as "value" // ok 
var str3: string = "my string" // assignment of healthy man  
var str4: string = {} // error
var str5: string = {} as string // OK
var str6: string = {} as "anything" // OK!!

type Duumvirate = 1 | 2 
var int: Duumvirate = 3 as 1 // OK!!!

// idea of unions
type Tristate =  "TRUE"|"FALSE"|"NONE";

function foo(param: Tristate) {
    return "enhanced: " + param;
}

let validInput = "NONE";
foo(validInput as Tristate);