Typescript: Variable not recognized as string literal type when using ternary operator

This has been discussed several times.

First, you are assigning to a let variable, which means the compiler will assume that you may want to change the value in the future, therefore it does not assign a string literal type to the variable.

Also, in 2.0, there was never an inference of literals. The developer would have to do that explicitly. So to fix this:

function test(val: 'AAA'|'BBB'){
  //xxx
}

let a: 'AAA' | 'BBB' = true ? 'AAA' : 'BBB';
test(a);

But in TypeScript 2.1 it will interpret assignments to const in the most strict way possible, so you could do it like this:

function test(val: 'AAA'|'BBB'){
  //xxx
}

const a = true ? 'AAA' : 'BBB';
test(a);