Why can I create an index dynamically in Typescript?

Question

Why does the following code pass the Typescript compiler?

type O = {
  name: string
  city: string
}

function returnString(s: string) {
  return s
}

let o1: O = {
  name: "Marc",
  city: "Paris",
  [returnString("random")]: "London",
}

Answer 1

Why does this surprise you?

The code passes because, for one thing, it's valid plain-old JavaScript as of ES2015.

Even if it wasn't valid JavaScript, since this code can be translated to JavaScript that doesn't directly support computed key values, and it's a very nice, convenient thing to be able to do... why not support it?

As ES2015:

"use strict";
function returnString(s) {
    return s;
}
let o1 = {
    name: "Marc",
    city: "Paris",
    [returnString("random")]: "London",
};

Transpiled to ES5:

"use strict";
var _a;
function returnString(s) {
    return s;
}
var o1 = (_a = {
        name: "Marc",
        city: "Paris"
    },
    _a[returnString("random")] = "London",
    _a);

Answer 2

It's because of the : never type. I believe this is a very advanced typescript concept which is usually used to discriminate unions.

This is not a complete answer but it could serve you like a hint to understand the problem, it loses the scope of the string return value.

type O = {
  name: string
  city: string
}

declare const neverValue: never;

// This will fail
// function returnStringS(s: string): 's' {
//     return 's';
// }

// This will work
function returnStringS(s: string): string {
  return 's';
}

// This will work
function returnStringS(s: string): string {
  return s;
}


let o1: O = {
  name: "Marc",
  city: "Paris",
  [returnStringS("random")]: "London",
}

let o2: O = {
  name: "Marc",
  city: "Paris",
  [returnString("random")]: "London",
}

const some2: O = {
  name: "Marc",
  city: "Paris",
  [neverValue]: "London"
}

At the Typescript documentation TS never type is the key of this issue/concept.

The never type is a subtype of, and assignable to, every type; however, no type is a subtype of, or assignable to, never (except never itself). Even any isn’t assignable to never.