Returning an implicit type type of a Union in TypeScript

TypeScript gets compiled to JavaScript, and JavaScript (where the code runs and gives you the console.log output) has no concept of types – it runs just the plain JavaScript, which is:

function withImplicitReturnType(b) {
  if (b) {
    return 10;
  return "test";

The function, when called here, returns 10, so 10 gets logged.

The 10 | 'test' union type can only be seen in TypeScript, for example, in intellisense, but not in the running JavaScript after the code has been compiled.

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If you wanted to use that union, remember that the union is only a type, so it’ll be useful for type-checking, but it will not exist in the emitted JavaScript code.

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