`!` does not narrow discriminated unions when `strict` is `false`

[LancesLance56]

Search Terms

Tested on:

• TypeScript 6.0.3
• TypeScript 7.0.2

with "strict": false in compilerOptions

https://www.typescriptlang.org/play/?strict=false&jsx=0&ssl=13&ssc=2&pln=2&pc=1#code/C4TwDgpgBAShDOBXANsKBeAUFKAfKA3lAPYDWAXFMAE6IQDcUAbgIbJ2Xw0CWAdgOZQAvtjyESFKADM28BlAjVqxapx4Dh9TJgAmEAMbIW1aPuK8uUE0lSU4N4FszcpUABQBCaymAA6MgCUhKLeqL6KytSMAPTRUAAKypDUoFAA5BEqaVA6xAhQvMRoEAAe3FyYIs6ubqF+ZBjo6NKyEEEEIQg+4UoqMXEA6iqk8JVAA

type Result =
  | { ok: true; value: string }
  | { ok: false; error: string };

declare const result: Result;

if (!result.ok) {
  result.error; // Property 'error' does not exist
}

if (result.ok === false) {
  result.error; // Works
}

Actual behavior

When strict is false, !result.ok does not narrow the discriminated union, while result.ok === false does.

Expected behavior

if (!result.ok) should narrow result to the { ok: false; error: string } branch, just like if (result.ok === false).

Additional information about the issue

Not sure whether this is intended behavior or a bug. Either way would love to learn why

[MartinJohns]

This is working as intended. With strictNullChecks disabled every type may be null or undefined. So result.ok may be nullish, while you have the first version with a value property. Differently said, !result.ok may be true for a value typed { ok: true; value: string }.

const result: Result = { ok: null, value: "abc" };

if (!result.ok) {
  console.log(result);
}

[jcalz]

Your playground link doesn't reproduce the issue you're talking about (click it and see).

With --strictNullChecks turned off, you can use undefined or null just about everywhere. So the following assignment is allowed:

const result: Result = Math.random() < 0.99 ?
  { ok: undefined, value: "abc" } : // this is fine
  { ok: false, error: "" }

And therefore it is quite likely to have !result.ok (since !undefined is true) without an error property and TS is warning you of a real problem:

if (!result.ok) {
  result.error.toUpperCase(); // 99% chance of runtime error
}

Playground link

So TS is working as intended. You should probably keep --strictNullChecks (and --strict) turned on anyway.