Strict inequality operator (!==) type guard for `unknown` type

[kazatsuyu]

Search Terms

unknown
strict inequality operator
!==
type guard

Suggestion

unknown type with !== operator should be treated as a type guard.

Use Cases

Using a !== b instead of !(a === b).

Examples

function f1(x: unknown): string | undefined {
  if (!(x === undefined) && typeof x !== 'string') {
    throw new Error();
  }
  return x; // string | undefined
}

function f2(x: unknown): string | undefined {
  if (x !== undefined && typeof x !== 'string') {
    throw new Error();
  }
  return x; // currently x is `unknown`, but it could be `string | undefined`.
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[jack-williams]

Fix up #33488.