Type narrowing with assertions containing truthy literals

TypeScript Version: 4.2.0-dev.20201109

Search Terms:

type narrowing truthy
type narrowing assertion
conditional throw

Code

function test(foo?: number) {
  if (!foo) throw new Error();

  return foo.toFixed();
}

function testWithLiteralTrue(foo?: number) {
  if (!foo && true) throw new Error();

  return foo.toFixed();
}

function testWithTruthyLiteral(foo?: number) {
  if (!foo && "str") throw new Error();

  return foo.toFixed(); // Object is possibly 'undefined'.
}

Expected behavior:

In all three functions typescript would know foo is a number after the assertion.

Actual behavior:

In the last example, testWithTruthyLiteral, typescript complains foo may be undefined. This is the same behavior for all known truthy values except for literal true.

Playground Link:
https://www.typescriptlang.org/play?ts=4.2.0-dev.20201109#code/GYVwdgxgLglg9mABFApgZygRgBTDnAfgC5EwQBbAIxQCcBKRAbwChFEZhFsBCPOBqAAsacAO6kU4gKI0RNbHQDczVohoooIGkj4A6KHABiMAB4oAJguUBfFaEiwEydFABMufMVIVq9Jqo4uXnxEADJQ5BoQFAFhMQlpWTh5JRU2dU1tRD0DYzNLVNtme2h4JFQMAGYPQhIyKloGFjZAnj4wiIAiDBpO2JFxMElEGTkrNLUNLR18fSNTC3GiksdylwAWGq963yaAzjaQ8MRMfvihxLHU1Qzp7NnchYKbO3BSpwqoAFYtup9G-wtA7BOAdJjWM6DYajZLjG5TLI5eb5JbMIA

Related Issues:
https://github.com/microsoft/TypeScript/issues/29323

It sounds like from https://github.com/microsoft/TypeScript/issues/29323#issuecomment-454548280 that determining truthiness of expressions could lead to circular dependencies between compiler passes, but that literal true has been special cased. This should be able to be extended to truthy literals without the circular dependency concern.

And for context, like others that have brought up similar issues, this stems from wanting a truthy token that can be used for a minification hook without affecting the types or code execution.

Inizia con l’esempio TypeScript Playground collegato e confronta le tre funzioni usando il literal true e altri literal truthy. Leggi l’issue correlata #29323 e la discussione collegata sull’analisi della truthiness e sulle dipendenze tra i pass del compilatore. Il lavoro è completato quando i literal truthy restringono foo in modo coerente dopo il throw condizionale, con copertura per i casi segnalati.