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.

Bắt đầu với ví dụ TypeScript Playground được liên kết và so sánh ba hàm bằng literal true và các literal truthy khác. Đọc issue liên quan #29323 và cuộc thảo luận được liên kết trong đó về phân tích truthiness và các phần phụ thuộc giữa các compiler pass. Được xem là hoàn tất khi các literal truthy thu hẹp foo một cách nhất quán sau throw có điều kiện, với độ bao phủ cho các trường hợp đã được báo cáo.