Without --strictNullChecks, x && "a"contributes a "" type.

[ghost]

TypeScript Version: 2.7.0-dev.20171116

Code

function f(b: boolean): false | "a" {
	return b && "a";
}

Expected behavior:

No error. (Just like with --strictNullChecks.)

Actual behavior:

src/a.ts(2,2): error TS2322: Type '"" | "a"' is not assignable to type 'boolean | "a"'.
  Type '""' is not assignable to type 'boolean | "a"'.

[ahejlsberg]

This is working as intended for backwards compatibility reasons. Originally, the type of an && operation was simply the type of the right hand operand and a lot of old code depends on that. With literal types we change the falsy left hand side value into a falsy value in the same domain as the right hand side to best approximate the original behavior.

In general, you see a true reflection of the run-time behavior only in --strictNullChecks mode.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.