Boolean function returns null

[colltoaction]

TypeScript Version:

1.8.10

Code

// A self-contained demonstration of the problem follows...
function bool_fun(value: string) : boolean {
    return value && value == "bug";
}

console.log(bool_fun(null)); // should log false or throw a compiler error, but logs null
console.log(bool_fun("false"));
console.log(bool_fun("bug"));

A boolean value should always be true or false, but never null (even if it's a falsy value). This would break strict comparisons like ===. The spec is also very clear: https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#3.2.2

Expected behavior:
Either we have a compiler error or the console prints:

false
false
true

Actual behavior:
The console prints:

null
false
true

[basarat]

A boolean value should always be true or false, but never null (even if it's a falsy value).

Please compile with strictNullChecks and it will be a compile error 🌹

[colltoaction]

I just tried adding it to my tsconfig file without luck (but don't worry, I'll figure it out). Anyway, adding this to my own project won't help if other dependencies are not using it, right?

[kitsonk]

@basarat forgot that strictNullChecks is only in the pre-release version of TypeScript (typescript@next)

The challenge here is that TypeScript before 2.0 (which is in pre-release) did not considered all types to be inclusive of null and undefined to reflect the run-time behaviour of JavaScript. With strictNullChecks, those types are removed from other types and guarded against.

TypeScript is not changing the runtime behaviour of the code you wrote (which effectively

But you still have issues with the way you wrote your code, because there are several cases where JavaScript at runtime will short-circuit your logic and return something other than a boolean:

console.log(bool_fun(null));
console.log(bool_fun(undefined));
console.log(bool_fun(0));
console.log(bool_fun(''));

Of course the third one would be caught by TypeScript today and the first and second one with strictNullChecks but the forth one will pass all the type guarding and it is a tough logic error to do static analysis on in TypeScript. So if you always want something that returns a boolean, write it so it always returns a boolean and don't let runtime JavaScript implicit coercions (or lack there of) ruin your runtime behaviour:

function bool_fun(value: string) : boolean {
    return Boolean(value && value == "bug");
}

[colltoaction]

Thanks to both, that's very helpful!

On Tue, May 31, 2016, 06:19 Kitson Kelly notifications@github.com wrote:

@basarat https://github.com/basarat forgot that strictNullChecks is
only in the pre-release version of TypeScript (typescript@next)

The challenge here is that TypeScript before 2.0 (which is in pre-release)
did not considered all types to be inclusive of null and undefined to
reflect the run-time behaviour of JavaScript. With strictNullChecks,
those types are removed from other types and guarded against.

TypeScript is not changing the runtime behaviour of the code you wrote
(which effectively

But you still have issues with the way you wrote your code, because there
are several cases where JavaScript at runtime will short-circuit your logic
and return something other than a boolean:

console.log(bool_fun(null));console.log(bool_fun(undefined));console.log(bool_fun(0));console.log(bool_fun(''));

Of course the third one would be caught by TypeScript today and the first
and second one with strictNullChecks but the forth one will pass all the
type guarding and it is a tough logic error to do static analysis on in
TypeScript. So if you always want something that returns a boolean, write
it so it always returns a boolean and don't let runtime JavaScript implicit
coercions (or lack there of) ruin your runtime behaviour:

function bool_fun(value: string) : boolean {
return Boolean(value && value == "bug");
}

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[mhegazy]

We can do better here. #8949 contains the fix.