Using Array and Boolean in condition does not trigger a warning

TypeScript Version: 3.8.0-dev.20200115

Search Terms: overlap boolean vs array condition

Code

function isAllowed() {
  const myCondition = false;
  if (myCondition === false) {
    return [];
  }

  return true;
}

if (isAllowed()) {
  // boolean and array will results in a "true" condition
  console.log('allowed');
} else {
  // never reached
  console.log('not allowed');
}

Expected behavior:

• Seeing a message saying something like This condition can produce unexpected results since the types 'boolean' and 'array' have no overlap (not really good but you get the idea)

Actual behavior: I won’t say that it’s a bug, but something that could be improved. I know that it’s valid javascript but this a bug in the code that should/could be prevented by Typescript

• Typescript does not complain that comparing boolean and array can results in bad condition

Already fixable by

• It does complain if you write if (isAllowed() === true) but it’s easy to not do it.

Playground Link: http://www.typescriptlang.org/play/?ts=3.8.0-dev.20200115&ssl=1&ssc=1&pln=15&pc=1#code/GYVwdgxgLglg9mABDAzgQQDYbgdwKYAmAFAJSIDeAUIohAilIgLYCeAwggTLAogLyIoAJxB4A3NWTBERVhzBceSPisEi8ZKjRpC8UEEKQBtALoSaAX0qTd+w4mABDDCnGUrlGNKKpM2fMQkmpJ0YChwGHgAdNgA5kQA5M7+hAkkEhaIeC54FCH0EdFxiWBwjMm4qenulEA

[RyanCavanaugh]

So thinking these through (this is my gut check, please disagree):

• boolean | undefined - bad (conflates false and undefined)
• boolean | object - bad (conflates true and {})
• number | undefined - bad (conflates 0 and undefined)
• object | number - bad? (conflates 1 and {})
• boolean | number - bad? (conflates on both sides)
• object | undefined - good (falsiness is unambiguous)

The rule I can backport on to this intuition is “If there is more than one truthy value or more than one falsy value, those values should be of the same type”.

What’s your intuition?

[mwgamble]

In my experience, anyone attempting to do something like this:

const myArray = functionThatReturnsVariableLengthArray();
if (myArray) {
    // do stuff
}

are always making a mistake. The following is their intention:

if (myArray.length > 0) {

I don’t think it ever makes sense to evaluate an array (or any object) directly as a boolean.