Wrong error message (typeof with Array.isArray)

[MattMoony]

Even though I am well aware that this bug is not one of the most pressing issues at hand and that it is actually not a real problem at all, since all arrays will already produce a truthy value in the first part of the if-clause, I still think that it could lead to some confusion at first glance. I tried my best finding a post mentioning this very specific bug, but I couldn't.

TypeScript Version: 3.7.5

Search Terms: Array.isArray undefined

Code

function foo(bar: number|number[]) {
  if (typeof bar !== 'undefined' || (Array.isArray(bar) && bar.length > 0)) {
    return console.log(':)');
  }
  console.log(':(');
}

foo([1, 2, 3]);

Expected behavior:
Error: Property does not exist on type 'never'

Since Array.isArray(...) will, to the best of my knowledge, never produce a truthy value when given something of type undefined. However, it should produce an Error: Property does not exist on type 'never' error, since the second part of the clause will never be evaluated with something of type array (because that would already be truthy in the first half).

Actual behavior:
Error: Object is possibly 'undefined'

Playground Link:

Related Issues:

[RyanCavanaugh]

Duplicate #26914

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.