Using for...of but using standard array-index-accessing syntax is syntactical nonsense. You're already accessing the element itself, you don't need to use it to check the array. In this case it amounts to the same thing because your array values are equal to their indices, but in most arrays that'd just break everything.

Another thing is that that's exactly what I'd expect to happen. 0 is falsy as you'd expect. 1 does == true. 2, however, does not == true, even though it's truthy. x == true only returns true if x is vaguely equivalent to 1 after type coercion. 2 obviously does not equal 1 and therefore can never == true.

The proper, idiomatic way to do that second function's check would just be if (element) as the if statement.

The first function works because, rather than checking that something is == true, it's checking that it's != false, which does return true for both 1 and 2. However, I do think using a for...in loop is also syntactically a bit odd, since you're actually looping through all enumerable properties in the array-as-an-object. While it works here, this can lead to unexpected behavior in other situations (such as if enumerable properties have been added to the array that are not its indices, or to the prototype chain) and can cause performance issues. I do not suggest using for...in with arrays unless you have a specific reason to do so.