Javascript: testing for at least one non-empty array (if one of the arrays may be null)

Ok, so, there're a bunch of errors in the code examples, i'll try to to explain them all:

myBooleanOr = (myArray1.length || myArray2.length); //3
myBooleanAnd = (myArray1.length && myArray2.length); //Error

Here, the first line returns the first truthy value it encounters. Since myArray1 has a length > 0, it returns that value and never evaluates the second part of the condition, that's why you're not getting the error. Swap the checks and it will break.
The second line combines the two values to give a result, so it will always give an error when one of the two variables are null.

var bool = (typeof myArray1 === undefined); //false

typeof returns a string, if you compare it to the undefined constant it will always be false, the correct statement is typeof myArray1 === "undefined" as written in most of the posts you linked

var bool = (myArray2 ==! null);

the "strictly not equal" operator is !== and NOT ==!. You're doing a different operation and that's why you get surprising results.

Putting the right spaces in the syntax, this is your code var bool = (myArray2 == !null);

So you boolean-flip the value of null, which is falsy by nature, getting true, and then compare if myArray2 is loosely-equal to true ... since myArray2 is null, and that is falsy as we said, the comparison gives back a false.

That said, for the solution to the question, I'd propose a slightly longer syntax that is more explicit, clear to understand, and you can scale to check how many arrays you like without adding more complexity:

var myArray1 = [1,2,3]
var myArray2 = null

var arrays = [myArray1, myArray2]

var oneNotEmpty = arrays.some( a => typeof a != "undefined" && a != null && a.length > 0)

console.log("At least one array non-empty?", oneNotEmpty)

You could use isArray to check something is an array or not. Most modern browsers supports it. https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

And then you can combine isArray and array's length to check if something is a valid non empty array or not.

function isOneNotEmpty(arrayOne, arrayTwo){
	
	return (Array.isArray(arrayOne)? arrayOne.length > 0 : false) || (Array.isArray(arrayTwo)? arrayOne.length > 0 : false)
}

console.log(isOneNotEmpty([],null));
console.log(isOneNotEmpty([1,2,3,4],null));
console.log(isOneNotEmpty([3,4,5],[1]));

Testing for an array is simple enough:

    var blnIsPopulatedArray = (myArray != null
                            && typeof myArray == "object" 
                            && typeof myArray.length == "number"
                            && myArray.length > 0);

Will return false if 'myArray' isn't an array with at least one item, or true if it is an array with at least one item.

One solution is the following (with or without Boolean()):

Using Array.isArray() and array.length:

   var myBooleanAnd = Boolean(Array.isArray(myArray2) && myArray2.length) && Boolean(Array.isArray(myArray1) && myArray1.length) ; //false -> OK
   var myBooleanOr = Boolean(Array.isArray(myArray2) && myArray2.length) || Boolean(Array.isArray(myArray1) && myArray1.length) ; //true -> OK

It is also possible to use myArray1 !== null instead of Array.isArray(myArray1), but since the latter is a more specific test, the broader Array.isArray() method seems preferable.

UPDATE

I had previously suggested using the following:

    var myBooleanAnd = Boolean(myArray1 && myArray2); //returns false -> OK
    var myBooleanOr = Boolean(myArray1 || myArray2); //returns true -> OK

but since, as pointed out by @JLRishe, the following expression also returns TRUE, this is not a safe solution, since it will only work in situations where the arrays can never be empty.

   var bool = Boolean([] && []); //returns true -> false positive

function arrayIsEmpty(array) {
 if (!Array.isArray(array)) {
   return false;
 }
 if (array.length == 0) {
   return true;
 }
}