Again, the message is self-explanatory. You are calling two methods (isNaN and isInteger) for an object (summation) that do not have those methods. You most likely want to call isNan(x) instead of summation.isNaN(x).

The message is self-explanatory. You did not define the "index" variable.

function greetingForAllFriends(friends){
if(friends[index]!==null && friends!==null){
return("Hello, "+""+friends+"!")// ...
}
else {return null}}

say's index is not defined in the function???

addWord(array, string){
array.push(string)
array=this.array
}
gives
Test Passed: Value == ["hello"]
TypeError: Cannot call method 'push' of undefined
at addWord