Evaluate each test:
A). sum3([1, ' 2 ' ])
* Array length is 2 # line 4: arr[0] + arr[1] # 1 + ' 2 ' # ' 12 ' (string).
* Assertion: ' 12 ' == 12 is true (type coercion). However, this "test" is not logically correct: a numeric sum function should not be expected to return ' 12 ' as a string. It passes only because of weak equality coercion, so it is not a good/valid test of correct behavior.
B). sum3([ ' hello ' , 2, 3, 4])
* Length # 3 # line 5: ' hello ' + 2 + 3 # ' hello2 ' + 3 # ' hello23 ' .
* ' hello23 ' === NaN is always false because NaN is never equal to anything, not even itself. This assertion fails and also misunderstands how NaN comparisons work.
C). sum3([-3, 2])
* Length 2 # -3 + 2 = -1.
* Assertion: -1 === -1 # true. This is a correct and meaningful test.
D). sum3([0])
* Length 1 # returns arr[0] # 0.
* Assertion: 0 === 0 # true. Also a correct and meaningful test.
Therefore, the valid and logically correct tests here are C and D.