"there should be an value here, but there isn't at the moment."
undefined and null are both "non-value" values. They are similar, although they behave
slightly differently. For example, if you try to do sums with them:
10 + null // null behaves like zero
10 + undefined // undefined is not a number
null is coerced to be 0 , making the sum possible whereas undefined is coerced to NaN ,
making the sum impossible to perform.
manually by the programmer.
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