Javascript NaN

Question

var a = 10 + null;
var b = 10 + undefined;
console.log(a+ " " + b);

// outputs 10 NaN

From what I can see, only the string (represented by " ") is not a number...
So why isn't the output 10 NaN 10?
What happened to the 10 in var b?

Answer 1

null is an object that represents a nothing value where undefined is a type.
null when used is mathematical operations is treated like the value 0.
So for 10 + null is equivalent to 10 + 0, which is equal to the value 10.
Where undefined is of a different type than a number. When you try to perform a mathematical operation with undefined it cannot be coerced to a different type or a number type in this case so the result of the operation cannot be evaluated and returns "Not a Number" NaN.

https://codeburst.io/understanding-null-undefined-and-nan-b603cb74b44c

https://stackoverflow.com/questions/25799408/javascript-null-and-plus-operatior

Answer 2

ChaoticDawg The first article does a good job for the most part, but might fall short with some of the explanations. I may post some feedback later to the author to help fill in the gaps later. The two main answers in the 2nd link were fantastic.

Thanks for sharing those links. 👌

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Solus Building on the explanation by @ChaoticDawg, I'd like to respond to your stated questions:

1. So why isn't the output 10 NaN 10?

It might help to see the code rewritten to reflect the evaluated expressions up to the console.log(...) line:

var a = 10; //10 + null as a number
var b = NaN; // 10 + undefined

Now... putting the above together:

console.log(10 + " " + NaN);

// outputs 10 NaN

2. What happened to the 10 in var b?

The var b never received the value 10.

b was assigned NaN.