what's the result of 1 + undefined

Question

1 + undefined = ?  

1. first, String(undefined) get string "undefined"
2. second, 1 + "undefined" = "1undefined"

what's wrong?

I run it in chrome console ,it return NaN.

can you pls explain the result?

I think it should be "1undefined". tks

Answer 1

NaN is the result of a failed Number operation.

1 + undefined           // NaN
"1" + undefined         // "1undefined"
1 + "" + undefined      // "1undefined"
1 + ("" + undefined)    // "1undefined"
typeof NaN              // "number"
typeof undefined        // "undefined"
NaN === NaN             // false (it's not reflexive!)
undefined === undefined // true (it's reflexive)
NaN.toString()          // "NaN"

NaN means Not a Number where a number was expected. Any Number operation with NaN will result in NaN as well.

Answer 2

You expect a string concatenation, but this will only happen if you have at least one string. And in your example nothing is a string. 1 is not a string and undefined is not a string.