what's the result of 1 + undefined

Question

1 + undefined = ?  

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

what

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.