Question

39

what's the result of 1 + undefined

rated 0 times [ 41] [ 2] / answers: 1 / hits: 25255 / 12 Years ago, wed, february 20, 2013, 12:00:00

1 + undefined = ?

first, String(undefined) get string undefined

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

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deonkalvinw

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answered 12 Years ago mitchellg · Accepted Answer

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.