JavaScript: The result of undefined++ is NaN, not undefined?

Question

The 1st console.log output is 2. No doubt.
But why the 2nd console.log output is not undefined? Shouldn't output the undefined at first, then the variable b becomes NaN?

var a = 2;
console.log(a++);

var b;
console.log(b++);

Answer 1

Consider this:

b = "foo"
c = b++
console.log(c) 

Since the postfix ++ returns the value before incrementing, we expect c to be foo, however, it's NaN. Why is that?

This is by design, and described as follows in the standard (emphasis mine):

12.4.4.1 Runtime Semantics: Evaluation

UpdateExpression:LeftHandSideExpression++

1. Let lhs be the result of evaluating LeftHandSideExpression.

2. Let oldValue be ? ToNumber(? GetValue(lhs)).

3. Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 12.8.5).

4. Perform ? PutValue(lhs, newValue).

5. Return oldValue.

Translated to quasi-JavaScript, the above algorithm would be:

 function postIncrement(someVariable) {
     let oldValue = Number(someVariable.value); // Note "Number" here
     let newValue = oldValue + 1;
     someVariable.value = newValue;
     return oldValue;
 }

In other words, value++ returns not just the value, but the value converted to a number. The conversion takes place before incrementing, and, since Number("foo") is NaN, this is what we get.

Yet another JavaScript quirk to take note of!

Answer 2

You need to assign value to b or check for null condition before logging b. b is NaN because its b++ is check for number.

Answer 3

var a = 2;

Here you have declared a variable "a" with a value of "2", making its datatype "INTEGER".

console.log(a++);

The output is 2 and the variable "a" has a value of "3".

var b;

Here you have declared a variable "b" without any value, making its datatype "UNDEFINED".

console.log(b++);

The output is NaN because the variable "b" is null, and therefore increment operator cannot be applied.