javascript - Why a is undefined while b is 3 in var a=b=3?

Question

In the following code, I expected both a and b to be 3. However, a is undefined and b is 3. Why?

(function(){
    var a = b = 3;
})();

console.log(typeof a);//"undefined"
console.log(b);//3

Answer 1

The issue here is that most developers understand the statement var a = b = 3; to be shorthand for:

var b = 3;
var a = b;

But in fact, var a = b = 3; is actually shorthand for:

b = 3;
var a = b;

Therefore, b ends up being a global variable (since it is not preceded by the var keyword) and is still in scope even outside of the enclosing function.

The reason a is undefined is that a is a local variable to that self-executing anonymous function

(function(){
    var a = b = 3;
})();