I was playing with IIFE and I'm confused. The value of a is undefined but b is not. Why am I able to access the value of b outside of IIFE?

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

console.log("a defined? " + (typeof a !== 'undefined')); // false
console.log("b defined? " + (typeof b !== 'undefined')); // true

I was playing with IIFE and I'm confused. The value of a is undefined but b is not. Why am I able to access the value of b outside of IIFE?

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

console.log("a defined? " + (typeof a !== 'undefined')); // false
console.log("b defined? " + (typeof b !== 'undefined')); // true

• javascript
• scope
• iife

Share Improve this question Follow edited Mar 9, 2019 at 15:41 Ivar 6,8871212 gold badges5656 silver badges6767 bronze badges asked Mar 9, 2019 at 15:30 spratap124spratap124 17133 silver badges1212 bronze badges 3

• 5 b is an implicit global because it has not been declared. – Paul Commented Mar 9, 2019 at 15:32
• 2 Should be var a = 3, b = a; – Quentin Commented Mar 9, 2019 at 15:33
• 3 …or var a, b; a = b = 3; – Bergi Commented Mar 9, 2019 at 15:34

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2 Answers 2

Sorted by: Reset to default 12

Declaration syntax can be confusing, because it looks like the syntax for ordinary expressions. It is, however, different.

var a = b = 3;

is parsed as

var a = (something);

In this case, something is b = 3, so it's exactly as if your function looked like

b = 3;
var a = b;

The "naked" b = 3 without var or let or const creates an implicit global variable. Thus b is visible outside the function, while a is not.

There are four ways of declaring a variable in JavaScript:

• var, which will scope that variable to the declaring function.
• let/const, which will scope that variable to the declaring block.
• Implicit declaration, which will scope that variable globally (unless previously declared in a different scope, in which case it will reassign that instead).

var a = b = 3;

In this statement, we declare a in the function scope with the value of b = 3. The expression b = 3 has value 3, but implicitly declares the variable b as well, which means that b will be declared in the global scope.

Outside of the function, the variable b is declared (since it was implicitly declared globally) while a is not (since it was declared only in the scope of the function).

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You should avoid implicitly declared variables, though (and preferably use let and const instead of var), so the above code should have been written like this:

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

Or, if you actually want the variables declared outside of the function as well:

let a, b;
(function() {
  // NOTE: not implicit declaration since a and b are both already declared
  a = 3;
  b = a;
})();