Why does the following code result in logging of b while a is still undefined?

(function(){ var a=b=5; })();
console.log('b:'+b);
console.log('a:'+a);

Why does the following code result in logging of b while a is still undefined?

(function(){ var a=b=5; })();
console.log('b:'+b);
console.log('a:'+a);

• javascript
• global-variables
• iife

4 Answers 4

Because var a=b=5; statement defines only local a variable and in fact is interpreted like

var a = (b=5);

which equals to

b = 5;
var a = 5;

which assigns 5 to a global b variable and defines a local a variable.

The proper way to define 2 local variables without value repetition would be

var b = 5, a = b;

In JavaScript if you ommit the var keyword before a variable, it will be considered as global. So here b is a global variable and a is only a local to that function's scope. That's why you are getting the error while accessing the a.

Never mind, I figured it out myself as it wouldn't let me sleep.

There are 2 assignments happening within the IIFE whereas only 1 declaration.

The statement var a=b=5; declares the variable a with var but simply does assignment for the other variable b. b is actually never declared here, just assigned - making it a global variable.

Hence b is accessible to the log statement outside the function, which prints its value as 5.

In other words, under 'strict' mode, the code will look like this:

(function() {
   'use strict';
   var a = window.b = 5;
})();

console.log(b);

A variable can be defined in 2 ways:

• var a= 5 // first
• a=5 // Second

In first way, a is a local variable, but in second way, it bees a global variable.

So, when you do var a=b=5, b is a global variable and hence retains value.