what-below-code-will-output-5.md

Q - What will be the output of the below code and why

function display() {
  var a = (b = 10)
}
display()
console.log("b", typeof b === "undefined")
console.log("a", typeof a === "undefined")

Ans -

b false
a true

Explanations

This is because the assignment operator has right to left associativity in Javascript which means it will be evaluated from right to left so first, b will be assigned the value 10 and then it’s assigned to a. Therefore

function display() {
  var a = (b = 10)
}

is same as

function display() {
  var a = (b = 10)
}

which is same as

function display() {
 b = 10;
 var a = b;
}

So b becomes a global variable because there is no var keyword before it and a becomes a local variable. Therefore, outside the function, only b is available so typeof a === 'undefined' comes as true and typeof b === 'undefined' comes as false.

If we execute the above code in strict mode as shown below,

"use strict"
function display() {
  var a = (b = 10)
}
display()
console.log("b", typeof b === "undefined")
console.log("a", typeof a === "undefined")

It will throw an error because b becomes a global variable and strict mode does not allow creating global variables so you will get an error when you execute this code. That’s it for today. I hope you learned something new.