JavaScript Logic Guide

Question

I have small function logically output should be 1 but i am getting 10 .. Can any one help me out with this logic.

var a=1;
function foo() {
    if(!a){
        var a=10;
    }
    console.log(a);
}
foo();

Output is coming 10 not 1 . how.

I would suggest trying to do some research on your own first. — maxpaj, Oct 11 '17 at 15:48
@Lexi- that is the question . if i have a then dnt go in if condition , if not a is declare then declare a .. — app_dev, Oct 11 '17 at 15:49
Remove the `var` part and you get the output you are expecting — Lixus, Oct 11 '17 at 15:53

score 4 · Answer 1 · answered Oct 11 '17 at 15:49

4

It's because of something called "hoisting".

https://www.w3schools.com/js/js_hoisting.asp

JS interpret your code like this

var a=1;
function foo(){
  var a;
  if(!a){
    a=10;
  }
  console.log(a);
}
foo();

answered Oct 11 '17 at 15:49

Himan

379
1
7

George Katsanos · Answer 2 · 2017-10-11T15:57:43.460

In addition to hoisting as mentioned in other answers, keep in mind that when writing ES6 (by using let or const) your code will behave as you expect it to.

Why does this happen? Because let and const have block-level scoping and not function-level scoping.

let a = 1;

function foo() {
  if (!a) {
    let a = 10;
  }
  console.log(a);
}

foo();

This is why, when you use a JavaScript linting mechanism, like ESLint, you will see rules like vars-on-top ( https://eslint.org/docs/rules/vars-on-top ) specifically to avoid these types of issues.

JavaScript Logic Guide

2 Answers2