Let, const, and var are WEIRD in JavaScript

zo-mar · on May 8, 2020

I like the article overall. It’s definitely important to understand the differences between all three variable declarations. However, I think the article falls short when explaining how scope differs between var vs let and const.

It’s not that let and const are “locally scoped”, but rather they are block scoped. Whereas, var is globally or function scoped. This is a very important distinction, and makes JavaScript way more consistent to work with.

Some contrived examples…

In this example, x is globally scoped. Once the loop finishes, it persists. This means it can be accessed and even worst, modified outside of the for loop.

  var someArray = [1, 2]
  for(var x = 0; x <= someArray.length; x++) {
    // do some work
  }

  console.log(x) // outputs: 3
  x++
  console.log(x) // outputs: 4

---

In this example, x is scoped to the for loop block. Once the loop finishes, it’s destroyed. This results in x not being defined when you access it outside the for loop.

  var someArray = [1, 2]
  for(let x = 0; x <= someArray.length; x++) {
    // do some work
  }

  console.log(x) // output: x is not defined

---

Cheers!