Variables in JavaScript

Before we get into how each variable type differs from each other let's do a quick knowledge check.

What's the difference between var, let, and const?

Let's start off easy with const, const lets you define a variable that contains a set chunk of data that cannot be set to anything else. This is the only variable assigner that will not let you change the data stored in it.

NOTE: While you cannot change the value assigned to the const variable, if you assign an object of an array to it, you are able to change the data inside the object.

let and var both store data and that data can be changed to anything you want, including a different type of data. The difference between the two is with scope. The scope of the variable is where in a program the data the variable contains can be accessed. A global variable defined by not using const, let, or var can be accessed anywhere in the program. This might sound like it is a good thing however, this can cause memory issues.

Looking at the code below and its output, you can see when using var the data stored in j can be accessed outside of the for loop it was declared in. In the next example, the function only outputs the value of i when inside the loop. This is because the scope of i is defined inside of the for loop.

"use strict";
console.log("var:");
for (var j = 0; j < 2; j++) {
  console.log(j);
}

console.log(j);

console.log("let:");
for (let i = 0; i < 2; i++) {
  console.log(i);
}
console.log(i);
                    
> "var:"
> 0
> 1
> 2
> "let:"
> 0
> 1
> ReferenceError: i is not defined

Let's see what you learned about scope:

Learning what variables do and how the scope of variables operate is fundamental to know how your code runs and being about to effectively debug your code. If you liked this interactive lesson please let us know! There will be many more to come.