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What is the difference between 'Var' and 'Let' ?

frk-ozbk on February 26, 2020

First of all, I want to talk about Javascript Scopes. Scopes determine the accessibility of the variable. You can't access a variable from outside ...

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Lucas.T • Feb 26 '20 • Edited

Hey 👋,

I think you can add the fact that you can update the value of an let variable but never a const variable.

Let example:

let x = 0
x = 1
console.log(x) // 1

Const example:

const x = 0
x = 1
console.log(x) // TypeError cannot reassign x

Otherwise it is a nice article.

frk-ozbk • Feb 26 '20

Thank you,

I just want to focus on the differences between let and var in this post.

Amin • Feb 26 '20

Nice, neat and clear article!

I also like to add the fact that in a for loop, you can access the declared variables with var, when you can't with a let.

"use strict";

for (var number = 0; number < 5; number++) {
    // ...
}

console.log(number); // 5

for (let index = 0; index < 5; index++) {
    // ...
}

console.log(index); // ReferenceError: index is not defined

Simple use-case, but useful to know IMO.

frk-ozbk • Feb 26 '20

Thank you.
Actually the first difference I wrote explains this. For loops are actually block. So if you declare a variable with "Let" inside the "For" block, you can't access it from outside of the "For" block.
But if you declare with "Var" you can access it because variables declared with "Var" have "function scope".

Amin • Feb 26 '20

Indeed, variable are blocked scoped, except for those who are declared with the var keyword and your article explain the thing really well.

But if you look closely, for loops are kind of special, especially since the variable is declared inside the parens of the for loop, and not inside the body (the block).

So this might be kind of confusing for newcomers! Especially those who use the var keyword inside of the for loop. I thought this would be great to have that in your article.

There was a very special talk from the Google YouTube channel about that. It is really cool (and a cool show!)

Indeed, for loops are... complicated!

frk-ozbk • Feb 26 '20

Thank you.
I did not watch this video. I will watch it and try to add it.

frk-ozbk • Feb 27 '20

I watched the video. But I am not too sure whether I should add or not? I'm afraid it will cause confusion for beginners.

Tobias SN • Feb 26 '20 • Edited

The reason you couldn’t assign the let variable is because you were declaring it at the same time, which you had already done. After a variable has been declared, just do a = 4 instead of let a = 4. Same goes for var, which you shouldn’t be using anyway.

However, if you do want a variable that can’t be re-assigned after the first time, use const instead of let.

lesha 🟨⬛️ • Feb 27 '20 • Edited

I guess writing an article about something is like a school homework assignment (but for grownups). Glad you figured it out, but...

At a risk of sounding salty, how many more of these articles do we really need?

frk-ozbk • Feb 27 '20 • Edited

I do not agree with you.
The Javascript community is big. There are too many articles about javascript. If I check every topic that I want to create an article about it, I can't write anything.
Right now I am writing an article about "This" keyword next time I will try to write about a more advanced topic. My only concern is that I will write something false which also I think my biggest benefit. Because it forces me to read more articles about what I wrote

Thank you for your feedback.

Guilherme Augusto Stella • Feb 26 '20

Thanks for sharing. It was short and sweet! I'ill send this with my team.

Also thanks to Lucas.T for the comment above.

frk-ozbk • Feb 26 '20

Thank you.