let or var?

[nelsonic]

asked by @SimonLab in https://github.com/FAC-GM/app/pull/465#issuecomment-222940039 #discuss

[wwalser]

I read your comment in the other thread. I just thought I'd add that I prefer using const and let in my own code. I agree with you in a sense, var could be considered clearer.

Q: "What should I use?"
A: "var, always."

However, I think my overall code is clearer when I use const and let properly. Variables are always defined using const, unless they will change after being defined. To a new reader, the very presence of a let tells them something about the code that they are about to read. "This specific variable is about to change". It also causes me to lean toward clarity when as I'm writing code. "I've just used let, do I need to do that? Can it be avoided?". I enjoy Hapi partly because the identity of a route is often declared right along side the route itself. The route implementation lives besides the identity of the route. Similarly, const and let affords me the ability to embed additional behavioral identity into my variable declarations.

My $0.02. I'm looking forward to where this repo goes 👍 .

[nelsonic]

@wwalser your $0.02 are very much welcome.
I agree that using let and const could make code clearer, but in my experience of reviewing the code of many developers that more often than not people have missunderstood the point of both of these ... 😞

const is a whole other discussion: #13

Let's focus on let ...

The point of let is that it allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used.
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/let

The examples on MDN are a good reference.

The idea of declaring a block-level variable with the same name (e.g: x) as another block-level variable within the same function is like calling your twin children the same name, sure you know they are distinct but confusion is inevitable.

All non-global variables are already _functionally scoped_ so provided you don't try to have multiple block/expression-scoped variables with the same name in the same function, I don't see any advantage to using let. (but I'd be delighted if someone can explain the advantages to me!)

And, in my experience they are confusing to inexperienced developers end up using let everywhere a var would be functionally equivalent because they see the "cool kids" on Medium using let everywhere ...

_Last, but not least_, there's the fact that let is not widely adopted/supported by the Browsers people are actually using: http://caniuse.com/#feat=let 32% of browsers in use don't support let

so to write JS that works in those browsers we have to "transpile" our code which totally defeats the objective because the "transpiler" just uses var everywhere.

So the question we could ask is: once IE < 11 "goes away" should we adopt let ?
To which I reply: let's come back to this when that happens.

Until let is 100% supported we can simplify things by just using var and a new variable name for "block scope".