Kevin Powell - Community•3w ago

const keyword vs let keyword in JavaScript

Hello guys, sorry to disturb you all; I've just started to learn JavaScript and the use of the const keyword and let keyword seems confusing, like I don't really know when to use which.... can't I always use the let keyword ? Is that a bad idea? If so, can someone just clarify when we should use the const keyword and when we should use the let keyword please

21 Replies

ἔρως•3w ago

you can use let for everything you should use const when you know that the value won't change

ἔρως•3w ago

this is what happens

ἔρως•3w ago

however, you can change an object if it is assigned to a constant

ἔρως•3w ago

but you can't assign anything else a few good uses for consts are dom elements and anonymous functions

Jochem•3w ago

I'd recommend inverting that. Use const, unless you know your value will change, or you can't set the value at declaration time

ἔρως•3w ago

i dont recommend that for new developers for one reason: people make mistakes, and may try to assign to a constant without noticing it, and then ask why it doesn't work

Jochem•3w ago

The error is pretty self explanatory

ἔρως•3w ago

yeah, but do people even read those before asking questions?

Faker•3w ago

yep got it, thanks guys !

ἔρως•3w ago

you're welcome

Faker•3w ago

what do you mean by "we can't set the value at declaration time"... can you give an example please

Jochem•3w ago

they should, and I honestly don't think that's a reason to advise a suboptimal practice. Cause you can always just teach them to read that error message next, and they have a good habit to build on

Faker•3w ago

ah you mean like a counter?

Jochem•3w ago

sometimes, you need to declare a variable in a certain scope, but the value depends on a condition:

let test;
if (someCondition) {
  //do some processing on some values
  //maybe even an async fetch call if this is inside an async function
  test = someFunctionResult();
} else {
  //do some different processing
  test = someOtherResult();
}

let test;
if (someCondition) {
  //do some processing on some values
  //maybe even an async fetch call if this is inside an async function
  test = someFunctionResult();
} else {
  //do some different processing
  test = someOtherResult();
}

if you declare the test variable inside those blocks, you'll end up with the variable going out of scope at the closing bracket, so you have to have the declaration outside my own way of working with let and const is to just always use const and then my IDE yells at me if I assign to it. At that point, you think "Hmm, should I be assigning to this variable?" and change it to let as needed

Faker•3w ago

yeah I see, JS is block-level scope I think...that's why we should declare the variable test outside of the block ?

Jochem•3w ago

yeah

Faker•3w ago

Yep I see, it's clearer now, ty !!

Jochem•3w ago

there's cases with for or while loops too, and a bunch of others with callbacks and closures, but it's usually a scope issue

ἔρως•3w ago

true, but it's easier to use constants when you know you need them, instead of using them for everything

Jochem•3w ago

And safer to use them by default

ἔρως•3w ago

i agree but to achieve "true" type safety, its better to just use typescript

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const keyword vs let keyword in JavaScript