Scopes & Reassigning

Const and Let inside of scopes and reassigning them

Constant scope

The constant variable is defined outside the if-statement but can still be accessed inside of it.

index.js

const variable = 'test';

if (true) {
  console.log(variable);
} 

terminal

$ node .
"test"

Reassign in different scope

In here, variable is defined at the top of your code, and is reassigned a different value inside an if-statement, the new value is still accessable outside (and bellow) the if-statement.

Truely if-statement

index.js

let variable = 'test';

if (true) {
  variable = 'maybe';
}

console.log(variable);

terminal

$ node .
"maybe"

Falsely if-statement

index.js

let variable = 'test';

if (false) {
  variable = 'maybe';
}

console.log(variable);

terminal

$ node .
"test"

Constant in wrong scope

The constant variable is defined in a lower scope than the console.log and is therefor not accessable and will throw a run time error.

index.js

if (false) {
  const variable = 'maybe';
}

console.log(variable);

terminal

$ node .
Uncaught ReferenceError: variable is not defined

Reassigning

Giving a variable a new value is not possible with const, only with let. Even if this sounds like an advantage for the let-keyword, you shouldn't use it if you don't need to as it uses more ram.

const without reassigning

const username = "Luna"; ✅

const reassigning

const username = "Luna";
username = "mwlica"; // ❌

let without reassinging

let username = "Luna"; // ❌

let reassigning

let username = "Luna";
username = "mwlica"; // ✅