Why do some variables not need var or let or const in front?

JavaScript
#1

when i am doing these lessons ill see that sometimes I need to create a variable and that it only accepts it If i don’t put anything before it, and other times it requires let or var or const in front of it like this:


function fun1() {
oopsGlobal = 5; // Assign 5 to oopsGlobal Here

}

      
```js

// Declare the myGlobal variable below this line
let myGlobal = 10;

function fun1() {
oopsGlobal = 5; // Assign 5 to oopsGlobal Here

}

// Only change code above this line

function fun2() {
var output = "";
if (typeof myGlobal != "undefined") {
  output += "myGlobal: " + myGlobal;
}
if (typeof oopsGlobal != "undefined") {
  output += " oopsGlobal: " + oopsGlobal;
}
console.log(output);
}

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Challenge: Global Scope and Functions

Link to the challenge:

#2

Can you be mores specific? What line?

If a variable is already declared, you don’t need a var/let/const. If you are not in strict mode, I think you can use a variable without declaring it, but it is bad practice.

#3

You don’t need to use var/let/const to declare a variable, at least outside of strict mode, but you absolutely should use them. And really you should only be using let and const because they are better about scoping and errors and help you out. But like kevin said in strict mode its not going to like variables without var/let/const and will toss an error, and can lead to buggy code with unintended consequences like the linked test is trying to show. And you don’t need var/let/const if all you are doing is using or assigning an already declared variable.

let something;
console.log(something)//undefined
//declared the variable but did not assign value
something = 10;
console.log(something)//10
//now we are storing 10 in the variable something without using let
something = 20
//still ok because im not re declaring just assigning a new value
let somethingElse = something;
console.log(somethingElse, something)//20 20
//something did not need a let but somethingElse did

Or you can also declare them in a chain with only one let.

let something, anotherThing, somethingElse;
someOtherThing = 'oops im undeclared because im on a new line without a var/let/const' 
something = 10, anotherThing = 20, somethingElse = 30;
console.log(something, anotherThing, somethingElse, someOtherThing)//10 20 30 and a long string

Use let multiple times on variables with the same name, but in different scopes.

let something = 10;//global

{
    let something = 50
console.log(something)//50 in block scope
}

function func(){
    let something = 20;
    console.log(something)//20 in function scope
    {
    let something = 30
    console.log(something)//30 block scoped
    }
    console.log(something)//20 in function scope still
}
console.log(something)//still 10 in global

But doing it twice in the same scope is a no go, and probably why you had problems with it only accepting it without let/const.

let something = 10;
let something = 20;
//not cool because they are in the same scope and tosses an error

And why you should not use var.

var something = 10;
console.log(something)//10 global
var something = 20;
console.log(something)//20 and no error even in strict mode

{
    var something = 30
console.log(something)//30 block scope
}

function func() {
    var something = 50;
    console.log(something)//50 in function
}

console.log(something)//30 from block scope in global