Uncommon Javascript Knowledge 1

#1, !!

Converts Object to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true.

!oObject  // inverted boolean
!!oObject // non inverted boolean so true boolean representation

So !! is not an operator, it's just the ! operator twice.

#2, ??

JavaScript nullish coalescing operator.  
It returns the right operand (rightExpression) if the left operand (leftExpression) is null or undefined.

let count;
let result = count || 1 ;
console.log(result); // 1  

let count = 0;
let result = count || 1; 
console.log(result); // 0  

#3, style your console.log

console.log("%cThis is a green text. %cThis is red text and bold", "color:green", "color:red; font-weight:bold")

#4. short-hand to check and use variable

console.log(user?.user_info?.emailAddress)

or

if(user && user.user_info) {
  console.log(user.user_info.emailAddress)
}

it also works for function or array as well   

yourObj.maybeFunctionName?.( ) 

yourObj.maybeArrayName?.[0]

your function will be called if it is defined and exist. 

#5. object short-hand for name and value. When your key and value are the same variable name, you don't have to repeat it. 

const color = "whatever"
const size = "whatever size is"
var Car = {
  color,
  size,
  notSameName: color
}

var Cart = {
  color: color,
  size: size,
  notSameName: color
}

#6. you should always load the javascript at the head instead of loading in the bottom of the page. To do that, you use the keyword 'defer'

<script src="script.js" defer></script>