Combining Conditions

You already know &&, || and ! — they take booleans and give back a boolean. An
if wants a boolean. So they go straight into the parentheses.

This AND That

let age = 25;
let hasTicket = true;

if (age >= 18 && hasTicket) {
  letThemIn();
} else {
  turnThemAway();
}

&& needs both sides true. Flip hasTicket to false and the train goes right, no
matter how old they are.

This OR That

let day = "Sunday";

if (day === "Saturday" || day === "Sunday") {
  sleepIn();
} else {
  setAlarm();
}

|| only needs one side. The first comparison fails, the second passes, so it’s a lie-in.

Not

let isOpen = false;

if (!isOpen) {
  showClosedSign();
}

! flips it, so !false is true. Read it out loud: “if it is not open.” Shorter and
clearer than isOpen === false.

The Trap: Repeat the Whole Comparison

if (day === "Saturday" || "Sunday") {          // wrong — always true
if (day === "Saturday" || day === "Sunday") {  // right

|| needs a full comparison on each side. "Sunday" alone is just a non-empty string,
and that’s always truthy — so the first version runs no matter what day it is.