Logical

and, or, not

const age = 20;
const hasTicket = true;

console.log(age >= 18 && hasTicket);
console.log(age < 18 || hasTicket);
console.log(!hasTicket);

true
true
false

They return values, not booleans

This is the part worth understanding. && and || do not produce true or false: they produce one of the operands.

console.log('a' && 'b');
console.log('' && 'b');
console.log('' || 'fallback');
console.log('set' || 'fallback');

b

fallback
set

&& returns the first falsy value, or the last one if none are falsy. || returns the first truthy value, or the last one if none are truthy.

Short circuiting

Both stop as soon as the answer is known, so the right side may never run. That is useful and occasionally surprising.

function shout() {
  console.log('this ran');
  return true;
}

false && shout();
true || shout();

console.log('nothing above printed');

nothing above printed

It is also how a guard is written: user && user.name never touches name when user is missing.

Nullish coalescing

?? looks like || but only steps in for null and undefined. That difference matters whenever zero or an empty string is a real answer.

const count = 0;

console.log(count || 10);
console.log(count ?? 10);

10
0

Combining them

&& binds tighter than ||, and mixing ?? with either without brackets is a syntax error, which the language does on purpose to stop ambiguous code.

const isAdmin = false;
const isOwner = true;
const isBanned = false;

console.log((isAdmin || isOwner) && !isBanned);

true