ProgramGuru

Logical Operators: AND, OR and NOT

A permission can depend on more than one answer

A school trip requires both a signed form and an available seat. Either a parent or a guardian may sign, and a blocked account should not be admitted. These rules combine separate yes-or-no answers.

AND requires both conditions, OR requires at least one, and NOT reverses a boolean answer. JavaScript uses &&, || and !.

  1. Translate a rule

    Choose AND for joint requirements and OR for accepted alternatives.

  2. Trace evaluation

    Explain why the second condition may not run.

  3. Handle operands

    Distinguish boolean results from the values returned by JavaScript && and ||.

Both trip requirements must hold

  1. Signed form

    The consent requirement is satisfied.

    true

  2. Seat available

    No seat remains.

    false

  3. Combine with AND

    One failed requirement prevents qualification.

    true && false → false
Read these stages alongside the complete example. They explain the relationship; they do not execute code.

OR means at least one, not exactly one

With boolean operands, AND is true only when both are true. OR is true when one or both operands are true. NOT changes true to false and false to true. A rule requiring exactly one alternative needs a different expression.

Group conditions to show the intended policy. “A parent or guardian signed, and a seat is free” becomes a grouped OR followed by AND. Read each component before reading the combined expression.

Before starting, review comparisons that produce booleans. These examples use JavaScript; other languages can use different syntax and rules.

Read the permission rule in pieces

The signed parent or guardian condition is true because the guardian signed. The seat condition is false, so the combined AND is false.

NOT blocked is true because blocked is false. It answers a separate question; it does not override the missing seat.

JavaScript · local teaching example

const parentSigned = false;
const guardianSigned = true;
const seatAvailable = false;
const blocked = false;
console.log((parentSigned || guardianSigned) && seatAvailable);
console.log(!blocked);

Expected output

false
true

Some conditions never need to run

JavaScript evaluates these expressions from left to right. With false on the left of AND, the whole condition cannot become true, so the function on the right is not called.

The function below prints when it runs. After the first expression, only false appears. In the second, true on the left means the right condition is needed, so “Seat checked” appears before the final true. This is called short-circuit evaluation.

JavaScript · local teaching example

function checkSeat() {
  console.log('Seat checked');
  return true;
}
console.log(false && checkSeat());
console.log(true && checkSeat());

Expected output

false
Seat checked
true

JavaScript can return an operand instead of a boolean

JavaScript || returns its first truthy operand, or its final operand if none is truthy. && returns its first falsy operand, or its final operand if all are truthy. These results are not automatically booleans.

Here a blank name uses the fallback text. Zero also falls back under ||, which can be wrong for a valid zero mark. Nullish coalescing ?? falls back only for null or undefined, preserving zero.

JavaScript · local teaching example

console.log('' || 'Unnamed student');
console.log(0 || 10);
console.log(0 ?? 10);

Expected output

Unnamed student
10
0

Apply the rule to a small change

A guardian has signed and a seat is available, but the student is blocked. Write the complete eligibility rule.

Worked answer

Require a signed parent or guardian, an available seat and NOT blocked. The first two requirements pass, but the blocked condition prevents qualification.

JavaScript · local teaching example

const parentSigned = false;
const guardianSigned = true;
const seatAvailable = true;
const blocked = true;
console.log((parentSigned || guardianSigned) && seatAvailable && !blocked);

Expected output

false

Recognize mistakes and limits

A right-hand side skipped by short-circuiting does not perform its side effects. Avoid placing essential saving or logging only there unless conditional execution is intended.

Bitwise & and | are different from logical && and ||. Also distinguish a fallback for missing data from a fallback for every falsy value, especially zero and false.

For the language or tool rules used here, see JavaScript operator rules.

Check the core idea

Which operator matches “both a signed form and a free seat”?
Choose one answer. You can try again.

Answer and explanation

&&

Yes. AND requires both boolean conditions.

Apply the idea to a new situation

JavaScript · code for this question

function checkSeat() {
  console.log('Seat checked');
  return true;
}
console.log(false && checkSeat());
console.log(true && checkSeat());
Does false && checkSeat() call checkSeat in JavaScript?
Choose one answer. You can try again.

Answer and explanation

No, the false left operand already determines the result.

Yes. AND short-circuits, so the right-hand call is skipped.

Combine requirements without hiding their meaning

Use AND for joint requirements, OR for allowed alternatives and NOT to reverse a condition. Group the intended policy and remember that short-circuiting can skip work. JavaScript operand-return rules need care outside boolean inputs.

Connect this idea with choosing a branch and truthiness and fallback values.

Reaching the recap and completing the quizzes are separate milestones.

    Next lesson: Assignment and Compound Assignment Operators →