Operators and Expressions

Operators are symbols that perform operations on values. An expression is any piece of code that produces a value. Mastering operators is essential for writing logic in JavaScript.

Arithmetic Operators

let a = 10;
let b = 3;

console.log(a + b);   // 13  addition
console.log(a - b);   // 7   subtraction
console.log(a * b);   // 30  multiplication
console.log(a / b);   // 3.333...  division
console.log(a % b);   // 1   remainder (modulo)
console.log(a ** b);  // 1000  exponentiation (10 to the power 3)

The % (modulo) operator is useful for checking divisibility:

if (number % 2 === 0) {
  console.log("Even number");
}

Comparison Operators

console.log(5 == "5");   // true  (loose equality — coerces types)
console.log(5 === "5");  // false (strict equality — no coercion)
console.log(5 != "5");   // false (loose not-equal)
console.log(5 !== "5");  // true  (strict not-equal)
console.log(10 > 5);     // true
console.log(10 < 5);     // false
console.log(10 >= 10);   // true
console.log(10 <= 9);    // false

Always prefer === and !== over == and != to avoid confusing type coercions.

Logical Operators

// && (AND) — true only if both sides are true
console.log(true && false);  // false
console.log(true && true);   // true

// || (OR) — true if either side is true
console.log(false || true);  // true
console.log(false || false); // false

// ! (NOT) — inverts the value
console.log(!true);   // false
console.log(!false);  // true

Short-Circuit Evaluation

// && stops at first falsy value
let user = null;
let name = user && user.name;  // null (short-circuits, never reads .name)

// || returns first truthy value
let displayName = user || "Guest";  // "Guest"

// ?? (nullish coalescing) — only falls back on null/undefined
let count = 0;
let result = count ?? 10;   // 0  (0 is not null/undefined)
let result2 = count || 10;  // 10 (0 is falsy, so falls through)

Assignment Operators

let x = 10;
x += 5;   // x = x + 5  => 15
x -= 3;   // x = x - 3  => 12
x *= 2;   // x = x * 2  => 24
x /= 4;   // x = x / 4  => 6
x **= 2;  // x = x ** 2 => 36
x %= 7;   // x = x % 7  => 1