Operators

Arithmetic, comparison, logical, and assignment operators the building blocks of expressions.

Overview

Operators are symbols that perform operations on values (called operands). JavaScript has a rich set of operators covering arithmetic, comparison, logical operations, assignment, and more. Arithmetic operators (+, -, *, /, %) work on numbers. The + operator is special: when used with strings it performs concatenation instead. This dual behavior is a common source of bugs when types are mixed unexpectedly. Comparison operators return a boolean (true or false). Always use strict equality (===) and strict inequality (!==) rather than loose equality (==, !=) which does type coercion. For example, 0 == false is true due to coercion, while 0 === false is false. Logical operators && (AND), || (OR), ! (NOT) combine boolean expressions. In modern JS they also have useful short-circuit behavior used for default values and conditional execution.

Key Points

•Arithmetic: + - * / % (modulo) ** (exponentiation)
•Always use === and !== for comparisons to avoid type coercion surprises
•&& returns the first falsy value, or the last value if all are truthy
•|| returns the first truthy value, or the last value if all are falsy
•?? (nullish coalescing) returns the right side only when the left is null or undefined useful for defaults
•Compound assignment: +=, -=, *=, /=, %= are shorthands for updating a variable

Examples

Arithmetic operators

console.log(10 + 3);   // 13
console.log(10 - 3);   // 7
console.log(10 * 3);   // 30
console.log(10 / 3);   // 3.3333...
console.log(10 % 3);   // 1  (remainder)
console.log(2 ** 8);   // 256 (exponentiation)

Comparison and strict equality

console.log(5 > 3);    // true
console.log(5 < 3);    // false
console.log(5 >= 5);   // true
console.log("5" == 5); // true  ← coercion, avoid
console.log("5" === 5);// false ← strict, use this
console.log(5 !== 3);  // true

Logical operators and short-circuiting

console.log(true && false);  // false
console.log(true || false);  // true
console.log(!true);          // false

// Short-circuit uses:
const name = null;
console.log(name ?? "Guest");    // "Guest" (null/undefined fallback)
console.log(name || "Guest");    // "Guest" (any falsy fallback)

?? only falls back for null/undefined. || falls back for any falsy value (0, "", false, NaN too).

Common Mistakes

Using = (assignment) where you meant === (comparison) inside conditions
Using + to add a number and a string "5" + 3 gives "53", not 8
Using || for defaults when 0 or "" should be valid values use ?? instead

Operators

Overview

Key Points

Examples

Common Mistakes

Exercises

Knowledge Check

What does "5" + 3 evaluate to in JavaScript?