4. Operators and Expressions in JavaScript

What Are Operators?

Operators are special symbols that tell JavaScript to perform an action — like adding numbers, comparing values, or joining strings.

An *expression* is any piece of code that produces a value. For example, `2 + 2` or `name + ' is learning JS'` are both expressions.

Arithmetic Operators

• `+` — Addition
• `-` — Subtraction
• `*` — Multiplication
• `/` — Division
• `%` — Modulus (remainder after division)
• `**` — Exponentiation (power)

let a = 10;
let b = 3;

console.log(a + b);  // 13
console.log(a - b);  // 7
console.log(a * b);  // 30
console.log(a / b);  // 3.333...
console.log(a % b);  // 1
console.log(a ** b); // 1000

String Concatenation

You can use the `+` operator to join (concatenate) strings together.

let firstName = "Ada";
let lastName = "Lovelace";

console.log(firstName + " " + lastName);  // Ada Lovelace

Assignment Operators

Assignment operators store values in variables, sometimes while performing an operation at the same time.

• `=` — Assign a value
• `+=` — Add and assign
• `-=` — Subtract and assign
• `*=` — Multiply and assign
• `/=` — Divide and assign

let x = 10;
x += 5;  // same as x = x + 5
console.log(x); // 15

Comparison Operators

Comparison operators are used to compare two values and return a Boolean (`true` or `false`).

• `==` — Equal to (compares value only)
• `===` — Strict equal to (compares value and type)
• `!=` — Not equal to (value only)
• `!==` — Strict not equal to (value and type)
• `>` — Greater than
• `<` — Less than
• `>=` — Greater than or equal to
• `<=` — Less than or equal to

console.log(5 == '5');   // true (same value)
console.log(5 === '5');  // false (different type)
console.log(8 > 3);      // true
console.log(4 <= 2);     // false

Logical Operators

Logical operators are used to combine or invert conditions. They’re often used in `if` statements.

• `&&` — AND (both must be true)
• `||` — OR (at least one must be true)
• `!` — NOT (reverses the result)

let age = 20;
let hasID = true;

console.log(age >= 18 && hasID); // true
console.log(age < 18 || hasID);  // true
console.log(!hasID);             // false

Type Coercion Gotchas

JavaScript can automatically convert types in some cases — this is called *type coercion*. It can cause unexpected results if you’re not careful.

console.log('5' + 5); // '55' (string concatenation)
console.log('5' - 2); // 3   (string converted to number)
console.log(true + 1); // 2   (true becomes 1)

Mini Challenge

Write a small script that checks if someone is old enough to vote. If their age is 18 or more, log 'You can vote!'. Otherwise, log 'You’re too young.'

// Example:
let age = 17;

if (age >= 18) {
  console.log("You can vote!");
} else {
  console.log("You're too young.");
}

Key Takeaway

Operators form the logic engine of your code. They let you combine, compare, and evaluate data — powering everything from math to decisions.

MDN Docs: Operators and Expressions