3.4 Expressions and Operators

An expression combines values, variables, and operators to compute a result. Programs use expressions to calculate, compare, join text, and update data.

Arithmetic operators

JavaScript supports common arithmetic operations:

• +: addition
• -: subtraction
• *: multiplication
• /: division
• %: remainder after division
• **: exponentiation

console.log(21 / 4);       // 5.25
console.log(22 % 3);       // 1
console.log(2 ** 3);       // 8

Unlike Python's //, JavaScript does not have a separate floor-division operator. Use Math.floor() when you need the integer part of a division result.

console.log(Math.floor(21 / 4));  // 5

Reversing a three-digit integer

Floor division and modulo are often used together to split an integer into digits.

let num = Number(prompt("Enter a 3-digit integer:"));

let a = Math.floor(num / 100);
let b = Math.floor(num / 10) % 10;
let c = num % 10;

console.log("Reversed:", c * 100 + b * 10 + a);

One possible run:

Enter a 3-digit integer: 520
Reversed: 25

The result is displayed as 25 because the reversed number is 025, and leading zeros are not shown when printing a number.

Compound operators

Compound assignment operators make variable updates shorter.

• a += b means a = a + b
• a -= b means a = a - b
• a *= b means a = a * b
• a /= b means a = a / b
• a %= b means a = a % b

JavaScript also has increment and decrement:

i++;  // i = i + 1
i--;  // i = i - 1

String concatenation

+ is not only for numbers. It can also join strings.

let s = "Hello" + "World";
s += "!";
console.log(s);

Output:

HelloWorld!

When one side is a string, + tends to concatenate. This is why "18" + 1 becomes "181". Convert text to a number first when you want arithmetic.