Check if one Number is Multiple of another in JavaScript

# Check if One Number is Multiple of Another

Use the modulo % operator to check if one number is a multiple of another number.

The operator returns the remainder when one number is divided by another number. The remainder will only be zero if the first number is a multiple of the second.

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const num1 = 15;
const num2 = 5;

const remainder = num1 % num2;
console.log(remainder); // 👉️ 0

if (remainder === 0) {
  // ✅ This runs
  console.log('👉️ num1 is a multiple of num2');
} else {
  console.log('⛔️ num1 is NOT a multiple of num2');
}

We used the modulo operator to get the remainder of dividing one number by another.

Here are some more examples of using the modulo % operator.

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console.log(13 % 5); // 👉️ 3
console.log(12 % 5); // 👉️ 2
console.log(11 % 5); // 👉️ 1
console.log(10 % 5); // 👉️ 0

The number 10 is a multiple of 5, so the division has a remainder of 0.

The modulo % operator always takes the sign of the first number (the dividend).

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console.log(-13 % 5); // 👉️ -3
console.log(12 % -5); // 👉️ 2

You might be wondering what happens if we get a remainder of -0.

Would the if condition fail because we only check for 0.

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console.log(-15 % 3); // 👉️ -0

However, 0 is equal to -0 in JavaScript so our condition would still work.

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console.log(0 === -0); // 👉️ true

# Creating a reusable function

If you need to check if a number is a multiple of another number often, extract the logic into a reusable function.

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function checkIfMultiple(num1, num2) {
  return num1 % num2 === 0;
}

console.log(checkIfMultiple(15, 5)); // 👉️ true
console.log(checkIfMultiple(15, 4)); // 👉️ false
console.log(checkIfMultiple(15, 3)); // 👉️ true

if (checkIfMultiple(15, 5)) {
  // ✅ This runs
  console.log('👉️ num1 is a multiple of num2');
} else {
  console.log('⛔️ num1 is NOT a multiple of num2');
}

The checkIfMultiple() function takes 2 numbers as parameters and checks if the first number is a multiple of the second.

# Checking if a number is not a multiple of another number

If you need to check if a number is not a multiple of another number, check if there is a remainder after using the modulo operator.

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function checkIfNotMultiple(num1, num2) {
  return num1 % num2 !== 0;
}

console.log(checkIfNotMultiple(15, 5)); // 👉️ false
console.log(checkIfNotMultiple(15, 4)); // 👉️ true
console.log(checkIfNotMultiple(15, 3)); // 👉️ false

if (checkIfNotMultiple(15, 5)) {
  console.log('⛔️ num1 is NOT a multiple of num2');
} else {
  // ✅ This runs
  console.log('👉️ num1 is a multiple of num2');
}

The checkIfNotMultiple() function takes 2 numbers as parameters and checks if the first number is not a multiple of the second.