Exercise 8 - JavaScript Notes

Exercise 8: Sum of Multiples of 3 and 5

Here is another simple but great exercise that trains your programming brain.

Exercise Description

I want you to create a function called sum. We give it a limit, and this function will return the sum of all the multiples of 3 and 5 from 0 up to this limit.

Example Scenario:

What are the multiples of 3 and 5 between 0 and 10?

Multiples of 3: 3, 6, and 9
Multiples of 5: 5 and 10 (We are including the number passed as the limit)

If you add these numbers together (3 + 6 + 9 + 5 + 10), the result will be 33. That is the goal of this function.

Solution:

1. Initialize the Loop

We need to start with a for loop. Let i equal 0. As long as i is less than or equal to the limit, we want to increment i.

for (i=0;i<=limit;i++){
    // logic goes here
}

2. check for Multiples

Now, we want to check to see if i is a multiple of 3 or 5 using the modulus operator (%).

If i % 3 === 0 OR (||)
If i % 5 === 0

3. Calculate the Sum

If the condition is met, we need to take i and add it to our sum. To do this, we first need to declare a variable sum and set it to 0.

4. Return the Result

Finally, we return the sum.

console.log(sum(10));

function sum(limit) {
    let sum = 0;

    for (let i = 0; i <= limit; i++) {
        if (i % 3 === 0 || i % 5 === 0)
            sum += i;
    }

    return sum;
}

Expected Output:

Note: If we save the changes with a limit of 10, we get 33 in the console.

Best Practice: Code Formatting and Readability

Now, pay attention to how I have formatted this code.

I have added extra vertical line breaks to separate the initialization from the actual logic, and from the return value.

function sum(limit) {
    let sum = 0; // Initialization
                 // <--- Vertical Break
    for (let i = 0; i <= limit; i++) {
        if (i % 3 === 0 || i % 5 === 0)
            sum += i; // Logic
    }
                 // <--- Vertical Break
    return sum;  // Return Value
}

Why do we do this?

If we didn't have these vertical breaks, this code would look a little bit squashed, and it becomes a little bit hard to read.

As a best practice, it's always good to separate the last line—the return statement.

To understand the concept behind (&&) and (||)

The reason we use the OR (||) operator instead of the AND (&&) operator comes down to exactly which numbers we want to include in our sum.

Here is the breakdown:

1. The Goal

We want to find numbers that belong to either group:

The group of numbers divisible by 3 (e.g., 3, 6, 9...)

The group of numbers divisible by 5 (e.g., 5, 10...)

2. Why || (OR) works

The || operator returns true if at least one side is true.

Take the number 3:

Is it divisible by 3? Yes.

Is it divisible by 5? No.

Result: Since one is true, we keep it.

Take the number 5:

Is it divisible by 3? No.

Is it divisible by 5? Yes.

Result: Since one is true, we keep it.

3. Why && (AND) would fail

The && operator returns true only if BOTH sides are true simultaneously.

Take the number 3 again:

Is it divisible by 3? Yes.

Is it divisible by 5? No.

Result: True && False is False. The number 3 would be skipped.

If you used &&, you would only sum numbers that are multiples of 15 (because $3 \times 5 = 15$). So, between 0 and 10, your result would be 0 because no number in that range is divisible by both 3 and 5 at the same time.

JavaScript Notes