JavaScript Fundamentals: For Loop and Modulus Operator

JavaScript Fundamentals: For Loop and Modulus Operator

Day 6 of #100DaysOfCode

Today is the 6th day of my #100DaysOfCode journey with JavaScript.

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This Article is a part of the JavaScript Fundamentals series.


Loops provide a quick and simple way to repeat an action. Programmers take advantage of this speed by creating scripts that do a task repeatedly until a predetermined condition is satisfied.

In pseudocode this might look a little like this:

while we have book
    read book
    while we have pages in this book
        read page
            while we have words on the page
                read word

For Loop

A for loop repeats until a specified condition evaluates to false.

  • Summation of numbers example:

Let's think about how to repeatedly add the digits 1, 2, 3, and 4: The first step is to add 1 to a sum. Then we go to 2, add this to the total, and so on until we get to 4. An iteration occurs each time we add to the total. When the value is more than 4, we iterate until we reach our exit condition.

let sum = 0;
for(let i = 1; i <= 4; i++) { 
    sum = sum + i; 

The for loop can be broken down into the initialization, condition, update and statement: Compare 👇🏼with👆🏼

for ([initialization]; [condition]; [update]) {

The Initialization is run once at the beginning of the loop. The Condition is checked before each iteration. The Update is run at the end of each iteration. The Statement is run as long as the Condition is true.

  • Factorial of numbers example:

In mathematics, a factorial is often denoted with an exclamation mark !. The sum of all positive integers greater than 0 up to and including the factorial number n is known as a factorial.

Let's take a look at a few examples of factorials:

4! = 4 * 3 * 2 * 1 = 24

3! = 3 * 2 * 1 = 6

2! = 2 * 1 = 2

Example: Taking in some integer value n, find the factorial for that number and return it.

function factorial(n){
let result = 1;
for (let i = 1; i <= n; i++){
result = result * i; // or result *= i;
return result;
  • Strings in loops example:

Let's add some exclamation marks to "Hello World"!

let str = "Hello World";
for(let i = 1; i <= 3; i++) {
    str += "!";
console.log(str); // Hello World!!!

Example: Let's create a function scream which will take in a value n and return a string with the letter "a" repeated that many times like scream(5); // "aaaaa"

function scream(n) {
    let str = "";
    let i = 1;
    for (let i = 1; i<=n; i++){
        str += "a"
    return str;

Modulus Operator

% operator is called the modulus operator. It will tell us the remainder of a division. When you divide 9 by 2 you get. Or 4 with a remainder of 1. The expression 9 % 2 evaluates to that remainder: 1

Let's take a look at a few examples:

console.log(8 % 3) // 2
console.log(9 % 2) // 1
console.log(7 % 4) // 3

Example: Let's modify our function to return a scream alternating between lower and capital case, like these:

console.log( scream(4) ); // aAaA console.log( scream(9) ); // aAaAaAaAa

function scream(n) {
    let str = "";

    for (let i = 1; i <= n; i++) {
        const remainder = i % 2;
        const isEven = remainder === 0;
        if (isEven) {
            str += "A";
        else {
            str += "a";
    return str;


Ending with an extra bit of information about JavaScript functions...

At the beginning of the for loop, the Initialization is once executed. Each iteration begins with a check of the Condition. The Statement will be executed if the expression evaluates to true. If it's untrue, the statement won't execute. Each iteration ends with the Update being executed. For the subsequent execution of the Statement, it may update a variable. As long as the Condition is true, the Statement is then executed. The actual work of the loop is completed here.

Today I learned about For Loop, Modulus Operator in JavaScript.

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