More.md

What are loops ?

Lets say you want to print "Hello World" 5 times. Your first instinct would be to console.log the string 5 times. But what if there would be a simple way to print this multiple times with just a few lines of code, this is where loops comes into play. They would help you to do repeated tasks/opertaions with just a few lines of code.

For Example :

// Instead of writing this :
console.log('Hello World!');
console.log('Hello World!');
console.log('Hello World!');
console.log('Hello World!');
console.log('Hello World!');

// You can write this :
for(var i = 1; i<=5; i++){
    console.log('Hello World');
}

Different types of loops :

• for - It loops through a block of code required number of times
• for/in - It loops through the properties of an object
• for/of - It loops through the values of an iterable object
• while- It loops through a block of code until a specific condition has not falsified
• do/while - It also loops a block of code until a specific condition has not falsified

1. For Loop:

Syntax of for loop :

for(intialization; condition; updation){
    //Block of code
}

• Initialization - This statement will execute only at the start of the loop
• Condition - This statement will set a condition which will be checked before the start of every loop and the loop will run until it gets falsified
• Updation - This statement will be executed after the completion of every loop

For Example :

let sum = 0;
//Sum of first 100 natural numbers :
for(var i = 1; i<=100; i++){
    sum += i;
}
console.log("The sum of first 100 natural numbers : " + sum);

Output :

The sum of first 100 natural numbers : 5050

Curious Corner :

Loop Scope : Using var in a loop and let in a loop may give different results outside the loop

For Example :

var i = 10;

for (var i = 0; i < 20; i++) {
 // block of code
}
// Here i is 20

let i = 10;

for (let i = 0; i < 20; i++) {
 // block of code
}
// Here i is 10

The reason is var re-declares the variable outside the scope of loop, this is alos known as Hoisting in JS. But let do not declares the variable outside the loop and thus the scope of let is limited within the paranthesis of the loop

2. For In Loop:

Syntax of for in loop :

for(key in object){
    //Block of code
}

• key - You can imagine it as the index value of each element in an array. But in case of objects, we have key-value pairs associated with every entry in the object. Thus we can iterate over every entry of an object using it's keys.

For Example :

• For in over objects

const student = {name:"Gurpreet", roll_no:"101", age:19};

for (let key in student) {
  console.log("Student "+key+" is "+student[key]);
}

Output :

Student name is Gurpreet
Student roll_no is 101
Student age is 19

• For in over arrays

const arr = [1,2,3,4,5];
let sum = 0;

for (let index in arr) {
  sum += arr[index];
}

console.log("The array sum is : " + sum);

Output :

The array sum is : 15

Curious Corner :

• Do not use for in over an array if the index order is important.

For Example :

var obj = {
"first":"first",
"2":"2",
"34":"34",
"1":"1",
"second":"second"
};

for (let key in obj) {
 console.log("Index: "+key+" and its value is: "+obj[key]);
}

Output:

Index: 1 and its value is: 1
Index: 2 and its value is: 2
Index: 34 and its value is: 34
Index: first and its value is: first
Index: second and its value is: second

• It is better to use a for loop, a for of loop, or Array.forEach() when the order is important.

3. For of Loop:

Syntax of for of loop :

for (value of iterable) {
  // block of code
}

• value - For every iteration the value of the next property is assigned to the variable
• iterable - An object which can be iterated

For Example :

• For of over arrays

const arr = ["Mango", "Orange", "Apple"];
let fruits = "";

for (let value of arr) {
  fruits += value + " ";
}

console.log("My favorite fuits are : " + fruits);

Output :

My favorite fuits are : Mango Orange Apple

• For of over strings

let str= "Attack on Titan";

let text = "";
for (let x of str) {
  if(x==' ')
    x='*';
    
  text += x;
}
console.log(text);

Output:

Attack*on*Titan

4. While Loop:

Syntax of while loop :

while (condition) {
  // code block to be executed
}

• condition - Loop will execute until the condition has been falsified

For Example :

let i = 1;

// Printing first 5 natural numbers
while (i <= 5) {
  console.log(i+" ");
  i++;
}

Output :

1 
2 
3
4
5

5. Do While Loop:

Syntax of do while loop :

do {
  // block of code
}
while (condition);

• condition - Loop will execute until the condition has been falsified

Note : do while loop is different from while loop because do while loop checks the condition after the execution of each loop.

For Example :

let students = ["Gurpreet", "Kailash", "Anant", "Pratul"];
let i=0;
do
{
  console.log(students[i]);
  i++;
}
while(i<4)

Output :

Gurpreet
Kailash
Anant
Pratul

Curious Corner :

You can iterate over an array with the help of Array.forEach() method as well instead of using loops. It was introduced with ES5 version.

Syntax :

array.forEach(function(currentValue, index))

• currentValue - Value of the current element
• index - Index of the current element

For Example :

let sum = 0;
const numbers = [1, 2, 3, 4];
numbers.forEach(myFunction);

function myFunction(value) {
 sum += value;
}
console.log("The sum is : "+sum);

Output :

The sum is : 10

Contributors: Harsh Mathur and Gurpreet Singh