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Types of Javascript Loops

For Loop Syntax

for loop is used to repeat block of code and perform different task with same code while condition is true. For loop has three section-

  1. initialize - initializing the variables and it executes only once when loop start.
  2. condition or expression - checking the condition in every interation.
  3. update the condition 
    for(initialize; condition; update)
{
    // repeating code 
}

Example

here is the example of for loop to print the number 0 to 10.

//printing number using for loop upto 10 (start with 0 and end with 10)
for (let i = 0; i < 11; i++) {
    document.write(i+" ");         
}

While Loop

while loop is used to execute the block of code while condition is true. In the while parenthesis we have to specify the condition. Loop must be terminate after performing the task. Infinite loop may freeze your pc.

while (condition) {
//    code 
}

Example Of While Loop

In this example, i variable is initialized with 1 and condition is i 'less than or equal to 50' and increment the variable i to update the value to print the number upto 50.

let i = 1;  //initialize a variable
while (i <= 50)   // condition
{
    document.write(" ", i);
    i++;  // increment 
}

Do While Loop

do while loop is same as while loop but it has conditional statement at the end. First it will execute the code then check the condition. syntax is here

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

Example:

print the value of variable 'a' as long as it is not equal to 10.

let a = 1;
do {
    document.writeln(a);
    a= a+1;   //increment
} while (a !=10);  //condition

For In Loop

for in loop is specially used for iterable object. It returns a key in every interation . Using their key we can access value. for in loop used to print the value of a object as long as it's key is available is object. syntax is here

for (let key in object) {
    let element = object[key];
    // code 
}

Example:

In this example accessing the key and value of an object using for in loop.

// course is an object which has key and value 
const course = {100:"javascript", 200:"php", 300:"python", 400:"c++" ,500:"java"};
for (let id in course) 
{    // in every iteration id contain the key of object course such as 100, 200, 300, 400, 500
        document.writeln(course[id]);
}

Accessing Array Element Using For In Loop

let array = [10, 20, 30, 40, 50];
for (let index in array) {
    document.write(array[index]);
}

For Of Loop

for of loop is used in iterable data structure such as array, string, map and more. It return the value of iterable object.

let array = [10, 20, 30, 40, 50];
// for of loop return value of array in every iteration
for (let value of array)  
{
    document.write(" ",value);
}

Example Of For Of Loop

In this example accessing value of an iterable object using for of loop.

 // language is an object which has key and value 
const language = ["javascript", "php", "python", "c++", "java"];
for (let value of language) {    // in every iteration value contain the value of object 
    document.write(" ", value);
}