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Loops In Javascript

The purpose of this article is to explain loops in JavaScript.

Purpose Of Loops

The purpose of the loop is to execute a statement or code block repeatedly till the boundary condition is not reached.

Types Of Loops

There are 4 types of loops: -

For loop
While loop
Do while loop
For in loop

For Loop

The For loop executes a code block a specified number of times.

Syntax Of For Loop

for (variable_name = start_value; variable_name <= end_value; variable_name = variable_name + increment_value) {
	//	Code of block to be executed
}

Consider a scenario where we want to print a number from 1 to 10 then the for loop will be: -

for (i = 1; i <= 10; i = i + 1) {
	//	Code of print numbers
}

Try It

Here the loop starts with the value i as 1. The value is incremented by 1 after every iteration till the value of i reaches 10. The table below explains how the for loop works.

Value of variable i	Is condition i <= 10 satisfied	Action
1	No	Execute the code block and increment the value of i
2	No	Execute the code block and increment the value of i
3	No	Execute the code block and increment the value of i
4	No	Execute the code block and increment the value of i
5	No	Execute the code block and increment the value of i
6	No	Execute the code block and increment the value of i
7	No	Execute the code block and increment the value of i
8	No	Execute the code block and increment the value of i
9	No	Execute the code block and increment the value of i
10	Yes	Exit loop

While Loop

While loop executes a code block until a particular condition is not satisfied.

Syntax Of While Loop

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

If the above scenario needs to be executed from a while loop, then the code becomes: -

i = 1;
while (i <= 10) {
	// Code to execute
	i++;
}

Try It

Do-While Loop

The do-while loop is like a while loop. The difference is that a while loop first checks the condition, and if it evaluates to true, the code inside the loop is executed.

But in the do-while loop, the code segment inside the do block gets executed at least once before checking the condition. This loop executes the code block until the condition mentioned in the while loop becomes false.

Syntax Of Do-While Loop

do {
	//    code to be executed
}
while ( condition );

In the above syntax, the code inside the do block gets executed and after the execution, the condition inside the while is checked. If the condition is true, then again, the code inside the do block executes otherwise the loop exits.

Consider the code below: -

var i = 2;
do {
	//  Code to execute
}
while ( i == 1 );

Try It

In the above code: -

Code inside the do block gets executed
While the condition is checked. Condition fails
Loop exits

For-In Loop

For-in loops through the properties of an object.

Syntax Of For-In Loop

for (variable_name in object) {
	//    code to be executed
}

Consider the following code: -

var obj = new Object();
obj['name'] = 'karan';
obj['address'] = 'India';
obj['tech'] = '.net';

Here obj is a variable of type Object. In the object, we have three properties i.e. name, address, and tech, and value is assigned to each property. See the figure below.

Picture showing a sample object in JavaScript with attributes

for in loop given below prompts the following alerts name, address, and tech.

for (x in obj) {
	alert(x);
	// alert(obj[x]); // this will prompt karan, India and .net
}

Try It

Posted By -	Karan Gupta

Posted On -	Monday, July 16, 2012

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