How Can I Use For Loop In Java Script With Examples

In Java Script A for-loop (or simply for loop) is a control flow statement for specifying iteration, which allows code to be executed repeatedly. So today we use for loops in java script

Different Kinds of Loops

JavaScript supports different kinds of loops:

• for - loops through a block of code a number of times

• for/in - loops through the properties of an object

• for/of - loops through the values of an iterable object

• while - loops through a block of code while a specified condition is true

• do/while - also loops through a block of code while a specified condition is true

The For Loop

The for loop has the following syntax:

for (statement 1; statement 2; statement 3) {
// code block to be executed
}

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

<script>
    var text = "";
    var i;
    for (i = 0; i < 5; i++) {
        text += "The number is " + i + "<br>";
    }
    document.getElementById("demo").innerHTML = text;
</script>

From the example above, you can read:

Statement 1 sets a variable before the loop starts (var i = 0).

Statement 2 defines the condition for the loop to run (i must be less than 5).

Statement 3 increases a value (i++) each time the code block in the loop has been executed.

Statement 1

Normally you will use statement 1 to initialize the variable used in the loop (i = 0).

This is not always the case, JavaScript doesn't care. Statement 1 is optional.

You can initiate many values in statement 1 (separated by comma):

<script>
    var lang = ["JavaScript", "PHP", "Web", "Python"];
    var i, len, text;
    for (i = 0, len = lang.length, text = ""; i < len; i++) {
        text += lang[i] + "<br>";
    }
    document.getElementById("demo").innerHTML = text;
</script>

<script>
    var lang = ["JavaScript", "PHP", "Web", "Python"];
    var i = 2;
    var len = lang.length;
    var text = "";

    for (; i < len; i++) {
        text += lang[i] + "<br>";
    }

    document.getElementById("demo").innerHTML = text;
</script>

Statement 2

Often statement 2 is used to evaluate the condition of the initial variable.

This is not always the case, JavaScript doesn't care. Statement 2 is also optional.

If statement 2 returns true, the loop will start over again, if it returns false, the loop will end.

If you omit statement 2, you must provide a break inside the loop. Otherwise the loop will never end.

Statement 3

Often statement 3 increments the value of the initial variable.

This is not always the case, JavaScript doesn't care, and statement 3 is optional.

Statement 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.

Statement 3 can also be omitted (like when you increment your values inside the loop):

<script>
    var lang = ["JavaScript", "PHP", "Web", "Python"];

    var i = 0;
    var len = lang.length;
    var text = "";

    for (; i < len; ) {
        text += lang[i] + "<br>";
        i++;
    }
    document.getElementById("demo").innerHTML = text;
</script>

The For/In Loop

The JavaScript for/in statement loops through the properties of an object:

<script>
    var txt = "";
    var person = {fname:"Kiran", lname:"V", age:25};
    var x;
    for (x in person) {
        txt += person[x] + " ";
    }
    document.getElementById("demo").innerHTML = txt;
</script>

The For/Of Loop

The JavaScript for/of statement loops through the values of an iterable objects

for/of lets you loop over data structures that are iterable such as Arrays, Strings, Maps, NodeLists, and more.

The for/of loop has the following syntax:
 

for (variable of iterable) {
// code block to be executed
}

variable - For every iteration the value of the next property is assigned to the variable. Variable can be declared with const, let, or var.

iterable - An object that has iterable properties.

Looping over an Array

<script>
    var lang = ["JavaScript", "PHP", "Web", "Python"];
    var x;

    for (x of lang) {
        document.write(x + "<br >");
    }
</script>

Looping over a String

<script>
    var txt = 'JavaScript';
    var x;

    for (x of txt) {
        document.write(x + "<br >");
    }
</script>

Complete code of For Loop In Java Script

<!DOCTYPE html>
<html>
<head>
    <title>JavaScript For Loops</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
</head>
<body>
<div class="container">
    <br>
    <br>
    <div class="text-center">
        <h1>JavaScript For Loops</h1>
    </div>
    <br>
        <div class="well">
            <h1></h1>
            <h1 id="demo"></h1>
        </div><br>
</div>
</body>
</html>
<script>
    var lang = ["JavaScript", "PHP", "Web", "Python"];
    var i, len, text;
    for (i = 0, len = lang.length, text = ""; i < len; i++) {
        text += lang[i] + "<br>";
    }
    document.getElementById("demo").innerHTML = text;
</script>