JavaScript array indexOf

JavaScript array indexOf

The JavaScript array indexOf method, returns the starting position of an element within an array or string.

The syntax of this method is as follows: array.indexOf(element)

Where element is therefore the element whose position you want to know.

As with the includes method, also in this case, you can optionally specify the position from which to start, adding a second parameter as an argument of indexOf.

array.indexOf(element, start).

The method returns a numeric value that corresponds to the position of the element in the array or string.

JavaScript array indexOf – example 1

We know the position of an element in an array using the indexOf () method.

So let’s suppose we have the following array:

var objectsSchools = ['eraser', 'notebook', 'pencil'];

We then apply the indexOf method to know, for example, the position of the notebook element.

var result = objectsSchools.indexOf('notebook');

In the browser console I will display the value 1 which corresponds to the position of the element.

JavaScript array indexOf – example 2

Display all even-numbered elements of an array using the indexOf method.

This algorithm can also be solved in other ways. Here we will use the indexOf method to analyze another scope of the following method.

Let’s create an array of integers. Then, with a for loop, we analyze the position of each element, checking if it is an even number. To do this, we use, as seen in previous years, the modulo (%) operator and calculate the remainder of the division of the number divided by 2.

So only if it is even we display the number, for simplicity, in the browser console.

So here is the example code of the JavaScript indexOf method:

var arrayNumbers = [6,12,34,67,89,98,5,2,7,13];
console.log (arrayNumbers);
for (var i = 0; i < arrayNumbers.length; i ++) {
  if (arrayNumbers.indexOf(arrayNumbers[i]) % 2 == 0) {

We also recall that the same algorithm could be solved simply in this way:

for (var i = 0; i < arrayNumbers.length; i ++) {
  if (arrayNumbers[i] % 2 == 0) {


In this lesson we have dealt with two simple examples of using the JavaScript array indexOf method, in the next lessons I will propose other examples.

Some useful links

JavaScript tutorial

JavaScript Calculator

Object in JavaScript

For in loop

Caesar cipher decoder

Slot Machine in JavaScript

JavaScript array length

JavaScript array length

The length property used on arrays in JavaScript is useful for determining how many elements are contained in an array.

This property is also used to determine the length of a string.

The syntax is very simple: ArrayName.length.

Where with nomeaArray we indicate the array of which you want to calculate the length.

JavaScript array length – example 1

Let’s look at a very simple first example of this JavaScript property.

Given an array, determine its length.

So let’s suppose we have an array containing the following elements:

var schoolObjects = ['eraser', 'notebook', 'pencil'];

We then apply the length property of JavaScript on this array and we will get as a result 3, which corresponds to the number of elements.

var lengthArray = schoolObjects.length;

Consequently, if I want, for example, to print the first and last element of the array, I can write:


Or, if I don’t know the length of the array, I can also write:

console.log(schoolObjects[lengthArray - 1]);

In fact, by subtracting the value 1 from the length of the array, I always get the last element.

JavaScript array length – example 2

We create an array of elements and then display each element using the for loop.

This second example is similar to the previous one but uses the length property on the array as a condition of the for loop.

So I create the array in this way:

var schoolObjects = ['eraser', 'notebook', 'pencil'];

Then I scroll through the array through a simple for loop, stopping when the variable counter i is less than days.length.

In fact, as observed in the previous example, the last element has the position days.length – 1, which in our specific case has the value 2.

Here is the useful for loop to scroll through the array and print each element on the web page.

for (var i=0; i < schoolObjects.length; i++) {
  document.write(schoolObjects[i] + “
”); }


In this lesson we have analyzed some examples of using the JavaScript array length property, in the next lessons we will deal with other examples with the use of this property.

Some useful links

JavaScript tutorial

JavaScript Calculator

Object in JavaScript

For in loop

Caesar cipher decoder

Slot Machine in JavaScript

JavaScript add attribute

JavaScript add attribute

We study some JavaScript methods to add an attribute to elements.

In particular, in this lesson we will use createAttribute() and setAttributeNode().

In the previous article we saw how to create new elements such as paragraphs, levels, titles, etc …

JavaScript add attribute – createAttribute

With the createAttribute() method you can add one or more attributes to the elements created.

The syntax is therefore this: element.createAttribute(attribute_name);

where attribute_name is any attribute of the created tag.

JavaScript add attribute – example 1

After creating the attribute with createAttribute (), we will use the setAttributeNode () method, to associate the attribute with an element.

So let’s take an example to better understand how it works.

Here I will insert the link:

Here I insert the link:

Then try to click on the button above, the link to the Coding Creativo website will appear. If you click on the link, a new page will open, which you can close anyway to return to the tutorial.

Development example on JavaScript add attribute

So first I create the layer where to display the link I want to create. Immediately below I create the button that must be clicked to bring up the new link.

<div id="link">Here I insert the link: </div>
<input type="submit" onclick="insert()" value="insert">

I will now proceed to create the JavaScript code. I then create an insert() function that will be called when the onclick event on the button is activated.

Then, inside the function, I create the new element a with the createElement() method, as we did in the previous tutorial.

After between the opening and closing tags I write the Creative Coding text using the innerHTML property.

I create a variable to which I assign the attribute created with the createAttribute () method, in this case href. Then I give him the value of the website:

Finally, with setAttributeNode() I associate the newly created attribute with the a tag.

Then I create the blank = _target attribute in the same way to make the site open in a new page.

Finally, with the getElementById() method I get the level id and with appendChild() I add the created tag with its attributes.

function insert() {
  var link = document.createElement("a");
  link.innerHTML = "Coding Creativo ";
  var att = document.createAttribute("href");
  att.value = "";
  var newpage = document.createAttribute("target");
  newpage.value = "_blank";

JavaScript add attribute – example 2

Let’s now take another example with the use of createAttribute ().

Here I will insert the image:

Here I insert the image:

Then try to click on the button, an image of 200px size will appear. If you click again, as in the other cases, the tag is reproduced.

Development example on JavaScript add attribute

First of all, let’s create the layer where the image and the button appear:

<div id="image">Here I insert the image: </div>
<input type="submit" onclick="insert()" value="insert">

Then in a similar way to the previous example I create the javascript code:

function insert() {
  var image = document.createElement("img");
  var att = document.createAttribute("src");
  att.value = "img/angol.jpg";
  var widthImage= document.createAttribute("width");
  widthImage.value = "200";

This time I used the src attribute to indicate the path to the image and the width attribute for the width.

Clearly you have to indicate the path of your image.


These are just some JavaScript examples of how to add a new attribute where we used createAttribute () and setAttributeNode ().

JavaScript createTextNode

JavaScript createTextNode

We continue our tutorial in JavaScript using the createTextNode method, which is the method that allows us to insert text into a node.

In fact, after having created a new element or on an already existing element, it may be necessary to insert some text inside it.

To do this, you can use, for example, the JavaScript createTextNode() method, where inside the round brackets I specify the new text to be inserted.

The syntax of this method is as follows: element.createTextNote().

createTextNode JavaScript – example

In this example we will create a new paragraph, where we will insert any text inside. The new paragraph will be created when the button is clicked.

Try it out by clicking on the button below, you can click it also several times.


We explain in detail the procedure for making this example.

In the HTML code we insert a ‘Add new paragraph’ button as before and under it we add a level (div), where the new generated paragraph will appear.

Our add () function will carry out very simple instructions:

  • will first create the new paragraph element,
  • then it will write some text within this paragraph using the createTextNode method of JavaScript
  • and then using the appendChild () method will add this newly created content to the desired location. The desired point is obtained thanks to the getElementById method.

So, here is the html code to create the example:

<button onclick="add()">Add a new paragraph</button>
<div id="text"></div>

And JavaScript code to create the example:

function add() {
  var paragraph = document.createElement("p");
  var text = document.createTextNode("Coding Creativo");


So, summarizing in this lesson we learned how to create elements in the DOM with JavaScript and how to use the createElement(), createTextNode() and appendChild() methods.

Some useful links

JavaScript tutorial

JavaScript Calculator

Object in JavaScript

For in loop

Caesar cipher decoder

Slot Machine in JavaScript

Introduction to JavaScript language

Learn JavaScript – basic concepts