Custom JavaScript FileReader (Source code)

Table of Contents

1. Project Introduction
2. HTML Code
3. CSS Code
4. JavaScript Code
5. Conclusion
6. Preview

A FileReader is a JavaScript API that allows web developers to read the contents of files from a user's local computer. This can be incredibly useful for websites that require users to upload files, such as image sharing websites, file storage services, or document editors.

Building a custom FileReader with HTML, CSS, and JavaScript can give web developers complete control over the file upload process, allowing them to create custom interfaces, add features such as progress bars, and tailor the experience to the needs of their users.

In this tutorial, we will guide you through the process of building a custom FileReader from scratch. We will start by explaining the basics of the FileReader API and its uses. Next, we will create the HTML and CSS for our custom FileReader. After that, we will dive into the JavaScript code needed to create and handle the FileReader object, including uploading files and reading file contents. Finally, we will test our custom FileReader to ensure it works as expected.

By the end of this tutorial, you will have the knowledge and skills needed to create your own custom FileReader with HTML, CSS, and JavaScript. Whether you are building a file upload feature for your website or simply want to learn more about the FileReader API, this tutorial is for you.

Explanation of the FileReader API

The FileReader API is a JavaScript API that provides a way for web developers to read the contents of files from a user's local computer. It is a part of the HTML5 specification and is supported by modern web browsers.

The FileReader API allows web developers to access the contents of a file without having to upload it to the server first. This can be useful for websites that require users to upload files, such as image sharing websites, file storage services, or document editors.

FileReader API methods and properties

The FileReader API provides several methods and properties that can be used to read files. Some of the most commonly used methods include:

• readAsArrayBuffer(): This method reads the contents of the file as an ArrayBuffer object.
• readAsDataURL(): This method reads the contents of the file as a base64-encoded string.
• readAsText(): This method reads the contents of the file as a text string.
• abort(): This method aborts the reading operation.

The FileReader API also provides several properties that can be used to access information about the file being read, such as its name, type, and size.

1. readyState: This property represents the current state of the FileReader object, which can be one of the following values:
   • EMPTY: No file has been selected yet.
   • LOADING: The file is currently being read.
   • DONE: The file has been read completely.
2. result: This property contains the result of the reading operation. The type of the result depends on the method used to read the file.
3. error: This property contains the error that occurred during the reading operation, if any.
4. onloadstart: This property is called when the reading operation starts.
5. onprogress: This property is called repeatedly while the file is being read to report the progress of the operation.
6. onload: This property is called when the reading operation is successfully completed.
7. onabort: This property is called if the reading operation is aborted.
8. onerror: This property is called if an error occurs during the reading operation.

Let's start making your own custom FileReader using HTML, CSS, and JavaScript step by step.

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Prerequisites:

Before starting this tutorial, you should have a basic understanding of HTML, CSS, and JavaScript. Additionally, you will need a code editor such as Visual Studio Code or Sublime Text to write and save your code.

Step 1 (HTML Code):

To get started, we will first need to create a basic HTML file. In this file, we will include the main structure for our JavaScript filereader.

After creating the files just paste the following below codes into your file. Make sure to save your HTML document with a .html extension, so that it can be properly viewed in a web browser.

The first line, <!DOCTYPE html>, is a declaration that specifies the document type as HTML5. The <html> element is the root element of the HTML document and encloses all other elements.

Within the <head> element, there are three child elements:

1. The <title> element, which sets the title of the webpage to "JavaScript FileReader".
2. Two <link> elements that reference external CSS files. The first link references the Font Awesome icon library, which is used to display an upload icon. The second link references a local stylesheet named "styles.css".

The <body> element contains the main content of the webpage.

Inside the <div> element with a class of "container", there are three child elements:

1. An <h1> element that displays the heading "File Reader".
2. A <p> element that prompts the user to select a text file to read.
3. A <label> element that contains a file input element (<input type="file">) and an icon from Font Awesome. This label is styled to look like a button and is used to open the file dialog when clicked. The accept attribute specifies which file types are allowed to be selected, in this case .txt, .md, .html, .css, and .js files.

Below the label, there is a <div> element with a class of "file-info" that contains two child elements:

1. A <p> element that displays "Selected file:".
2. A <div> element with an id of "file-name" that initially displays "No file selected!".

Finally, there is a <pre> element with an id of "file-content". This element is initially empty and will be used to display the contents of the selected file.

At the end of the document, there is a <script> element that references an external JavaScript file named "script.js". This file contains the code that uses the FileReader API to read the selected file and display its contents in the <pre> element.

This is the basic structure of our JavaScript filereader using HTML, and now we can move on to styling it using CSS.

Step 2 (CSS Code):

Once the basic HTML structure of the JavaScript filereader is in place, the next step is to add styling to the filereader using CSS.

Next, we will create our CSS file. In this file, we will use some basic CSS rules to create our JavaScript filereader.

Here is a detailed explanation of each style rule:

The body selector sets the background color to a light grayish-blue (#E8EBEF) and the font family to "Segoe UI", Tahoma, Geneva, Verdana, sans-serif.

The .container selector sets the width to 90% of the parent element, with a maximum width of 600px. It centers the container with a 50px margin on top and bottom and a 0px margin on the left and right. It also sets a 20px padding, a white background, and a box shadow with a slight blur and opacity. Lastly, it aligns the text to the center.

The h1 selector sets the font size to 2.5em, the color to a dark gray (#222), and the top margin to 0.

The p selector sets the font size to 1.2em, the color to a light gray (#555), and the bottom margin to 20px.

The .custom-file-upload selector styles an HTML element with the class "custom-file-upload". It sets the display to inline-block, adds padding and bold font, and sets the background color to a blue shade (#3578E5) with rounded corners. It also adds a cursor pointer and a smooth transition effect when hovered over. Additionally, it styles the hover state by changing the background color to a darker shade of blue (#2A66C2). Lastly, it sets the display of the input element with a type of "file" to none, so it is not visible.

The .fa-cloud-upload selector styles an HTML element with the class "fa-cloud-upload" by adding a margin to the right of 10px.

The .file-info p selector styles all p elements that are descendants of an HTML element with the class "file-info" by setting the display to inline-block.

The #file-name selector styles an HTML element with an ID of "file-name" by setting the display to inline-block and the font weight to bold.

The pre selector styles the preformatted text by adding a 1px solid border, a 10px padding, a light gray background color (#F8F8F8), a font size of 1.2em, a line height of 1.4, and wrap text inside the element. It also adds a horizontal scrollbar when the content overflows, and sets the text alignment to initial. Lastly, it adds a margin of 20px on the top.

This will give our JavaScript filereader an upgraded presentation. Create a CSS file with the name of styles.css and paste the given codes into your CSS file. Remember that you must create a file with the .css extension.

Step 3 (JavaScript Code):

Finally, we need to create a filereader function in JavaScript.

This JavaScript code defines an event listener for a file input element and sets the text content of certain HTML elements based on the selected file. Here is a detailed explanation of each line of code:

The first line declares a variable fileInput and assigns it the value of the HTML element with the id "file-input".

The second line declares a variable fileContent and assigns it the value of the HTML element with the id "file-content".

The third line declares a variable fileName and assigns it the value of the HTML element with the id "file-name".

The fileInput.addEventListener() method adds an event listener to the file input element, listening for the "change" event. When this event occurs (i.e., when the user selects a file), the function inside the parentheses will be executed.

The const file = event.target.files[0] line gets the selected file from the event object and assigns it to a file variable.

The if (!file) block checks if there is no selected file. If there is no file, it sets the text content of fileName to "No file selected!" and returns from the function.

The if (file.type && !file.type.startsWith("text/")) block checks if the selected file is a valid text file. If the file is not a valid text file, it displays an alert message and returns from the function.

The if (file) block checks if there is a selected file. If there is a file, it sets the text content of fileName to the name of the selected file. Otherwise, it sets the text content of fileName to an empty string.

The const reader = new FileReader() line creates a new instance of the FileReader object.

The reader.readAsText(file) method reads the contents of the selected file as a text string.

The reader.onload method is called when the file has been successfully read. It sets the text content of fileContent to the result of the readAsText() method.

Create a JavaScript file with the name of script.js and paste the given codes into your JavaScript file and make sure it's linked properly to your HTML document, so that the scripts are executed on the page. Remember, you’ve to create a file with .js extension.

Conclusion:

In this tutorial, we have learned how to build a custom FileReader with HTML, CSS, and JavaScript. We started by understanding the basics of the FileReader API and its uses, followed by creating the HTML and CSS for our custom FileReader. Next, we wrote the JavaScript code to create and handle the FileReader object, including uploading files and reading file contents. Finally, we tested our custom FileReader to ensure it worked as expected.

By building our own custom FileReader, we have gained a deeper understanding of the file upload process and how it can be customized to fit our specific needs. We have also learned how to use the FileReader API, which can be applied to many other use cases beyond file uploading.

We encourage you to experiment with your custom FileReader and add your own features to it. For example, you could add a progress bar to show the upload progress, or you could modify the styling to match the theme of your website.

Overall, building a custom FileReader is a powerful tool for web developers to improve the file uploading experience for their users. We hope this tutorial has been helpful in guiding you through the process of creating your own custom FileReader.