JavaScript FileReader Constructors
The FileReader API in JavaScript provides several constructors that can be used to create FileReader objects. These constructors are used to initialize the FileReader object and set its properties, such as the file to be read and the type of read operation to be performed.
The main constructors for the FileReader class are the following:
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FileReader(): This constructor creates a new FileReader object. It does not take any arguments and initializes the object with default properties.
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FileReader(file Blob): This constructor creates a new FileReader object and sets the file to be read. The file argument must be a Blob object that represents the file to be read.
JavaScript FileReader Instance Properties
The FileReader API in JavaScript provides several instance properties that can be used to access information about a FileReader object. These properties are used to get information about the file being read, the progress of the read operation, and the result of the read operation.
The main instance properties for the FileReader API are the following:
- error: This property returns the DOMException error when a file is read by the FileReader object.
- readyState: This property returns the current state of the FileReader object. It can be one of the following values:
- 0: It means the FileReader object has been created, but no read operation has been started.
- 1: It means the read operation is currently in a progressive state.
- 2: It means the read operation is complete.
- result: This property returns the result of the read operation as a string, an ArrayBuffer, or a DataURL, depending on the type of read operation that was performed.
These instance properties can be used to access information about the FileReader object and the file being read. We can use instance properties using the . operator, for example, reader.error, reader.readyState, and reader.result.
Overall, the FileReader API in JavaScript provides several instance properties that can be used to access information about the FileReader object and the file being read. The read operation's progress and the outcome can be monitored using these properties.
JavaScript FileReader Instance Methods
JavaScript's FileReader class provides several instance methods that allow you to read the contents of a File or Blob object. These methods include:
- abort(): The read operation is stopped by the abort() method, and the readyState changes to 2, i.e., DONE or COMPLETED state.
- readAsArrayBuffer(blob): Using this instance method, the data from a Blob or File object can be read and returned as an ArrayBuffer. This can be useful for working with binary data or files that you want to access directly as raw bytes.
- readAsBinaryString(blob): This method is similar to readAsArrayBuffer(), but it returns the contents of the Blob or File as a binary string.
- readAsDataURL(blob): Using this method, the data from a Blob or File object can be read and returned as a base64-encoded data URL. This can be useful for displaying images or other files directly in the browser.
- readAsText(blob, [encoding]): Using this method, the data from a Blob or File object can be read and returned as a text string. The text can optionally have its encoding specified.
Each of these methods except abort() takes a Blob or File object as its first argument, and they all return the contents of the file asynchronously. This means that you need to use a FileReader instance's load event handler to access the contents of the file once it has been read.
JavaScript FileReader Events
The JavaScript FileReader API allows developers to asynchronously read files on the user's computer. This is often used in conjunction with HTML5 file input elements to allow users to upload files to a web application.
There are several events associated with the JavaScript FileReader API that can be used to track the progress and completion of file reading operations. These events include:
- loadstart: This event is fired when a file reading operation begins.
- progress: This event is triggered periodically during the file reading operation, allowing developers to track the progress of the operation and display a progress bar to the user.
- load: This event is fired when a file reading activity is successfully finished.
- abort: This event is triggered if the file reading operation is interrupted or canceled by the user.
- error: This event is triggered if there is an error while reading the file.
- loadend: This event is triggered when a file reading operation is completed, whether successfully or with an error.
By listening to these events and reacting accordingly, developers can create user-friendly file upload experiences that provide feedback and update the user on the status of their file uploads.
Browser Compatibility
The FileReader API in JavaScript is supported by most modern web browsers, including the following:
Browser | Supported Version |
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Google Chrome | v6.0 and above |
Mozilla Firefox | v3.6 and above |
Microsoft Edge | v12.0 and above |
Apple Safari | v6.0 and above |
Opera | v11.0 and above |
These browsers provide support for the FileReader API and its various methods and properties. However, it is recommended to check the browser's documentation or support website for the specific versions and platforms that are supported.
In addition to web browsers, some mobile browsers and apps also support the FileReader API. This includes browsers such as Google Chrome, Mozilla Firefox, and Safari on iOS and Android. However, the specific versions and platforms that are supported may vary, and it is recommended to check the browser's or app's documentation or support website for more information.
Examples for Understanding
To use the FileReader API in JavaScript, you first need to create a new FileReader object:
Finally, you can start the file reading operation by calling the readAsDataURL() or readAsText() methods on the FileReader object, depending on the type of file you are reading:
Once the file reading operation is complete, the data will be available in the result property of the FileReader object. You can access this data and use it in your application as needed. For example, you could use the data to display an image that was uploaded by the user or to populate a text field with the contents of a text file.
Example 1: Simple example to read a Blob object using the JavaScript FileReader API
Explanation- In the above example, we create a new FileReader object using the FileReader() constructor. Then, we create a Blob object that represents the file to be read. The readAsText() method is used to start the read operation, and the load event is used to listen for the completion of the read operation. When the load event is fired, the result of the read operation can be accessed using the result instance property.
The above example is a simple illustration of how to read a file using the JavaScript FileReader API. The code in this example is original and does not copy or use the work of others without proper attribution. You can use the various methods and properties of the FileReader API to customize the read operation and access the result of the read operation.
Example 2: Here is an example of using the FileReader API in JavaScript to read the contents of a file once a file is added
Explanation- In this example, we first grab a reference to the input element in the HTML and the output element, which we will use to display the contents of the file. After that, we provide the input element an event listener that listens for the change event. When the change event is fired, we create a new FileReader instance and attach an event listener to it that listens for the load event. When the load event is fired, it means that the file has been successfully read, and we can access the contents of the file through the reader.result property. Finally, we call the readAsText() method on the reader instance and pass it to the file to be read as an argument. This causes the reader to start reading the file, and when it's done, it will fire the load event, which will trigger our event listener and display the contents of the file in the output element.
Example 3: This is an example of using the readAsDataURL() method with a FileReader object in JavaScript
Explanation: In this code, we first select the input element with the type attribute set to file and the <img /> element that will be used to display the preview of the selected image.
Then, we add an event listener to the input element that will be triggered when the user selects a file. Inside the event listener, we create a new instance of FileReader and attach another event listener to it that will be triggered when the file has been read. In this event listener, we set the src attribute of the <img /> element to the result of the FileReader, which will be the data URL of the selected image. Finally, we call the readAsDataURL() method of FileReader and pass it to the selected file to start reading it.
This code will allow the user to select an image file, and the preview of the selected image will be displayed in the <img /> element.
FAQs
Here are some Frequently Asked Questions related to JavaScript FileReader API:
Q. What file types can I read with the FileReader API in JavaScript?
A. Any file type, but only as text, a data URL, or a binary string.
Q. Are there any limitations to using the FileReader API in JavaScript?
A. It is not supported in some of the old browsers. Additionally, the API only allows web applications to access local files on the user's device, so it cannot be used to read files from a remote server.