Can't Read Local File in Js File
Read files in JavaScript
How to select files, read file metadata and content, and monitor read progress.
— Updated
Being able to select and interact with files on the user's local device is one of the most commonly used features of the web. It allows users to select files and upload them to a server, for example, uploading photos, or submitting taxation documents, etc. But, it also allows sites to read and dispense them without ever having to transfer the data beyond the network.
The mod File System Admission API #
The File System Access API provides an like shooting fish in a barrel way to both read and write to files and directories on the user's local organisation. It's currently available in nigh Chromium-derived browsers similar Chrome or Border. To learn more than most information technology, run across the File System Admission API article.
Since the File Organisation Access API is not compatible with all browsers nevertheless, check out browser-fs-access, a helper library that uses the new API wherever information technology is available, just falls back to legacy approaches when it is non.
Working with files, the classic way #
This guide shows you how to:
- Select files
- Using the HTML input element
- Using a drag-and-drop zone
- Read file metadata
- Read a file'south content
Select files #
HTML input element #
The easiest way to allow users to select files is using the <input blazon="file"> element, which is supported in every major browser. When clicked, it lets a user select a file, or multiple files if the multiple attribute is included, using their operating system'due south built-in file pick UI. When the user finishes selecting a file or files, the chemical element's alter event is fired. Yous can access the list of files from event.target.files, which is a FileList object. Each item in the FileList is a File object.
<!-- The `multiple` attribute lets users select multiple files. -->
<input type = "file" id = "file-selector" multiple >
<script >
const fileSelector = document. getElementById ( 'file-selector' ) ;
fileSelector. addEventListener ( 'change' , ( effect ) => {
const fileList = event.target.files;
console. log (fileList) ;
} ) ;
</script > This example lets a user select multiple files using their operating system's congenital-in file selection UI then logs each selected file to the console.
Limit the types of files user tin can select #
In some cases, you may desire to limit the types of files users can select. For example, an paradigm editing app should but accept images, not text files. To practice that, you can add an take attribute to the input element to specify which files are accepted.
<input type = "file" id = "file-selector" accept = ".jpg, .jpeg, .png" > Custom elevate-and-drop #
In some browsers, the <input blazon="file"> element is also a driblet target, allowing users to elevate-and-drib files into your app. But, the drop target is small, and can be hard to employ. Instead, in one case you've provided the core functionality using an <input type="file"> chemical element, yous can provide a large, custom drag-and-drib surface.
Choose your drop zone #
Your drop surface will depend on the design of your awarding. You lot may only desire function of the window to exist a driblet surface, or potentially the unabridged window.
Squoosh allows the user to drag-and-drop an image anywhere into the window, and clicking select an image invokes the <input type="file"> element. Any y'all choose every bit your drop zone, brand sure information technology's articulate to the user that they can elevate-and-drib files onto that surface.
Define the drop zone #
To enable an element to exist a elevate-and-drib zone, you lot'll need to listen for ii events, dragover and drop. The dragover effect updates the browser UI to visually indicate that the drag-and-drib activity is creating a re-create of the file. The drop consequence is fired after the user has dropped the files onto the surface. Like to the input element, you can access the list of files from outcome.dataTransfer.files, which is a FileList object. Each item in the FileList is a File object.
const dropArea = certificate. getElementById ( 'drib-area' ) ; dropArea. addEventListener ( 'dragover' , ( event ) => {
effect. stopPropagation ( ) ;
event. preventDefault ( ) ;
// Style the drag-and-drop as a "copy file" functioning.
event.dataTransfer.dropEffect = 'copy' ;
} ) ;
dropArea. addEventListener ( 'drop' , ( event ) => {
event. stopPropagation ( ) ;
effect. preventDefault ( ) ;
const fileList = event.dataTransfer.files;
console. log (fileList) ;
} ) ;
event.stopPropagation() and event.preventDefault() stop the browser's default beliefs from happening, and permit your code to run instead. Without them, the browser would otherwise navigate away from your folio and open the files the user dropped into the browser window.
Check out Custom drag-and-drop for a live demonstration.
What about directories? #
Unfortunately, today there isn't a good mode to go access to a directory.
The webkitdirectory aspect on the <input type="file"> element allows the user to cull a directory or directories. It is supported in some Chromium-based browsers, and possibly desktop Safari, but has conflicting reports of browser compatibility.
If drag-and-drib is enabled, a user may attempt to drag a directory into the drib zone. When the driblet event is fired, it will include a File object for the directory, but will exist unable to access whatsoever of the files within the directory.
The File object contains a number of metadata properties about the file. Most browsers provide the file name, the size of the file, and the MIME type, though depending on the platform, different browsers may provide different, or additional information.
function getMetadataForFileList ( fileList ) {
for ( const file of fileList) {
// Not supported in Safari for iOS.
const proper noun = file.proper noun ? file.name : 'Not SUPPORTED' ;
// Not supported in Firefox for Android or Opera for Android.
const type = file.blazon ? file.type : 'NOT SUPPORTED' ;
// Unknown cantankerous-browser back up.
const size = file.size ? file.size : 'NOT SUPPORTED' ;
console. log ( {file, proper name, type, size} ) ;
}
} You lot can see this in action in the input-blazon-file Glitch demo.
Read a file's content #
To read a file, apply FileReader, which enables yous to read the content of a File object into retentiveness. You lot can instruct FileReader to read a file as an array buffer, a data URL, or text.
function readImage ( file ) {
// Check if the file is an image.
if (file.type && !file.blazon. startsWith ( 'image/' ) ) {
console. log ( 'File is non an paradigm.' , file.type, file) ;
render ;
} const reader = new FileReader ( ) ;
reader. addEventListener ( 'load' , ( outcome ) => {
img.src = issue.target.event;
} ) ;
reader. readAsDataURL (file) ;
}
The instance above reads a File provided by the user, and so converts it to a data URL, and uses that data URL to brandish the paradigm in an img element. Check out the read-image-file Glitch to run across how to verify that the user has selected an image file.
Monitor the progress of a file read #
When reading large files, it may be helpful to provide some UX to signal how far the read has progressed. For that, use the progress event provided by FileReader. The progress outcome provides 2 backdrop, loaded, the amount read, and total, the total amount to read.
role readFile ( file ) {
const reader = new FileReader ( ) ;
reader. addEventListener ( 'load' , ( event ) => {
const upshot = outcome.target.consequence;
// Exercise something with issue
} ) ;
reader. addEventListener ( 'progress' , ( event ) => {
if (consequence.loaded && event.total) {
const percent = (outcome.loaded / issue.total) * 100 ;
console. log ( ` Progress: ${Math. circular (percent) } ` ) ;
}
} ) ;
reader. readAsDataURL (file) ;
} Hero image by Vincent Botta from Unsplash
Final updated: — Better article
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Source: https://web.dev/read-files/
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