Java Reading User Input to Find File
JavaScript read File Reading local files with JavaScript
This is a repost from my blog
For security and privacy reasons web apps do not have directly access to the files
on the user's device. If you need to read one or multiple local files, you can do
this through the usage of a file input and a FileReader. In this post nosotros will take a await
at how this works through a few examples.
TL;DR
- JavaScript does not take direct access to the local files due to security and privacy.
- We can offer the user the possibility to select files via a
fileinput chemical element that we can and then process. - The
fileinput has afilesproperty with the selected file(southward). - We tin can employ a
FileReaderto access the content of the selected file(s).
How it works
Every bit JavaScript in the browser cannot access the local files from the user's device,
we need to provide the user with a manner to select one or multiple files for the states to apply.
This can exist washed with the HTML file input element:
<input type= "file" id= "fileInput" >
If we want to allow the selection of multiple files, we can add the multiple aspect:
<input type= "file" id= "fileInput" multiple >
We can either use the change event of the input field to respond to a file pick
or add another UI element to let the user explicitly start the processing of the selected file.
Also note: The option of a file with the input element does non upload the file anywhere,
the only thing that happens is that the file becomes bachelor to the JavaScript on the page.
The file input has a files property that is a list (as at that place could be multiple selected files) of File objects.
<input type= "file" id= "fileInput" > <script> document . getElementById ( ' fileInput ' ). addEventListener ( ' modify ' , office selectedFileChanged () { panel . log ( this . files ); // volition contain data about the file that was selected. }); </script>
A File object looks like this:
{ proper noun : ' test.txt ' , // the name of the selected file size : 1024 , // the size in bytes type : ' text/plainly ' , // the assumed file type based on file extension. This might be incorrect. lastModified : 1234567890 , // timestamp of the terminal alter according to the user's system lastModifiedDate : ' Thu Jul 04 2019 09:22:51 GMT+0200 (Central European Summer Time) ' // a date object for the last modified timestamp }
Now we have the ability to select a file and run across the metadata, but how can we access the file content?
To go the actual content of a selected file, we demand a FileReader.
A file reader takes a File object and offers us methods to get admission to the data as:
- a string of text data
- a information URL
- a string of binary information
- an ArrayBuffer containing raw binary information
In the post-obit examples, we will use the readAsText and readAsDataURL methods to show the content of text and image files.
Example one: Reading text files
To show the file content every bit text, we change the change effect handler:
certificate . getElementById ( ' fileInput ' ). addEventListener ( ' change ' , function selectedFileChanged () { if ( this . files . length === 0 ) { console . log ( ' No file selected. ' ); return ; } const reader = new FileReader (); reader . onload = function fileReadCompleted () { // when the reader is washed, the content is in reader.outcome. panel . log ( reader . issue ); }; reader . readAsText ( this . files [ 0 ]); });
First we brand sure that at that place is a file that can be read. If the user cancels or otherwise
closes the file selection dialog without selecting a file, we take goose egg to read and exit our function.
Nosotros then keep to create a FileReader. The reader works asychronously in order
to not block the main thread and UI updates which is important when reading large files (like videos).
The reader emits a 'load' event (similar to images for example) that tells our lawmaking that the reading is finished.
The reader keeps the file content in its result property. The data in this property depends on which method we used to read the file.
In our example nosotros read the file with the readAsText method, so the result will exist a string of text.
Example two: Displaying an image from the user's device
If we want to display images, reading the file into a string isn't very helpful.
Conveniently the FileReader has a readAsDataURL method that reads the file into
an encoded string that can be used as the source for an <img> element. The code is pretty much the same as previously,
with the exceptions that nosotros read the file with readAsDataURL and display the result as an image:
document . getElementById ( ' fileInput ' ). addEventListener ( ' change ' , office selectedFileChanged () { if ( this . files . length === 0 ) { console . log ( ' No file selected. ' ); return ; } const reader = new FileReader (); reader . onload = function fileReadCompleted () { const img = new Image (); // creates an <img> element img . src = reader . result ; // loads the data URL as the image source document . body . appendChild ( img ); // adds the image to the body }; reader . readAsDataURL ( this . files [ 0 ]); });
Source: https://dev.to/g33konaut/reading-local-files-with-javascript-25hn
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