The FileReader API can read user device files and process content with JavaScript. It supports reading text files and binary files, such as 1. Read as text such as .txt, .csv, .json; 2. Read as Data URL for previewing pictures; 3. Read as ArrayBuffer for binary operations or sending raw data. It works asynchronously and does not block the main thread. Use methods include: 1. Get the file from <input type="file">; 2. Create a FileReader instance; 3. Call the corresponding reading method; 4. Handle the result in the onload event. For example, when previewing the image, read the file through readAsDataURL and assign the result to img.src in onload. Common events include onload indicating successful read, onerror processing error, and onprogress tracking progress. Note: 1. The results are only available in the onload callback; 2. Compatible with IE10 and modern browsers; 3. Unable to cancel the read operation; 4. Only one file can be read at a time per instance; 5. The file system cannot be accessed directly, and only files selected by the user can be processed.
The FileReader API lets you read files from the user's device and work with their contents in JavaScript — all without sending the file to a server. It's especially useful for previewing images, reading text files, or parsing data locally before upload.
What can the FileReader API do?
FileReader gives you methods to read different types of files, mainly text files and binary files like images or PDFs. Here are the most common use cases:
- Read as text – For
.txt,.csv,.json, etc. - Read as Data URL – Useful for previewing images.
- Read as ArrayBuffer – Good for binary manipulation or sending raw data.
It works asynchronously, meaning it doesn't block the main thread while reading large files.
How to use FileReader in practice
Here's a basic flow for using the FileReader API:
- Get a file from an
<input type="file">element. - Create a new
FileReader()instance. - Use one of its read methods based on your needs.
- Handle the result in the
onloadevent.
For example, if you want to show a preview of an image selected by the user:
const input = document.querySelector('input[type="file"]');
input.addEventListener('change', () => {
const file = input.files[0];
const reader = new FileReader();
reader.onload = function(e) {
const img = document.createElement('img');
img.src = e.target.result;
document.body.appendChild(img);
};
reader.readAsDataURL(file);
});This is a typical way to let users preview what they're about to upload.
Common events you should know
FileReader has several events that help manage the reading process:
-
onload– When the file is successfully read. -
onerror– If something goes wrong (like trying to read a corrupted file). -
onprogress– While the file is being read (useful for progress bars).
These events make it easier to handle different outcomes and give feedback to the user.
One thing to note: The result property of the FileReader object contains the file data after reading completes. So you only get access to it inside or after the onload callback.
Things to watch out for
There are a few gotchas when working with FileReader:
- It only works in modern browsers – IE10 and all modern browsers support it, but not older ones.
- You can't cancel a read operation – Once you call
readAs..., it runs until done. - Each FileReader instance reads one file at a time – If you need to read multiple files, create a new reader for each or reuse one with
abort()first.
Also, remember that FileReader doesn't actually access the file system — it only reads files the user explicitly selects via input elements or drag-and-drop.
Basically that's it.
The above is the detailed content of How does the FileReader API work?. For more information, please follow other related articles on the PHP Chinese website!
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