FileReader: result property
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
Note: This feature is available in Web Workers.
The result read-only property of the FileReader interface returns the
file's contents. This property is only valid after the read operation is complete, and
the format of the data depends on which of the methods was used to initiate the read
operation.
Value
An appropriate string or ArrayBuffer based on which of the reading methods
was used to initiate the read operation. The value is null if the reading
is not yet complete or was unsuccessful.
The result types are described below.
| Method | Description |
|---|---|
readAsArrayBuffer()
|
The result is a JavaScript
ArrayBuffer
containing binary data.
|
readAsBinaryString()
|
The result contains the raw binary data from the file in a
string.
|
readAsDataURL()
|
The result is a string with a data: URL
representing the file's data.
|
readAsText()
|
The result is text in a string. |
Examples
This example presents a function reader() which reads a file from a file input. It works by creating a FileReader object and creating a listener for load events, such that when then file is read, the result is obtained and passed to the callback function provided to reader().
The content is handled as raw text data.
// Given this HTMLInputElement of type="file":
// <input id="image" type="file" accept="image/*">
function reader(file, callback) {
const fr = new FileReader();
fr.onload = () => callback(null, fr.result);
fr.onerror = (err) => callback(err);
fr.readAsDataURL(file);
}
document.querySelector("#image").addEventListener("change", (evt) => {
// No files, do nothing.
if (!evt.target.files) {
return;
}
reader(evt.target.files[0], (err, res) => {
console.log(res); // Base64 `data:image/...` String result.
});
});
Given the asynchronous nature of FileReader, you could use a Promise-based approach. Here's an example for a file input with attribute multiple that returns a Promise.
// Given this HTMLInputElement:
// <input id="images" type="file" accept="image/*" multiple>
const reader = (file) =>
new Promise((resolve, reject) => {
const fr = new FileReader();
fr.onload = () => resolve(fr);
fr.onerror = (err) => reject(err);
fr.readAsDataURL(file);
});
async function logImagesData(fileList) {
let fileResults = [];
const frPromises = fileList.map(reader);
try {
fileResults = await Promise.all(frPromises);
} catch (err) {
// In this specific case, Promise.all() might be preferred
// over Promise.allSettled(), since it isn't trivial to modify
// a FileList to a subset of files of what the user initially
// selected. Therefore, let's just stash the entire operation.
console.error(err);
return;
}
fileResults.forEach((fr) => {
console.log(fr.result); // Base64 `data:image/...` String result.
});
}
// HTMLInputElement type="file" Event handler:
document.querySelector("#images").addEventListener("change", (evt) => {
// If no files, do nothing.
if (!evt.target.files) {
return;
}
logImagesData([...evt.target.files]);
});
Specifications
| Specification |
|---|
| File API # dom-filereader-result |
Browser compatibility
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