How to get a blob?
<input type="file" onchange="previewFile()">
function previewFile() {
const file = document.querySelector('input[type=file]').files[0];
const reader = new FileReader();
// Get blob?
console.log(file);
}
3 Answers
As pointed in the comments, file is a blob:
file instanceof Blob; // true
And you can get its content with the file reader API https://developer.mozilla.org/en/docs/Web/API/FileReader
Read more: https://developer.mozilla.org/en-US/docs/Using_files_from_web_applications
var input = document.querySelector('input[type=file]');
var textarea = document.querySelector('textarea');
function readFile(event) {
textarea.textContent = event.target.result;
console.log(event.target.result);
}
function changeFile() {
var file = input.files[0];
var reader = new FileReader();
reader.addEventListener('load', readFile);
reader.readAsText(file);
}
input.addEventListener('change', changeFile);<input type="file">
<textarea rows="10" cols="50"></textarea>
2 Comments
Samuel
@rafaelcastrocouto , @Daniel : if I want to send the blob over websocket, What object is required to be sent from abv code ? shall I send
ws.send(reader) ???
rafaelcastrocouto
Send the buffer (blob), you only need to parse (read) it for visualization purposes.
A file object is an instance of Blob but a blob object is not an instance of File
new File([], 'foo.txt').constructor.name === 'File' //true
new File([], 'foo.txt') instanceof File // true
new File([], 'foo.txt') instanceof Blob // true
new Blob([]).constructor.name === 'Blob' //true
new Blob([]) instanceof Blob //true
new Blob([]) instanceof File // false
new File([], 'foo.txt').constructor.name === new Blob([]).constructor.name //false
If you must convert a file object to a blob object, you can create a new Blob object using the array buffer of the file. See the example below.
const file = new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'});
//or const file = document.querySelector('input[type=file]').files[0];
const reader = new FileReader();
reader.onload = function(e) {
const blob = new Blob([new Uint8Array(e.target.result)], {type: file.type });
console.log(blob);
};
reader.readAsArrayBuffer(file);
As pointed out by @bgh you can also use the arrayBuffer method of the File object. See the example below.
const file = new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'});
//or const file = document.querySelector('input[type=file]').files[0];
file.arrayBuffer().then((arrayBuffer) => {
const blob = new Blob([new Uint8Array(arrayBuffer)], {type: file.type });
console.log(blob);
});
If your environment supports async/await you can use a one-liner like below
const fileToBlob = async (file) => new Blob([new Uint8Array(await file.arrayBuffer())], {type: file.type });
console.log(await fileToBlob(new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'})));
1 Comment
bgh
These days it's probably a better idea to use use the
arrayBuffer() method instead (developer.mozilla.org/en-US/docs/Web/API/Blob/arrayBuffer). Something like: let blob = new Blob(await file.arrayBuffer());async function FileToString (file) {
try {
let res = await file.raw.text();
console.log(res);
} catch (err) {
throw err;
}
}
file instanceof Blob; // true