FileReader: readAsDataURL() method
The readAsDataURL
method is used to read the contents of the specified
Blob
or File
. When the read operation is finished, the
readyState
becomes DONE
, and the
loadend
is triggered. At that time, the
result
attribute contains the data as a data: URL representing the
file's data as a base64 encoded string.
Note: The blob's result
cannot be
directly decoded as Base64 without first removing the Data-URL declaration preceding
the Base64-encoded data. To retrieve only the Base64 encoded string, first
remove data:*/*;base64,
from the result.
Syntax
js
readAsDataURL(blob)
Parameters
Return value
None (undefined
).
Examples
Reading a single file
HTML
html
<input type="file" onchange="previewFile()" /><br />
<img src="" height="200" alt="Image preview" />
JavaScript
js
function previewFile() {
const preview = document.querySelector("img");
const file = document.querySelector("input[type=file]").files[0];
const reader = new FileReader();
reader.addEventListener(
"load",
() => {
// convert image file to base64 string
preview.src = reader.result;
},
false
);
if (file) {
reader.readAsDataURL(file);
}
}
Result
Reading multiple files
HTML
html
<input id="browse" type="file" multiple />
<div id="preview"></div>
JavaScript
js
function previewFiles() {
const preview = document.querySelector("#preview");
const files = document.querySelector("input[type=file]").files;
function readAndPreview(file) {
// Make sure `file.name` matches our extensions criteria
if (/\.(jpe?g|png|gif)$/i.test(file.name)) {
const reader = new FileReader();
reader.addEventListener(
"load",
() => {
const image = new Image();
image.height = 100;
image.title = file.name;
image.src = reader.result;
preview.appendChild(image);
},
false
);
reader.readAsDataURL(file);
}
}
if (files) {
Array.prototype.forEach.call(files, readAndPreview);
}
}
const picker = document.querySelector("#browse");
picker.addEventListener("change", previewFiles);
Result
Specifications
Specification |
---|
File API # readAsDataURL |
Browser compatibility
BCD tables only load in the browser