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 URL representing the file's data as a base64 encoded string.
Syntax
instanceOfFileReader.readAsDataURL(blob);
Parameters
Example
HTML
<input type="file" onchange="previewFile()"><br> <img src="" height="200" alt="Image preview...">
JavaScript
function previewFile() {
var preview = document.querySelector('img');
var file = document.querySelector('input[type=file]').files[0];
var reader = new FileReader();
reader.addEventListener("load", function () {
preview.src = reader.result;
}, false);
if (file) {
reader.readAsDataURL(file);
}
}
Live Result
Example reading multiple files
HTML
<input id="browse" type="file" onchange="previewFiles()" multiple> <div id="preview"></div>
JavaScript
function previewFiles() {
var preview = document.querySelector('#preview');
var 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) ) {
var reader = new FileReader();
reader.addEventListener("load", function () {
var image = new Image();
image.height = 100;
image.title = file.name;
image.src = this.result;
preview.appendChild( image );
}, false);
reader.readAsDataURL(file);
}
}
if (files) {
[].forEach.call(files, readAndPreview);
}
}
FileReader() constructor was not supported by Internet Explorer for versions before 10. For a full compatibility code you can see our crossbrowser possible solution for image preview. See also this more powerful example.Specifications
| Specification | Status | Comment |
|---|---|---|
| File API The definition of 'FileReader' in that specification. |
Working Draft | Initial definition |
Browser compatibility
| Feature | Firefox (Gecko) | Chrome | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|
| Basic support | 3.6 (1.9.2)[1] | 7 | 10[2] | 12.02[3] | 6.0 |
| Feature | Firefox Mobile (Gecko) | Android | IE Mobile | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|
| Basic support | 32 | 3 | 10 | 11.5 | 6.0 |
[1] Prior to Gecko 2.0 beta 7 (Firefox 4.0 beta 7), all Blob parameters below were File parameters; this has since been updated to match the specification correctly. Prior to Gecko 13.0 (Firefox 13.0 / Thunderbird 13.0 / SeaMonkey 2.10) the FileReader.error property returned a FileError object. This interface has been removed and FileReader.error is now returning the DOMError object as defined in the latest FileAPI draft.
[2] IE9 has a File API Lab.
[3] Opera has partial support in 11.1.