Home >Web Front-end >JS Tutorial >FileReader implements local preview before uploading pictures
This time I will bring you FileReader to implement local preview before uploading pictures. FileReader to implement local preview before uploading pictures. .
Introduction When uploading and previewing images, if there are no special requirements, you can directly transfer the image to the background first. After success, you can get the URL and then render it on the page. This will not cause any problem when the image is relatively small, but it will be slower if it is larger. Only then can you see the preview, and junk files are also generated, so it is better to preview it locally before uploading.
When I was working on a project and looking for a plug-in, I knew that the pure front-end could realize local preview of images. However, when I was asked about it during the interview today, I was confused. Then I accidentally found the implemented demo on the computer desktop, and then checked based on the demo. I took a look at the API and briefly summarized it:
First we have to get the File object When you use the
input tagto upload an image, the event object will contain a collection of file objects
event.target.files
The core is the FileReader object According to MDN:
The FileReader object allows a web application to asynchronously read the contents of a file (or raw data buffer) stored on the user's computer, using a File or Blob object to specify the file or data to be read.
First, you need to get an instance object of FileReader as
constructorvar freader = new FileReader();
Use the readAsDataURL() method to read the specified content
freader.readAsDataURL(file);
The last is an
Event processing, which is equivalent to monitoring the reading progress. Here we render the image when the reading is completed, so use onloadfreader.onload = function(e) { console.log(e); myImg.setAttribute('src', e.target.result); }
After the rack is loaded, what you finally get is a base64 encoded src address. I will find out the specific reason next time and then write a special article about base64 encoding
Complete code
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<form action="">
<input type="file" name="files" id="fileTog" accept="image/*" multiple="multiple" onchange="changImg(event)">
<img alt="暂无图片" id="myImg" src="" height="100px" width="100px">
</form>
<script>
function changImg(e){
var myImg = document.getElementById('myImg');
for (var i = 0; i < e.target.files.length; i++) {
var file = e.target.files[i];
console.log(file);
if (!(/^image\/.*$/i.test(file.type))) {
continue; //不是图片 就跳出这一次循环
}
//实例化FileReader API
var freader = new FileReader();
freader.readAsDataURL(file);
freader.onload = function(e) {
console.log(e);
myImg.setAttribute('src', e.target.result);
}
}
}
</script>
</body>
</html>
Postscript
This incident exposed one of my problems in learning new things. I just check it out and read it once before I know it. This habit is particularly harmful. In the future, whenever I find a question, I not only need to check how to do it. Yes, you also need to understand why you can do this. The so-called knowing what it is and why it is. Also, you can type the usual code with your hands and try not to copy it. Copying it is fun for a while, but it is embarrassing to not be able to write it by hand.
I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on thephp Chinese website!
Recommended reading:How to export Excel from element UI
##vue-dplayer Detailed explanation of the steps to implement hls playback
The above is the detailed content of FileReader implements local preview before uploading pictures. For more information, please follow other related articles on the PHP Chinese website!