I am working with a file upload interface where users will be uploading images. My goal is to validate the height and width of the image on the client side. Is there a way to determine the size of an image using just the file path in JavaScript?
Important: If not, what other methods can be used to obtain the dimensions on the client side?
If you want to accomplish this task, you can utilize the File API from the W3C on browsers that support it. By using the File API and the readAsDataURL function with the FileReader interface, you can set the data URL as the src of an img element to then read its dimensions. Currently, Firefox 3.6 has support for the File API, while Chrome and Safari are either already equipped or soon will be.
During the transitioning period, your approach would involve:
Detecting if the browser supports the File API (easily done with:
if (typeof window.FileReader === 'function')If supported, proceed to read the data locally and display it in an image to obtain its dimensions.
If not supported, upload the file to the server (possibly through submitting the form from an iframe to prevent page redirection), and then communicate with the server to determine the image's size or retrieve the uploaded image.
Edit I have been planning to create a File API example for some time now, here's one:
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Show Image Dimensions Locally</title>
<style type='text/css'>
body {
font-family: sans-serif;
}
</style>
<script type='text/javascript'>
function loadImage() {
var input, file, fr, img;
if (typeof window.FileReader !== 'function') {
write("The file API isn't supported on this browser yet.");
return;
}
input = document.getElementById('imgfile');
if (!input) {
write("Could not locate the imgfile element.");
}
else if (!input.files) {
write("This browser does not seem to support the `files` property of file inputs.");
}
else if (!input.files[0]) {
write("Please select a file before clicking 'Load'");
}
else {
file = input.files[0];
fr = new FileReader();
fr.onload = createImage;
fr.readAsDataURL(file);
}
function createImage() {
img = document.createElement('img');
img.onload = imageLoaded;
img.style.display = 'none'; // If you don't want it showing
img.src = fr.result;
document.body.appendChild(img);
}
function imageLoaded() {
write(img.width + "x" + img.height);
img.parentNode.removeChild(img);
img = undefined;
}
function write(msg) {
var p = document.createElement('p');
p.innerHTML = msg;
document.body.appendChild(p);
}
}
</script>
</head>
<body>
<form action='#' onsubmit="return false;">
<input type='file' id='imgfile'>
<input type='button' id='btnLoad' value='Load' onclick='loadImage();'>
</form>
</body>
</html>
This code snippet functions smoothly on Firefox 3.6. It refrains from utilizing any library, hence the use of attribute-style event handlers and similar approaches.
While the previous sample code gets the job done, there is room for improvement.
let fileReader = new FileReader();
fileReader.onload = function(event) {
let image = new Image();
image.onload = function() {
console.log(this.width, this.height);
};
image.src = event.target.result;
};
fileReader.readAsDataURL(this.files[0]);
If you opt for a flash-based uploader like SWFUpload, you can access comprehensive information and carry out multiple queued uploads.
Personally, I highly suggest using SWFUpload even though I have no affiliation with them apart from being a satisfied user.
Alternatively, you could develop a silverlight control to select and upload your file.
It is not possible to modify the filename and file content that are transmitted to the server via HTTP. JavaScript does not have control over these specific fields.
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