File API

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Working with file uploads, especially on the front end, was always a hassle.  We didn't use to be able to drag and drop files, complete AJAX uploads, provide multiple files, and hell, we couldn't get any information about the file until it hit the server; you'd need to upload the damn file before you could reject it!

Nowadays we have the File API which provides us access to file information via JavaScript and an input[type=file] element.  Let's have a look at how the File API works!

Accessing Files

To get the list of files mapped to a given input[type=file], you use the files property:

// Assuming <input type="file" id="upload" multiple>

var uploadInput = document.getElementById('upload');

uploadInput.addEventListener('change', function() {
	console.log(uploadInput.files) // File listing!
});

Unfortunately the FileList doesn't have a forEach method like Array so we'll need to do some old school looping through the FileList:

for (var i = 0, fileCount = uploadInput.files.length; i < fileCount; i++) {
  console.log(files[i]);
}

It's important to note that FileList does have a length property.

Getting File Information

Each file in the FileList provides a good set of information on each file, including file size, MIME type, last modified date, and name:

{
	lastModified: 1428005315000,
	lastModifiedDate: Thu Apr 02 2015 15:08:35 GMT-0500 (CDT),
	name: "profile.pdf",
	size: 135568,
	type: "application/pdf",
	webkitRelativePath: ""
}

What's nice about getting this file information is that you can do some very basic validation before uploading the file.  For example, you can validate MIME type or total file size:

var maxAllowedSize = 500000;

for (var i = 0, fileCount = uploadInput.files.length, totalSize = 0; i < fileCount; i++) {
	totalSize += files[i].size;
	if(totalSize > maxAllowedSize) {
		// Notify the user that their file(s) are too large
	}

	if(files[i].type != 'application/pdf') {
		// Notify of invalid file type for file in question
	}
}

Total file size is too large or a file doesn't pass the test? Now you can present the user with a message without needing to upload and assess the file first.

That's my quick look at the File API. It's a sweet little API that can save you and your user some wasted upload time. There's lots more than can be done with the file API, much of which you can find on MDN.

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Discussion

  1. MaxArt

    Keep in mind that the type property isn’t reliable, because browsers normally base the value solely on the file’s extension. One can rename a .php file to .pdf (for example) and it wouldn’t be noticed by the File API.

  2. Ickata
    [].slice.call(uploadInput.files).forEach(function(file, i, files) {
    	
    });
  3. Even better:

    [].forEach.call(uploadInput.files, function(file, i, files) {
    	
    });
    
  4. in ES6 :

      var uploadInput = document.getElementById('upload');
      Array.from(uploadInput).forEach(function(i) {
        ...
      });
    

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!