Limit the size of a file upload (html input element)

You can't do it client-side. You'll have to do it on the server.

Edit: This answer is outdated!

When I originally answered this question in 2011, HTML File API was nothing but a draft. It is now supported on all major browsers.

I'd provide an update with solution, but @mark.inman.winning has already answered better than I could.

Keep in mind that even if it's now possible to validate on the client, you should still validate it on the server, though. All client side validations can be bypassed.

const input = document.getElementById('input')

input.addEventListener('change', (event) => {
  const target = event.target
  	if (target.files && target.files[0]) {

      /*Maximum allowed size in bytes
        5MB Example
        Change first operand(multiplier) for your needs*/
      const maxAllowedSize = 5 * 1024 * 1024;
      if (target.files[0].size > maxAllowedSize) {
      	// Here you can ask your users to load correct file
       	target.value = ''
      }
  }
})

<input type="file" id="input" />

If you need to validate file type, write in comments below and I'll share my solution.

(Spoiler: accept attribute is not bulletproof solution)

var uploadField = document.getElementById("file");

uploadField.onchange = function() {
    if(this.files[0].size > 2097152){
       alert("File is too big!");
       this.value = "";
    };
};

This example should work fine. I set it up for roughly 2MB, 1MB in Bytes is 1,048,576 so you can multiply it by the limit you need.

Here is the jsfiddle example for more clearence:
https://jsfiddle.net/7bjfr/808/

This is completely possible. Use Javascript.

I use jQuery to select the input element. I have it set up with an onChange event.

$("#aFile_upload").on("change", function (e) {

    var count=1;
    var files = e.currentTarget.files; // puts all files into an array

    // call them as such; files[0].size will get you the file size of the 0th file
    for (var x in files) {
    
        var filesize = ((files[x].size/1024)/1024).toFixed(4); // MB
    
        if (files[x].name != "item" && typeof files[x].name != "undefined" && filesize <= 10) { 

            if (count > 1) {
                
                approvedHTML += ", "+files[x].name;
            }
            else {
            
                approvedHTML += files[x].name;
            }

            count++;
        }
    }
    $("#approvedFiles").val(approvedHTML);

});

The code above saves all the file names that I deem worthy of persisting to the submission page before the submission actually happens. I add the "approved" files to an input element's val using jQuery so a form submit will send the names of the files I want to save. All the files will be submitted, however, now on the server-side, we do have to filter these out. I haven't written any code for that yet but use your imagination. I assume one can accomplish this by a for loop and matching the names sent over from the input field and matching them to the $_FILES (PHP Superglobal, sorry I don't know ruby file variable) variable.

My point is you can do checks for files before submission. I do this and then output it to the user before he/she submits the form, to let them know what they are uploading to my site. Anything that doesn't meet the criteria does not get displayed back to the user and therefore they should know, that the files that are too large won't be saved. This should work on all browsers because I'm not using the FormData object.