Prevent browser from loading a drag-and-dropped file

After a lot of fiddling around, I found this to be the stablest solution:

var dropzoneId = "dropzone";

window.addEventListener("dragenter", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
}, false);

window.addEventListener("dragover", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
});

window.addEventListener("drop", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
});

<div id="dropzone">...</div>

Setting both effectAllow and dropEffect unconditionally on the window causes my drop zone not to accept any d-n-d any longer, regardless whether the properties are set new or not.

For jQuery the correct answer will be:

$(document).on({
    dragover: function() {
        return false;
    },
    drop: function() {
        return false;
    }
});

Here return false will behave as event.preventDefault() and event.stopPropagation().

To allow drag-and-drop only on some elements, you could do something like:

window.addEventListener("dragover",function(e){
  e = e || event;
  console.log(e);
  if (e.target.tagName != "INPUT") { // check which element is our target
    e.preventDefault();
  }
},false);
window.addEventListener("drop",function(e){
  e = e || event;
  console.log(e);
  if (e.target.tagName != "INPUT") {  // check which element is our target
    e.preventDefault();
  }  
},false);

Note: Although the OP did not ask for an Angular solution, I came here looking for that. So this is to share what I found to be a viable solution, if you use Angular.

In my experience this problem first arises when you add file drop functionality to a page. Therefore my opinion is that the component that adds this, should also be responsible for preventing drop outside of the drop zone.

In my solution the drop zone is an input with a class, but any unambiguous selector works.

import { Component, HostListener } from '@angular/core';
//...

@Component({
  template: `
    <form>
      <!-- ... -->
      <input type="file" class="dropzone" />
    </form>
  `
})
export class MyComponentWithDropTarget {

  //...

  @HostListener('document:dragover', ['$event'])
  @HostListener('drop', ['$event'])
  onDragDropFileVerifyZone(event) {
    if (event.target.matches('input.dropzone')) {
      // In drop zone. I don't want listeners later in event-chain to meddle in here
      event.stopPropagation();
    } else {
      // Outside of drop zone! Prevent default action, and do not show copy/move icon
      event.preventDefault();
      event.dataTransfer.effectAllowed = 'none';
      event.dataTransfer.dropEffect = 'none';
    }
  }
}

The listeners are added/removed automatically when component is created/destroyed, and other components using the same strategy on the same page do not interfere with each other due to the stopPropagation().

Here's a little more modernized version of this answer using ES6 syntax.

let dropzoneId = 'dropzone'

const dragEventHandler = e => {
  if (e.target.id !== dropzoneId) {
    e.preventDefault()
    e.dataTransfer.effectAllowed = 'none'
    e.dataTransfer.dropEffect = 'none'
  }
}

// window.addEventListener("dragenter", dragEventHandler, false)
// window.addEventListener("dragover", dragEventHandler, false)
// window.addEventListener("drop", dragEventHandler, false)
['dragenter', 'dragover', 'drop'].forEach(ev => window.addEventListener(ev, dragEventHandler, false))

<div id="dropzone">...</div>

try this:

document.body.addEventListener('drop', function(e) {
    e.preventDefault();
}, false);

Preventing all drag and drop operations by default might not be what you want. It's possible to check if the drag source is an external file, at least in some browsers. I've included a function to check if the drag source is an external file in this StackOverflow answer.

Modifying Digital Plane's answer, you could do something like this:

function isDragSourceExternalFile() {
     // Defined here: 
     // https://stackoverflow.com/a/32044172/395461
}

window.addEventListener("dragover",function(e){
    e = e || event;
    var IsFile = isDragSourceExternalFile(e.originalEvent.dataTransfer);
    if (IsFile) e.preventDefault();
},false);
window.addEventListener("drop",function(e){
    e = e || event;
    var IsFile = isDragSourceExternalFile(e.originalEvent.dataTransfer);
    if (IsFile) e.preventDefault();
},false);

For what its worth, I use the following. Nice and explicit if not particularly elegant perhaps?

var myDropZone = document.getElementById('drop_zone');

// first, inhibit the default behaviour throughout the window
window.addEventListener('drop', () => { 
  event.preventDefault(); 
} );
window.addEventListener('dragover', () => { 
  event.dataTransfer.dropEffect = 'none'; // dont allow drops
  event.preventDefault(); 
} );

// Next, allow the cursor to show 'copy' as it is dragged over 
// my drop zone but dont forget to stop the event propagating

myDropZone.addEventListener('dragover', () => { 
  event.dataTransfer.dropEffect = 'copy';
  event.stopPropagation(); // important !!
  event.preventDefault();
} );

// In my drop zone, deal with files as they are dropped
myDropZone.addEventListener('drop', myDropHandler);

To build on the "check the target" method outlined in a few other answers, here is a more generic/functional method:

function preventDefaultExcept(predicates) {
  return function (e) {
    var passEvery = predicates.every(function (predicate) { return predicate(e); })
    if (!passEvery) {
      e.preventDefault();
    }
  };
}

Called like:

function isDropzone(e) { return e.target.id === 'dropzone'; }
function isntParagraph(e) { return e.target.tagName !== 'p'; }

window.addEventListener(
  'dragover',
  preventDefaultExcept([isDropzone, isntParagraph])
);
window.addEventListener(
  'drop',
  preventDefaultExcept([isDropzone])
);

I have an HTML object (embed) that fills the width and height of the page. The answer by @digital-plane works on normal web pages but not if the user drops onto an embedded object. So I needed a different solution.

If we switch to using the event capture phase we can get the events before the embedded object receives them (notice the true value at the end of the event listener call):

// document.body or window
document.body.addEventListener("dragover", function(e){
  e = e || event;
  e.preventDefault();
  console.log("over true");
}, true);

document.body.addEventListener("drop", function(e){
  e = e || event;
  e.preventDefault();
  console.log("drop true");
}, true);

Using the following code (based on @digital-plane's answer) the page becomes a drag target, it prevents object embeds from capturing the events and then loads our images:

document.body.addEventListener("dragover", function(e){
  e = e || event;
  e.preventDefault();
  console.log("over true");
}, true);

document.body.addEventListener("drop",function(e){
  e = e || event;
  e.preventDefault();
  console.log("Drop true");

  // begin loading image data to pass to our embed
  var droppedFiles = e.dataTransfer.files;
  var fileReaders = {};
  var files = {};
  var reader;

  for (var i = 0; i < droppedFiles.length; i++) {
    files[i] = droppedFiles[i]; // bc file is ref is overwritten
    console.log("File: " + files[i].name + " " + files[i].size);
    reader = new FileReader();
    reader.file = files[i]; // bc loadend event has no file ref

    reader.addEventListener("loadend", function (ev, loadedFile) {
      var fileObject = {};
      var currentReader = ev.target;

      loadedFile = currentReader.file;
      console.log("File loaded:" + loadedFile.name);
      fileObject.dataURI = currentReader.result;
      fileObject.name = loadedFile.name;
      fileObject.type = loadedFile.type;
      // call function on embed and pass file object
    });

    reader.readAsDataURL(files[i]);
  }

}, true);

Tested on Firefox on Mac.

I am using a class selector for multiple upload areas so my solution took this less pure form

Based on Axel Amthor's answer, with dependency on jQuery (aliased to $)

_stopBrowserFromOpeningDragAndDropPDFFiles = function () {

        _preventDND = function(e) {
            if (!$(e.target).is($(_uploadBoxSelector))) {
                e.preventDefault();
                e.dataTransfer.effectAllowed = 'none';
                e.dataTransfer.dropEffect = 'none';
            }
        };

        window.addEventListener('dragenter', function (e) {
            _preventDND(e);
        }, false);

        window.addEventListener('dragover', function (e) {
            _preventDND(e);
        });

        window.addEventListener('drop', function (e) {
            _preventDND(e);
        });
    },