I need to show a modal message like "Uploaded file exceeds X MB" when the user tries to upload a large file. I have set maximum size in the field settings and when I try to upload a large file Drupal shows a label with custom error. I need to trap this event and launch the js with modal message.

I have a;

 function hook_file_validate($file) {


It is invoked in ajax so I cannot make a drupal_add_js call with the javascript code to execute. How can I execute JS method for solving this problem?


It sounds like you're wanting to do file size upload validation with complicated method of server/client code. If all you need is size validation, you can do that client-side without the need of the server.

There are many ways to do this; one example would involve adding a JS onchange event handler to your file field.

MYMODLE_form_alter(&$form, &$form_state, $form_id) {
  if($form_id = "my_form") {
    $form['field_MYFILEFIELD']['#attributes']['onchange'] = "
      if (event.srcElement.files[0].size > (8 * 1048576)) { 
        alert('Uploaded file exceeds 8 MB'); 
        return false;  
  //@TODO: move the validation code to a function & use better modal

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