Load Javascript when a particular form is loaded

I have the code below in my own drupal module(myid):

myid.module file:

function myid_init() {     drupal_add_js(drupal_get_path("module", "myid") . "/js/myid.js"); }   function myid_menu() {       $items = array();      $items['admin/settings/myid'] = array(         'title' => 'myid Settings',         'description' => 'Configuration options for the myid module',         'page callback' => 'myid_admin_settings',         'access callback ' => 'user_access',         'access arguments' => array('administer site configuration'),     );     return $items; }  function myid_admin_settings() {     return drupal_get_form('myid_settings_form'); }  function myid_settings_form() {     $form = array();      $form['settings'] = array(         '#type' => 'item',          '#title' => t("Registrar's Signature"),         '#description' => t(''),            '#markup' => "<canvas id='myid_registrar_signature' width='500' height='100'></canvas>",             '#required' => TRUE,     );      return $form; } 

myid.js file:

1. $( document ).ready(function() {     2.     myid_retrieve_save_information(); 3. }); 4. function myid_retrieve_save_information(){ 5.     var cnv_rsignature = document.getElementById("myid_registrar_signature");     6.     var ctx_rsignature= cnv_rsignature.getContext('2d'); 7. } 

Why does every time I load the page, it gets the following error pointing to Line 6 in myid.js:

Uncaught TypeError: Cannot read property 'getContext' of null 

Is it because #myid_registrar_signature was not yet loaded when Line 6 was executed? Where did I went wrong?

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Load Javascript when a particular form is loaded

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