Wordpress插件：在页面之前加载JavaScript

Creating my first Wordpress Plugin and having an issue with JavaScript loading.

I'm defining a Class which runs several functions that include JavaScript via wp_enqueue_script.

Here is a sample JS file Demonstrating Problem:

//The alert will fire
alert('javascript is firing');
// This will return an error saying 
//Cannot read property 'setAttribute' of undefined
document.getElementsByTagName("body")[0].setAttribute("ng-app", "app");

plugin.php:

<?php
/*
Plugin Name: Wordpress Plugin
Plugin URI: http://www.wordpressplugin.io
Description: A Wordpress Plugin
Author: Me
Version: 1.0
Author URI: http://www.me.com
*/

define('Wordpress Plugin', '1');

class wordpressPlugin {

function __init(){
    add_action( 'wp_enqueue_scripts', array( $this, 'angularScripts'));
}

function angularScripts(){
      wp_enqueue_script('app', plugin_dir_url( __FILE__).'js/app.js',array('jquery'), null, false);
}



//Create instance of class
$wpNG = new wordpressPlugin();
// Activate Plugin
add_action( 'init', array( $wpNG, '__init' ), 1000 );
?>

app.js

alert('hello');
document.getElementsByTagName("body")[0].setAttribute("ng-app", "app");

The alert will fire but the document.getElements will not run and return an error of undefined. But, if I wrap this within a window.onload function the document.getElements will run fine. Doing a window.onload is not the desirable.

How does the JavaScript need to be set up to run without needing window.onload?