Register WordPress Scripts

function-admin.php

<?php
// Add custom JS script to WordPress admin.
function custom_scripts() {

	wp_register_script( 'my-admin-script', 'http://example.com/custom.js', array( 'jquery' ), '1.0', true );
	wp_enqueue_script( 'my-admin-script' );

}

// Hook into the 'admin_enqueue_scripts' action
add_action( 'admin_enqueue_scripts', 'custom_scripts' );

function-login.php

<?php
// Add custom JS script to WordPress login.
function custom_scripts() {

	wp_register_script( 'my-login-script', 'http://example.com/custom.js', array( 'jquery' ), '1.0', false );
	wp_enqueue_script( 'my-login-script' );

}

// Hook into the 'login_enqueue_scripts' action
add_action( 'login_enqueue_scripts', 'custom_scripts' );

function.php

<?php
// Register Scripts – jQuery, jQuery Migrate, Bootstrap & Custom Script
function custom_scripts() {
	wp_enqueue_style( 'style', get_stylesheet_uri() );

	wp_register_style( 'bootstrap', '//maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css', array(), '4.0.0', 'all' );
	wp_enqueue_style( 'bootstrap' );

	wp_register_script( 'popper', '//cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js', array( 'jquery' ), '1.12.9', true );
	wp_enqueue_script( 'popper' );

	wp_register_script( 'bootstrap', '//maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js', array( 'jquery', 'popper' ), '4.0.0', true );
	wp_enqueue_script( 'bootstrap' );

	wp_register_script( 'script', get_template_directory_uri() .'/assets/js/script.js', array( 'jquery' ), '1.0', true );
	wp_enqueue_script( 'script' );

	if ( is_singular() && comments_open() && get_option( 'thread_comments' ) ) {
		wp_enqueue_script( 'comment-reply' );
	}
}

// Hook into the 'wp_enqueue_scripts' action
add_action( 'wp_enqueue_scripts', 'custom_scripts' );