[jquery] How do I add a simple jQuery script to WordPress?

The solutions I've seen are from the perspective of adding javascript features to a theme. However, the OP asked, specifically, "How exactly do I add it for a single WordPress page?" This sounds like it might be how I use javascript in my Wordpress blog, where individual posts may have different javascript-powered "widgets". For instance, a post might let the user change variables (sliders, checkboxes, text input fields), and plots or lists the results.

Starting from the JavaScript perspective:

1. Write your JavaScript functions in a separate “.js” file

Don’t even think about including significant JavaScript in your post’s html—create a JavaScript file, or files, with your code.

2. Interface your JavaScript with your post's html

If your JavaScript widget interacts with html controls and fields, you’ll need to understand how to query and set those elements from JavaScript, and also how to let UI elements call your JavaScript functions. Here are a couple of examples; first, from JavaScript:

var val = document.getElementById(“AM_Freq_A_3”).value;

And from html:

<input type="range" id="AM_Freq_A_3" class="freqSlider" min="0" max="1000" value="0" oninput='sliderChanged_AM_widget(this);'/>

3. Use jQuery to call your JavaScript widget’s initialization function

Add this to your .js file, using the name of your function that configures and draws your JavaScript widget when the page is ready for it:

jQuery(document).ready(function( $ ) {
    your_init_function();
});

4. In your post’s html code, load the scripts needed for your post

In the Wordpress code editor, I typically specify the scripts at the end of the post. For instance, I have a scripts folder in my main directory. Inside I have a utilities directory with common JavaScript that some of my posts may share—in this case some of my own math utility function and the flotr2 plotting library. I find it more convenient to group the post-specific JavaScript in another directory, with subdirectories based on date instead of using the media manager, for instance.

<script type="text/javascript" src="/scripts/utils/flotr2.min.js"></script>
<script type="text/javascript" src="/scripts/utils/math.min.js"></script>
<script type="text/javascript" src="/scripts/widgets/20161207/FreqRes.js"></script>

5. Enqueue jQuery

Wordpress registers jQuery, but it isn’t available unless you tell Wordpress you need it, by enqueuing it. If you don’t, the jQuery command will fail. Many sources tell you how to add this command to your functions.php, but assume you know some other important details.

First, it’s a bad idea to edit a theme—any future update of the theme will wipe out your changes. Make a child theme. Here’s how:

https://developer.wordpress.org/themes/advanced-topics/child-themes/

The child’s functions.php file does not override the parent theme’s file of the same name, it adds to it. The child-themes tutorial suggest how to enqueue the parent and child style.css file. We can simply add another line to that function to also enqueue jQuery. Here's my entire functions.php file for the child theme:

<?php
add_action( 'wp_enqueue_scripts', 'earlevel_scripts_enqueue' );
function earlevel_scripts_enqueue() {
    // styles
    $parent_style = 'parent-style';
    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );

    // posts with js widgets need jquery
    wp_enqueue_script('jquery');
}