messenger_ninja.user.js

// ==UserScript==
// @name         messenger ninja
// @namespace    https://gist.github.com/akinov/95cf1c427ce26198a5bfc9c6b70ec109
// @version      0.1
// @description  ninja!
// @author       akinov
// @grant        none
// @match        https://www.messenger.com/*
/* load jQuery */
// @require  https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js
// ==/UserScript==

(function() {
    'use strict';

    const nekoSrc = 'https://pbs.twimg.com/profile_images/424484505915621376/EOwsjaMZ_400x400.png',
          nekoName = '猫田 猫男';

    const $toggeBtn = $('<div>🎭</div>').css({
        position: 'fixed',
        right: '10px',
        bottom: '10px',
        'z-index': '1000'
    }).on('click', toggleNinja);
    $('body').append($toggeBtn);

    function toggleNinja() {
        $('._1t2u ._4ld- > img, ._1t2u ._1jt6 img, .uiScrollableAreaContent ._55lt img').each(function() {
            const $this = $(this);
            if (!$this.attr('origin-src')) {
                $this.attr('origin-src', $this.attr('src'));
            }

            toggleImg($this);
        });

        $('._3oh- a, .uiScrollableAreaContent ._1ht6').each(function() {
            const $this = $(this);
            if (!$this.attr('origin-text')) {
                $this.attr('origin-text', $this.text());
            }
            toggleText($this);
        });
    }

    function toggleImg($this) {
        if ($this.attr('origin-src') == $this.attr('src')) {
            $this.attr('src', nekoSrc);
        } else {
            $this.attr('src', $this.attr('origin-src'));
        }
    }

    function toggleText($this) {
        if ($this.attr('origin-text') == $this.text()) {
            $this.text(nekoName);
        } else {
            $this.text($this.attr('origin-text'));
        }
    }
})();

What exactly does this script do?