Shadow Root getElementsByClassName

I am using LitElement to create custom Web Components. I am fairly new at it and decided to try making image slideshow. I used W3Schools slideshow as reference while modifying it to work as LitElement. The problem is, that when I am trying to use document.getElementByClassName I am not getting anything. I am familiar with this issue since I am working with Shadow DOM so I changed it to this.shadowRoot.getElementsByClassName. Unfortunately, I get told that what I am trying to use is not a function. How Do I get elements by class name when I am working with LitElement and shadow dom? In case you want to see how my component looks like, here is the code:

import { LitElement, html} from '@polymer/lit-element';

class ImageGalleryElement extends LitElement {

static get properties() { return {
    slideIndex: { type: Number },
  }};

constructor(){
    super();

    this.slideIndex=1;
    this.showSlides(this.slideIndex);
}

// Next/previous controls
plusSlides(n) {
    this.showSlides(this.slideIndex += n);
}

// Thumbnail image controls
currentSlide(n) {
    this.showSlides(this.slideIndex = n);
}

showSlides(n) {
    var i;
    console.dir(this.shadowRoot);
    var slides = this.shadowRoot.getElementsByClassName("mySlides");
    console.dir(slides);
    var dots = this.shadowRoot.getElementsByClassName("dot");
    if (n > slides.length) {this.slideIndex = 1} 
    if (n < 1) {this.slideIndex = slides.length}
    for (i = 0; i < slides.length; i++) {
        slides[i].style.display = "none"; 
    }
    for (i = 0; i < dots.length; i++) {
        dots[i].className = dots[i].className.replace(" active", "");
    }
    slides[this.slideIndex-1].style.display = "block"; 
    dots[this.slideIndex-1].className += " active";
}


render(){
return html`
    <style>
        * {box-sizing:border-box}

        /* Slideshow container */
        .slideshow-container {
            max-width: 1000px;
            position: relative;
            margin: auto;
        }

        /* Hide the images by default */
        .mySlides {
            display: none;
        }

        /* Next & previous buttons */
        .prev, .next {
            cursor: pointer;
            position: absolute;
            top: 50%;
            width: auto;
            margin-top: -22px;
            padding: 16px;
            color: white;
            font-weight: bold;
            font-size: 18px;
            transition: 0.6s ease;
            border-radius: 0 3px 3px 0;
            user-select: none;
        }

        /* Position the "next button" to the right */
        .next {
            right: 0;
            border-radius: 3px 0 0 3px;
        }

        /* On hover, add a black background color with a little bit see-through */
        .prev:hover, .next:hover {
            background-color: rgba(0,0,0,0.8);
        }

        /* Caption text */
        .text {
            color: #f2f2f2;
            font-size: 15px;
            padding: 8px 12px;
            position: absolute;
            bottom: 8px;
            width: 100%;
            text-align: center;
        }

        /* Number text (1/3 etc) */
        .numbertext {
            color: #f2f2f2;
            font-size: 12px;
            padding: 8px 12px;
            position: absolute;
            top: 0;
        }

        /* The dots/bullets/indicators */
        .dot {
            cursor: pointer;
            height: 15px;
            width: 15px;
            margin: 0 2px;
            background-color: #bbb;
            border-radius: 50%;
            display: inline-block;
            transition: background-color 0.6s ease;
        }

        .active, .dot:hover {
            background-color: #717171;
        }

        /* Fading animation */
        .fade {
            -webkit-animation-name: fade;
            -webkit-animation-duration: 1.5s;
            animation-name: fade;
            animation-duration: 1.5s;
        }

        @-webkit-keyframes fade {
            from {opacity: .4} 
            to {opacity: 1}
        }

        @keyframes fade {
            from {opacity: .4} 
            to {opacity: 1}
        }
    </style>
    <!-- Slideshow container -->
    <div class="slideshow-container">

        <!-- Full-width images with number and caption text -->
        <div class="mySlides fade">
            <div class="numbertext">1 / 3</div>
            <img src="../../img/img-snow-wide" style="width:100%">
            <div class="text">Caption Text</div>
        </div>

        <div class="mySlides fade">
            <div class="numbertext">2 / 3</div>
            <img src="../../img/img-nature-wide" style="width:100%">
            <div class="text">Caption Two</div>
        </div>

        <div class="mySlides fade">
            <div class="numbertext">3 / 3</div>
            <img src="../../img/img-mountains-wide" style="width:100%">
            <div class="text">Caption Three</div>
        </div>

        <!-- Next and previous buttons -->
        <a class="prev" @click="${this.plusSlides(-1)}" >&#10094;</a>
        <a class="next" @click="${this.plusSlides(1)}">&#10095;</a>
    </div>
    <br>

    <!-- The dots/circles -->
    <div style="text-align:center">
        <span class="dot" @click="${this.currentSlide(1)}"></span> 
        <span class="dot" @click="${this.currentSlide(2)}"></span> 
        <span class="dot" @click="${this.currentSlide(3)}"></span> 
    </div>
`;
}
} 
// Register the element with the browser
customElements.define('image-gallery-element', ImageGalleryElement);

Answers:

Answer

The getElementsByClassName() method works only on a HTML Document or element.

The shadowRoot is a Document Fragment by inheritance, not a Document nor a HTML element.

Instead you should use querySelectorAll().

Update

It's the same behavior for getElementById().

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