Why the property 'classList' cannot be read?

Question

I am trying to add and remove a class depending on whether a variable is equal to 0. Here is my code:

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

class SelectCounter extends LitElement {
static get properties() {
    return {
        numPassengers: {type: Number},
    };
}

constructor() {
    super();
    this.numPassengers = 0;
    this.disableState();
}

createRenderRoot() {
    return this;
}

disableState() {
    this.btn = document.querySelector('.minus');
    if (this.numPassengers === 0) {
        this.btn.classList.add('counter__btn--disable');
    }
}

render(){
    return html`
        <div class="counter">
            <a class="counter__btn minus" href="#"></a>
            <span class="counter__label">${this.numPassengers}</span>
            <a class="counter__btn" href="#"></a>
        </div>
    `;
}
}
customElements.define('select-counter', SelectCounter);

I've tried it in several ways but I don't understand why it happens. Any idea?

Apparently the selector didn't return an element. Either log that out to be sure or update your check to `if (this.btn && this.numPassengers === 0) {`. — isherwood, Feb 17 '20 at 16:41

Umbo · Accepted Answer · 2021-07-10T08:39:13.783

You're calling disableState() in the constructor, when the DOM of the component is not ready yet so this:

this.btn = document.querySelector('.minus');

is null.

If you move that logic in firstUpdated() then it should work.

firstUpdated() {
    disableState();
}

disableState() {
    // Also, consider querySelecting inside this.renderRoot
    // to avoid picking elements with the same class in
    // other parts of the document
    this.btn = this.renderRoot.querySelector('.minus');
    // ...
}

Note that you can do the same thing in a more declarative way, without the need to select the element:

render() {
    return html`
        <div class="counter">
            <a class="counter__btn minus ${this.numPassengers === 0 ? 'counter__btn--disable' : ''}" href="#"></a>
            <span class="counter__label">${this.numPassengers}</span>
            <a class="counter__btn" href="#"></a>
        </div>
    `;
}

or by using lit-html's classMap directive:

import {classMap} from 'lit-html/directives/class-map.js';

// ...

render() {
    return html`
        <div class="counter">
            <a class=${classMap({
                counter__btn: true,
                minus: true,
                'counter__btn--disable': this.numPassengers === 0,
            })} href="#"></a>
            <span class="counter__label">${this.numPassengers}</span>
            <a class="counter__btn" href="#"></a>
        </div>
    `;
}

Why the property 'classList' cannot be read?

1 Answers1