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I'm making a web ponent that will show "Hello {name}!" where {name} es from name="foo". When I try it I don't get any errors but it just displays "Hello null!".
index.html:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="./script.js"></script>
</head>
<body>
  <hello-world name="Joe"></hello-world>
</body>
</html>

script.js:

class HelloWorld extends HTMLElement {
  constructor() {
    super();

    this.attachShadow({ mode: 'open' });

    const p = document.createElement('p');

    p.innerHTML = `Hello ${this.getAttribute('name')}!`;

    this.shadowRoot.append(p);
  }
}

customElements.define('hello-world', HelloWorld);

In any situation lets assume that name will always have an input.

I'm making a web ponent that will show "Hello {name}!" where {name} es from name="foo". When I try it I don't get any errors but it just displays "Hello null!".
index.html:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="./script.js"></script>
</head>
<body>
  <hello-world name="Joe"></hello-world>
</body>
</html>

script.js:

class HelloWorld extends HTMLElement {
  constructor() {
    super();

    this.attachShadow({ mode: 'open' });

    const p = document.createElement('p');

    p.innerHTML = `Hello ${this.getAttribute('name')}!`;

    this.shadowRoot.append(p);
  }
}

customElements.define('hello-world', HelloWorld);

In any situation lets assume that name will always have an input.

  • javascript
  • html
  • web-ponent
  • custom-element
  • native-web-ponent
Share Improve this question edited Jan 14, 2022 at 20:52 connexo 57k15 gold badges111 silver badges147 bronze badges asked Dec 2, 2021 at 14:23 TacoSnackTacoSnack 1791 silver badge12 bronze badges 3
  • nvm fixed with <script defer src="./script.js"> – TacoSnack Commented Dec 2, 2021 at 14:31
  • 1 That is a hack, your are making the JS execute after all DOM is parsed. Don't do DOM access in the constructor like connexo explains. – Danny '365CSI' Engelman Commented Dec 2, 2021 at 15:33
  • @Danny'365CSI'Engelman Strictly speaking, access its own attributes is not accessing the DOM; the reason not to do this is that in the dynamic creation case new HelloWorld or document.createElement('hello-world') the attribute won't be present. Relying on attributes in the constructor can easily make the element dysfunctional for those use-cases, even though in this specific case it won't even throw an error. – connexo Commented Dec 2, 2021 at 15:36
Add a ment  | 

1 Answer 1

Reset to default 6

You must not read attributes in the constructor, otherwise you're violating the specification:

The element's attributes and children must not be inspected, as in the non-upgrade case none will be present, and relying on upgrades makes the element less usable.
From: https://html.spec.whatwg/multipage/custom-elements.html#custom-element-conformance

You must delay this kind of work until the connectedCallback triggers, or, in the case of attributes, configure a proper attributeChangedCallback.

Conforming to the spec will also solve your initial issue, and it will greatly enhance the functionality and usefulness of your web ponent.

Please note that it's also not the smartest of ideas to pick an attribute name that already exists in the HTML specification, as a universal attribute.

class HelloWorld extends HTMLElement {
  p = document.createElement('p');
  constructor() {
    super();
    this.attachShadow({ mode: 'open' }).append(this.p);
  }
  
  static get observedAttributes() { return ['greeting-name']; }
  attributeChangedCallback(attr, oldVal, newVal) {
    if (oldVal === newVal) return; // nothing to do
    switch (attr) {
      case 'greeting-name':
        this.p.textContent = `Hello ${newVal || 'world'}!`;
        break;
    }
  }
  
  connectedCallback() {
    if (!this.getAttribute('greeting-name')) { this.setAttribute('greeting-name', 'world'); }
  }
}

customElements.define('hello-world', HelloWorld);
<hello-world greeting-name="Joe"></hello-world>

<hello-world id="foo"></hello-world>

<input type="text" oninput="document.getElementById('foo').setAttribute('greeting-name', this.value)" placeholder="Type a name" />

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