Run Code When a Component Is Created

The constructor() method fires when a component instance is created. Don’t add attributes to the host element during construction. You can add attributes to the host element in any other lifecycle hook.

The constructor flows from parent to child.

These requirements from the HTML: Custom elements spec apply to the constructor().

  • The first statement must be super() with no parameters. This call establishes the correct prototype chain and value for this. Always call super() before touching this.
  • Don’t use a return statement inside the constructor body, unless it is a simple early-return (return or return this).
  • Don’t use the document.write() or document.open() methods.
  • Don’t inspect the element's attributes and children, because they don’t exist yet.
  • Don’t inspect the element’s public properties, because they’re set after the component is created.

You can add attributes to the host element during any stage of the component lifecycle other than construction.

This example is illegal, because it adds an attribute to the host element in the constructor().

This example is legal, because it adds an attribute to the host element in the connectedCallback().

See Also