Implementing attributeChangedCallback in Typescript?

Question

In this tutorial attributeChangedCallback is implemented like this:

// attribute change
attributeChangedCallback(property, oldValue, newValue) {

  if (oldValue === newValue) return;
  this[ property ] = newValue;

}

And I'm trying to convert it to Typescript and so far I have this:

// attribute change
attributeChangedCallback(property:keyof HelloWorldComponent, oldValue:any, newValue:any) {
    if (oldValue === newValue) return;
    this[ property ] = newValue;  
  }  
}

However this still creates the error:

Cannot assign to 'ATTRIBUTE_NODE' because it is a read-only property.ts(2540)
Cannot assign to 'CDATA_SECTION_NODE' because it is a read-only property.ts(2540)
Cannot assign to 'COMMENT_NODE' because it is a read-only property.ts(2540)
Cannot assign to 'DOCUMENT_FRAGMENT_NODE' because it is a read-only property.ts(2540)
Cannot assign to 'DOCUMENT_NODE' because it is a read-only property.ts(2540)

Thoughts?

Ugwu Somto · Accepted Answer · 2024-07-04 21:09:52Z

4

Seems you are trying to assign to properties that are read-only and typescript always defaults to a type that includes read-only properties when it can't correctly infer the type of the property parameter

This is a better fix

class HelloWorldComponent extends HTMLElement {

  private attr1: string | null = null;
  private attr2: string | null = null;

  attributeChangedCallback(property: string, oldValue: string | null, newValue: string | null) {
    if (oldValue === newValue) return;

    switch (property) {
      case 'attr1':
        this.attr1 = newValue;
        break;
      case 'attr2':
        this.attr2 = newValue;
        break;
      // Handle other attributes if needed
      default:
        throw new Error(`Unhandled attribute: ${property}`);
    }
  }
}

answered Jul 4, 2024 at 21:09

Ugwu Somto

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1 Comment

Ole Over a year ago

Thanks - Great Answer! Welcome to SO Ugwu!!

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