Lightning Web Components – Fix Required/Default Value Issue in LWC Flow Screen Component

It simply means that you need to set the value in just one of two places. Both will be demonstrated here, as well as what's not:

export default class AppName extends LightningElement {
  @api message1 = 'Default message'; // allowed
  @api message2;
  constructor() {
    super(); // must be called, and must be called before using 'this'
    this.message2 = 'New default message'; // also allowed
  }
  onclickhandler() {
    this.message1 = 'Try to change on click...'; // not allowed
  }
}

Note that the constructor is always called constructor, unlike Java and other languages where the constructor is the class' name.

After defining it either inline or via constructor, you are no longer allowed to modify the property at all. It becomes read-only. If you want to be able to communicate to the parent component, the get/set combination is required.

Generic SObject and SObject[] are not yet supported at the interface between LWC and Flow (TLDR: You can't get there from here). We're working on it right now, though, and hope to get it into Summer '20. You can get an early taste for some of the experience by looking at how we implemented it for Apex Invocable Actions in Spring '20

We don't have any short-term plans to support Object and Object[] in Flow, but if you have a use case where you want to work with very freeform data structures, take a look at the support for Apex-Defined Types. Basically, if you design classes in Apex that match your data structure, you can then pass instances of that type across Flow, LWC, and Invocable Action boundaries. A good intro to that is here.