Allow component fields to be declared const (Think PropTypes in React)

[maxamante]

Recently, I've implemented a component that has a field that needs to be declared const. Ideally, this field would be set on create and all changes to it would be thrown away. @TehShrike expressed in Gitter the desire for functionality like PropTypes/duck-type checking. At the suggestion, I thought of 'PropAttrs', that would allow a user to declare the field's desired properties and Svelte would perform checks on create/update. Thoughts?

[Rich-Harris]

I wonder if this is another argument for supporting TypeScript/Flow annotations? (See #418.) We could do this sort of thing:

<script language='typescript'>
  interface Data {
    readonly foo: string;
  };

  export default <Options>{
    data() {
      return <Data>{
        foo: 'x'
      };
    }
  };
</script>

Anyway, for posterity's sake, here's the workaround mentioned in Gitter — using helpers: https://svelte.technology/repl?version=1.40.1&gist=93912bc8b9ad55b97492e33d3d9d833b

[arxpoetica]

@maxamante how are things going for you on this issue?

[maxamante]

Hello. @arxpoetica Rich's workaround helped me figure out a solution. You can close if you need to.

[arxpoetica]

Thx