example.ts

class Data {
    a?: string
    b?: number


  static fromJson(json: Record<string, any>): Data {
    return {a: json.a, b: json.b}
  }
}

class UIProperty {
  key: keyof Data
  label: string
  value?: string | number
  
  // It's possible to simplify the types of this constructor somewhat, but the dart way is actually quite a lot better than typescript imo
  constructor({key, label, value}: {key: keyof Data, label: string, value?: string | number}) {
    this.key = key
    this.label = label
    this.value = value
  }
}

async function getData() {
  return Data.fromJson({a: 'a', b: 2})
}

function createPropertyList() {
  // Typescript will correctly infer the type of list to be UIProperty[]
  let list = [
      // Typescript will also understand that key: 'a' is a key of Data and that this is type-safe
      new UIProperty({ key: 'a', label: 'Label A' }),
      new UIProperty({ key: 'b', label: 'Label B' }),
      
      // This would however not compile, since 'c' is not a keyof Data
      new UIProperty({ key: 'c', label: 'Label C' }),
  ]
  
  getData().then(data => {
    for (let prop of list) {
      // This is also typesafe, since prop.key is a key of data
      prop.value = data[prop.key]
    }
  })
}