function makeFoo<T>(fn: () => T): () => T {
  // do some work here ...
  return function() {
    // ... or here
    return fn() 
  }
}
const obj = {
  name: 'test',
  foo: makeFoo(() => obj.name.length)
}

const obj = {
  name: 'test',
  foo: makeFoo(() => obj.name.length) as prop  // or a better name
}

function getter<T, R>(t: T, k: keyof T, value: R) {
  Object.defineProperty(t, k, {value, enumerable: true})  
  return value
}
const obj = {
  name: 'test',
  get foo() {
    return getter(obj, 'foo', makeFoo(() => obj.name.length))
  }
}