Question: do "previous mapper results" rely on order of keys

Given the following code snippet from the readme:

const Composed = adopt({
  cart: <Cart />,
  user: <User />,
  shippingRates: ({ user, cart, render }) => (
    <ShippingRate zipcode={user.zipcode} items={cart.items}>
      {render}
    </ShippingRate>
  )
})

How do you know in which order to apply the render prop components? Does this rely on the order of keys returned from Object.keys(mapper) call here?

As far as I know, object property order is not guaranteed. Asking because I’d love to use this library, but wondering if this could lead to some nasty (maybe cross browser) bugs.

Example of what could go wrong

const First = ({ children }: FirstProps) => children("foo");
const Second = ({ first, children }: SecondProps) => children(`${first}bar`);
const Display = ({ first, second }: DisplayProps) => (
  <div>
    First: {first}
    <br />
    Second: {second}
  </div>
);

const Composed = adopt({
  first: <First />,
  second: ({ first, render }) => <Second first={first}>{render}</Second>
});

const ComposedInverse = adopt({
  second: ({ first, render }) => <Second first={first}>{render}</Second>
  first: <First />,
});

const App = () => (
  <div style={styles}>
    <h3>Correct order</h3>
    <Composed>
      {({ first, second }) => <Display first={first} second={second} />}
    </Composed>

    <h3>Inversed</h3>
    <ComposedInverse>
      {({ first, second }) => <Display first={first} second={second} />}
    </ComposedInverse>
  </div>
);

Result

[lucasconstantino]

@edorivai thanks for the heads up, it is very much appreciated. This lib reached a point where it should definitely address these uncommon but very relevant problems, if not by fixing them, at least providing people with information for the future 😉

[edorivai]

I actually wanted to take a stab at this, only to find out that Typescript doesn’t support Map out of the box.

Meaning we have 3 options:

1. Ship a polyfill along with this library on npm
2. Target ES6
3. Implement it with something other than Map

What do you guys think?