could be instantiated with a different subtype of constraint '{}' typescript error

Question

I am seeing this error a hell of a lot after upgrading to typescript 3.5.

could be instantiated with a different subtype of constraint '{}' typescript error

I have no idea what it means and it is always referring to the empty {} type.

If you look at this playground on line 34, I have no idea what could be instantiated with a different subtype or what even the subtype could be.

This is a typescript generated message and I think that {} is the default type added to the error message.

This github issue exists but I'm still struggling to understand it

georg · Accepted Answer · 2019-09-22 11:36:28Z

1

I don't understand what the rest of the code does, but the problem at hand boils down to this:

function map2<A, B>(x: A|undefined, fn: (a: A) => B): B|undefined  {
  if (x === undefined) {
      return x;
  } else {
      return fn(x);
  }
}

This doesn't check, because A can also include undefined and TS cannot decide whether to return A or undefined in the first branch.

This can be easily fixed by telling TS that A is never undefined:

function map2<A extends {}, B>(x: A|undefined, fn: (a: A) => B): B|undefined  {
  if (x === undefined) {
      return x;
  } else {
      return fn(x);
  }
}

answered Sep 22, 2019 at 11:36

georg

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