Return type after assign to a const is different to return type without assign?

[unkempt marlin]

I have the following code where the return type of the function a is inferred to be {a?: undefined} | {a: number}, and the return type of the function b is inferred to be {}. Can someone explain why, and if I can get TypeScript to infer what I believe is the more correct type in the second case?

function a() {
  if (Math.random()) {
    return {};
  }
  return {a: 0};
}

function b() {
  if (Math.random()) {
    return {};
  }
  const t: {a: number} = {a: 0};
  return t;
}

[sharp sentinel]

extracraftx#0

Playground Link

Preview:```ts
function a() {
if (Math.random()) {
return {}
}
return {a: 0}
}

function b() {
if (Math.random()) {
return {}
}
const t: {a: number} = {a: 0}
return t
}```

[wooden skiff]

is there a reason you don't want to annotate the return type? i wouldn't rely on inference in this case since you care about the specific type

[unkempt marlin]

Yes, this is just a minimal example with two branches, but my actual function is going to be at least a couple more branches with more complex objects, so I would like to avoid explicitly specifying the return type

In any case, I feel this behaviour is odd right? There's no actual difference in the types of the return statements

Also this infers how I would expect:

function b() {
  if (Math.random()) {
    return {a: 0};
  }
  const t = {a: 0, b: ''};
  return t;
}

[wooden skiff]

i think in a the fact that the value is being returned can be used while inferring a type for the value {a: 0} (and that in turn can feed back to affect the overall return type). in b there's already a type for the returned value, TS sees that it's assignable to {} (the contextually-determined return type at that moment), and therefore doesn't narrow {}

just a guess though. you'd have to dig through the implementation to be sure. type inference works in mysterious ways

[wooden skiff]

if you don't want to annotate the overall return type, consider only annotating the default value that you return?

function c() {
  if (Math.random()) {
    const defaultValue: { a?: number } = {}
    return defaultValue;
  }
  const t: {a: number} = {a: 0};
  return t;
}

(i'm not sure if this transfers to your real use case though)

[unkempt marlin]

Thanks, was considering this already, it will have to do for now. I'll try digging into the implementation to try and figure out why this is happening. If anyone has any insights though would be appreciated

[tulip radish]

@unkempt marlin Compiler internals channel, or a github issue is probably the best way to get the attention of someone who knows about that. If you want to pester them over it that is.