Narrow set has if true

[CarsonF]

If the set includes the item then it's safe to narrow it to the type of the set.

const colors = new Set(['red', 'blue'] as const);
const inputColor = 'red' as string;
if (colors.has(inputColor)) {
  inputColor // => 'red' | 'blue'
}

[funtaps]

It is not really safe:

const colors = new Set(['red', 'blue'] as const);
const f = (s: Set<string>) => {
  s.add("asd");
}
f(colors);
console.log(colors.entries());

[CarsonF]

Mmm see your problem is you are mutating 😛

Jokes aside, that's not relevant to this.
TS thinks entries() returns

IterableIterator<["red" | "blue", "red" | "blue"]>

Which it's obviously now wrong about.
The fact that you can skate around some of the type safety is well known with TS.
TS tries to be pragmatically safe not absolutely safe.

If you declare a Set of X though and then check to see if Y is within it, you can confirm that Y is in X.
I believe this narrowing is safe.

[russelldavis]

@CarsonF I agree w/ that assessment. Unfortunately there's a different issue that makes this unsafe. Type predicates also narrow in the false case. Which means this can happen: Playground link

const colors = new Set<"red" | "blue">()
const inputColor = 'red' as 'red' | 'blue' | 'green'
if (!colors.has(inputColor)) {
  inputColor // unsafe: type is 'green', but value is 'red'
}

To fix this, typescript would need to add new language features. Discussed here: microsoft/TypeScript#51678 (comment)

[DeepDoge]

@russelldavis

const colors = new Set(['red', 'blue'] as const);
const inputColor = 'red' as 'red' | 'blue' | 'green'
if (!colors.has(inputColor)) {
  inputColor // unsafe: type is 'green', but value is 'red'
}

The example you showed is expected and correct behavior. Issue shown in the link you provided is different.

In your example:
If inputColor can be "red", "blue" or "green" and if its not "red" or "blue", then it's "green".
In that case based on the runtime logic, code never reaches inside of the if statement.
If it can reach there then inputColor clearly not "red" or "blue".
So "green" is the correct type.

const colors = new Set(['red', 'blue'] as const); // Set<"red" | "blue">
const inputColor = 'red' as 'red' | 'blue' | 'green'
if (!colors.has(inputColor)) {
   inputColor // type is 'green', but value is `never` because code never reaches here.
}

The actual issue pointed in the link you shared is:
When array/set T is not a literal, value is can remove the whole string type from value.

const colors = new Set(["red", "blue"]); // Set<string>
const inputColor = "red" as "red" | "blue" | "green";
if (!colors.has(inputColor)) {
  inputColor; // type is `never` while it shouldn't be
}

But, this can currently be avoided by checking if the array/set T is a literal or not. And if its not literal, it can not narrow and should just return boolean, you can do this with overloads.

Like this:

interface Set<T> {
  has<V>(value: IfTrue<IsLiteral<T>, T | (WidenLiteral<T> & {}), never>): value is T;
  has<V>(value: IfTrue<IsLiteral<T>, never, T | (WidenLiteral<T> & {})>): boolean;
}

But I would also do further changes and allow unknown as has() argument too.
(Which was the main point of my issue #180)

Playground

interface Set<T> {
  has<V>(
    value: V & IfTrue<IsLiteral<T>, V extends T ? T | (WidenLiteral<T> & {}) : V, never>,
  ): value is T;
  has<V>(
    value: V & IfTrue<IsLiteral<T>, never, V extends T ? T | (WidenLiteral<T> & {}) : V>,
  ): boolean;
}

// When T is not literal.
{
  const colors = new Set(["red", "blue"]);
  const inputColor = "red" as "red" | "blue" | "green";
  if (!colors.has(inputColor)) {
    inputColor; // "red" | "blue" | "green"
    // ^?
  } else {
    inputColor; // "red" | "blue" | "green"
    // ^?
  }
}

// When T is literal
{
  const colors = new Set(["red", "blue"] as const);
  const inputColor = "red" as "red" | "blue" | "green";
  if (!colors.has(inputColor)) {
    inputColor; // "green"
    // ^?
  } else {
    inputColor; // "red" | "blue"
    // ^?
  }
}

{
  const colors = new Set(["red", "blue"] as const);
  const inputColor = "red" as unknown;
  if (!colors.has(inputColor)) {
    inputColor; // unknown
    // ^?
  } else {
    inputColor; // "red" | "blue"
    // ^?
  }
}

{
  const colors = new Set(["red", "blue"] as const);
  const inputColor = "red" as "red" | 123;
  if (!colors.has(inputColor)) {
    inputColor; // 123
    // ^?
  } else {
    inputColor; // "red"
    // ^?
  }
}

{
  const colors = new Set(["red", "blue", 0 as number] as const); // Set<"red" | "blue" | number>
  const inputColor = "red" as "red" | 123;
  if (!colors.has(inputColor)) {
    inputColor; // "red" | 123
    // ^?
  } else {
    inputColor; // "red" | 123
    // ^?
  }
}

Also, as a note, everything we did here for has() can also be copy/pasted for delete() too, because delete also returns true if it can find the value it's trying to delete.

[russelldavis]

@DeepDoge Sorry about that -- the issue is real (and is the same issue I linked to), but my example code didn't properly illustrate it. I've updated my comment and playground link with a proper example. I just changed new Set(['red', 'blue'] as const) to new Set<"red" | "blue">(). With that change, the set has the same type as before, but is now empty.

If you run that updated code, you'll see that inputColor has the type green but a value of red, and the line does get reached. The one case where this problem doesn't occur is if the set contains every value in the union (which happened to be true in the original example, but won't always be true). This is also talked about in the linked comment in the typescript repo.

[DeepDoge]

@russelldavis Oh alright, this makes sense now. Set accepts "red" | "blue" but empty, which makes the type to be wrong, true.

[mattpocock]

Closing for reasons described above. Thanks for the discussion, that was really useful.