Mutating a Set can change the size - but lib.es2015.collection.d.ts declares size as a readonly property #62468
Description
🔎 Search Terms
"Set", "size", "readonly", "mutate"
🕗 Version & Regression Information
- I was unwilling to test this on prior versions
⏯ Playground Link
💻 Code
function computeNumber(): number {
const s = new Set<number>([34]);
if (s.size === 1) {
s.delete(34);
if (s.size === 1) {
return 5;
}
return s.size || "a string";
}
return 5;
}🙁 Actual behavior
The code in the snippet passes the type checker even though this function returns a string instead of a number.
The size property of Set is typed in such a way that TS thinks that it can never change on a set object.
🙂 Expected behavior
The snippet should fail to type check because it returns a string whereas the function type requires a number to be returned instead.
The size property of Set should be typed in such a way that TS knows that it can change when set member functions are called.
Additional information about the issue
It appears that lib.es2015.collection.d.ts types the size property in such a way that TypeScript thinks it will never change.
In my actual code this causes a false positive of @typescript-eslint/no-unnecessary-condition in code like this:
if (newSelectedThreadIndexes.size === 0) {
// call newSelectedThreadIndexes.add conditionally
// [...]
if (newSelectedThreadIndexes.size === 0) { // <-- "Unnecessary conditional, comparison is always true, since `0 === 0` is true"
// No thread could be found, so do not isolate this process.
return;
}