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Description
Bug Report
A not-uncommon JS idiom is [].concat(x || [], y || [], z || []) as a faster, simpler, more backwards-compatible [x, y, z].flatMap(x => x || []). However, when I try, I get type errors.
🔎 Search Terms
array concat concat overload
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about array concat
⏯ Playground Link
Playground link with relevant code
💻 Code
const a = [].concat(
true ? 'a' : [],
false ? 'b' : [],
);
const x = [].concat(
false ? 'b' : [],
true ? 'a' : [],
);
🙁 Actual behavior
No overload matches this call.
Overload 1 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
Argument of type 'never[] | "a"' is not assignable to parameter of type 'ConcatArray<never>'.
Type 'string' is not assignable to type 'ConcatArray<never>'.
Overload 2 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
Argument of type 'never[] | "a"' is not assignable to parameter of type 'ConcatArray<never>'.
Type 'string' is not assignable to type 'ConcatArray<never>'.(2769)
🙂 Expected behavior
No type errors (except perhaps that the variable may be any until i provide an explicit type for the resulting array), because this is valid JavaScript since at least ES3.