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Array concat has wrong types #47351
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.
Activity
ajafff commentedon Jan 8, 2022
As a workaround you can use
new Array<string>().concat( true ? 'a' : [], false ? 'b' : [])ljharb commentedon Jan 8, 2022
That would create a sparse array, which is deoptimized in engines, so that would never be a good workaround (new Array should never, ever be used).
MartinJohns commentedon Jan 8, 2022
([] as string[])works as well to help the type-checker.MartinJohns commentedon Jan 8, 2022
Duplicate of #36554 (comment). Used search terms:
never array concatljharb commentedon Jan 8, 2022
Thanks for the workaround suggestion! Unfortunately that doesn’t help me in JS, using JSDoc.
fatcerberus commentedon Jan 8, 2022
Your examples work as-is for me in JS, even with
checkJson; that said, the equivalent JSDoc-based workaround would beBut this won't work in a
.tsfile (or your playground) because JSDoc isn't used for typing there.MartinJohns commentedon Jan 8, 2022
Bruce Pascoe (@fatcerberus) The examples don't work with
strictNullChecksenabled. The playground strangely turns this flag off when you switch to JS mode, I fell for it the first time as well.SancheZz commentedon Jan 9, 2022
Jordan Harband (@ljharb) you can use
spreadoperator instead of theconcatmethod as a workaround.MartinJohns commentedon Jan 9, 2022
SancheZz He's intentionally not using the spread operator. See his first sentence, where he mentions:
ljharb commentedon Jan 9, 2022
Also, spread has different semantics - a string is spread into code units, a Map into entries, a Set into values, but the precise semantic I want here is
IsConcatSpreadable, which only concat, flat, and flatMap have.