Split: Just return string[] when S or Delimiter is non-literal

* Use `IsStringLiteral` for conditional branches, which is more appropriate than `string extends ..` * Add test cases where `S` or `Delimiter` is not `string` itself but also not a literal (e.g., Uppercase<string>) Co-authored-by: Som Shekhar Mukherjee <[email protected]>
sindresorhus · Jan 23, 2025 · 784facc · 784facc
1 parent 617a52b
commit 784facc
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Showing 2 changed files with 9 additions and 17 deletions.
diff --git a/source/split.d.ts b/source/split.d.ts
@@ -1,5 +1,5 @@
-import type {ArrayValues} from './array-values';
-import type {Substrings} from './internal';
+import type {IsStringLiteral} from './is-literal';
+
 /**
 Represents an array of strings split using a given character or character set.
 
@@ -24,11 +24,9 @@ array = split(items, ',');
 export type Split<
 	S extends string,
 	Delimiter extends string,
-> = string extends S
-	? string[]
-	: string extends Delimiter
-		? [S] | SplitHelper<S, ArrayValues<Substrings<S>>>
-		: SplitHelper<S, Delimiter>;
+> = IsStringLiteral<S | Delimiter> extends true
+	? SplitHelper<S, Delimiter>
+	: string[];
 
 type SplitHelper<
 	S extends string,

diff --git a/test-d/split.ts b/test-d/split.ts
@@ -45,21 +45,15 @@ expectType<
 
 // Split non-literal string by literal string.
 expectType<string[]>(split(null! as string, 'foobar'));
+expectType<string[]>(split(null! as Uppercase<string>, 'foobar'));
 
 // Split non-literal string by non-literal string.
 expectType<string[]>(split(null! as string, null! as string));
+expectType<string[]>(split(null! as `pre${string}`, null! as `${number}`));
 
 // Split literal string by non-literal string.
-expectType<
-| ['ABCDEF']
-| ['', '']
-| ['', 'F'] | ['A', '']
-| ['', 'EF'] | ['A', 'F'] | ['AB', '']
-| ['', 'DEF'] | ['A', 'EF'] | ['AB', 'F'] | ['ABC', '']
-| ['', 'CDEF'] | ['A', 'DEF'] | ['AB', 'EF'] | ['ABC', 'F'] | ['ABCD', '']
-| ['', 'BCDEF'] | ['A', 'CDEF'] | ['AB', 'DEF'] | ['ABC', 'EF'] | ['ABCD', 'F'] | ['ABCDE', '']
-| ['A', 'B', 'C', 'D', 'E', 'F']
->(split('ABCDEF', null! as string));
+expectType<string[]>(split('ABCDEF', null! as string));
+expectType<string[]>(split('ABCDEF', null! as Capitalize<string>));
 
 // Recursion depth at which a non-tail recursive implementation starts to fail.
 const fiftyZeroes = '00000000000000000000000000000000000000000000000000';