Join: Support const tuple values (#533)

sindresorhus · Dec 25, 2022 · db81191 · db81191
1 parent 00b13f8
commit db81191
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Showing 2 changed files with 13 additions and 2 deletions.
diff --git a/source/join.d.ts b/source/join.d.ts
@@ -21,13 +21,13 @@ const path: Join<[1, 2, 3], '.'> = [1, 2, 3].join('.');
 @category Template literal
 */
 export type Join<
-	Strings extends Array<string | number>,
+	Strings extends Readonly<Array<string | number>>,
 	Delimiter extends string,
 > = Strings extends []
 	? ''
 	: Strings extends [string | number]
 		? `${Strings[0]}`
-		: Strings extends [
+		: Strings extends readonly [
 			string | number,
 			...infer Rest extends Array<string | number>,
 		]

diff --git a/test-d/join.ts b/test-d/join.ts
@@ -21,3 +21,14 @@ expectNotAssignable<'foo.bar.baz'>(emptyDelimiter);
 const emptyInput: Join<[], '.'> = '';
 expectType<''>(emptyInput);
 expectNotAssignable<'foo'>(emptyInput);
+
+// Typeof of const tuple
+const tuple = ['foo', 'bar', 'baz'] as const;
+const joinedTuple: Join<typeof tuple, ','> = 'foo,bar,baz';
+expectType<'foo,bar,baz'>(joinedTuple);
+
+// Typeof of string[]
+const stringArray = ['foo', 'bar', 'baz'];
+const joinedStringArray: Join<typeof stringArray, ','> = '';
+expectType<string>(joinedStringArray);
+expectNotAssignable<'foo,bar,baz'>(joinedStringArray);