Fixed #103

mattpocock · mattpocock · commit 107dfc2ebcc2 · 2023-03-08T15:59:40.000Z
diff --git a/.changeset/empty-elephants-tell.md b/.changeset/empty-elephants-tell.md
@@ -0,0 +1,25 @@
+---
+"@total-typescript/ts-reset": minor
+---
+
+Changed the array.includes on readonly arrays to NOT be a type predicate. Before this change, this perfectly valid code would not behave correctly.
+
+```ts
+type Code = 0 | 1 | 2;
+type SpecificCode = 0 | 1;
+
+const currentCode: Code = 0;
+
+// Create an empty list of subset type
+const specificCodeList: ReadonlyArray<SpecificCode> = [];
+
+// This will be false, since 0 is not in []
+if (specificCodeList.includes(currentCode)) {
+  currentCode; // -> SpecificCode
+} else {
+  // This branch will be entered, and ts will think z is 2, when it is actually 0
+  currentCode; // -> 2
+}
+```
+
+Removing the type predicate brings ts-reset closer towards correctness.
diff --git a/readme.md b/readme.md
@@ -188,21 +188,6 @@ users.includes("bryan");
 
 This means you can test non-members of the array safely.
 
-It also makes `.includes` a type predicate, meaning you can use it to narrow wider types to a set enum:
-
-```ts
-import "@total-typescript/ts-reset/array-includes";
-
-const users = ["matt", "sofia", "waqas"] as const;
-
-const isUser = (input: string) => {
-  if (users.includes(input)) {
-    // input is narrowed to "matt" | "sofia" | "waqas"
-    console.log(input);
-  }
-};
-```
-
 ### Make `Set.has()` less strict
 
 ```ts
diff --git a/src/entrypoints/array-includes.d.ts b/src/entrypoints/array-includes.d.ts
@@ -4,7 +4,7 @@ interface ReadonlyArray<T> {
   includes(
     searchElement: T | (TSReset.WidenLiteral<T> & {}),
     fromIndex?: number,
-  ): searchElement is T;
+  ): boolean;
 }
 
 interface Array<T> {
diff --git a/src/tests/array-includes.ts b/src/tests/array-includes.ts
@@ -50,15 +50,6 @@ doNotExecute(async () => {
   arr.includes({ a: 1 });
 });
 
-doNotExecute(async () => {
-  let arr = [1, 2, 3] as const;
-
-  let member = 1;
-  if (arr.includes(member)) {
-    type tests = [Expect<Equal<typeof member, 1 | 2 | 3>>];
-  }
-});
-
 doNotExecute(async () => {
   const arr: Array<"1" | "2" | "3"> = ["1", "2", "3"];
 

Original file line number	Diff line number	Diff line change
`@@ -4,7 +4,7 @@ interface ReadonlyArray<T> {`
`4`	`4`	`includes(`
`5`	`5`	`searchElement: T \| (TSReset.WidenLiteral<T> & {}),`
`6`	`6`	`fromIndex?: number,`
`7`		`- ): searchElement is T;`
	`7`	`+ ): boolean;`
`8`	`8`	`}`
`9`	`9`
`10`	`10`	`interface Array<T> {`