Make languageNamed accept display names and extensions again

packages/quicktype-core/src/language/All.ts

@@ -66,28 +66,35 @@ all satisfies readonly TargetLanguage[];
 
 export function languageNamed<Name extends LanguageName>(
     name: Name,
+    targetLanguages?: readonly TargetLanguage[],
+): LanguageNameMap[Name];
+export function languageNamed(
+    name: string,
+    targetLanguages?: readonly TargetLanguage[],
+): TargetLanguage | undefined;
+export function languageNamed(
+    name: string,
     targetLanguages: readonly TargetLanguage[] = all,
-): LanguageNameMap[Name] {
-    const foundLanguage = targetLanguages.find((language) =>
-        language.names.includes(name),
+): TargetLanguage | undefined {
+    // Names take precedence over extensions, so that e.g. "js" resolves
+    // to JavaScript, not to Flow (whose extension is "js").
+    const lowerName = name.toLowerCase();
+    const foundLanguage = targetLanguages.find(
+        (language) =>
+            (language.names as readonly string[]).includes(lowerName) ||
+            language.displayName.toLowerCase() === lowerName,
     );
-    if (!foundLanguage) {
-        throw new Error(`Unknown language name: ${name}`);
-    }
 
-    return foundLanguage as LanguageNameMap[Name];
+    return (
+        foundLanguage ??
+        targetLanguages.find(
+            (language) => language.extension.toLowerCase() === lowerName,
+        )
+    );
 }
 
 export function isLanguageName(maybeName: string): maybeName is LanguageName {
-    if (
-        all.some((lang) =>
-            (lang.names as readonly string[]).includes(maybeName),
-        )
-    ) {
-        return true;
-    }
-
-    return false;
+    return languageNamed(maybeName) !== undefined;
 }
 
 export function isLanguageDisplayName(

test/unit/language-named.test.ts

@@ -0,0 +1,55 @@
+// languageNamed() must accept display names and file extensions, and must
+// return undefined for unknown names instead of throwing.
+//
+// Before quicktype 23.1.0, languageNamed() matched a language's names,
+// displayName, and extension, and returned undefined when nothing matched.
+// The 23.1.0 rewrite (da1238c0) made it match only the lowercase names
+// arrays and *throw* on a miss. That broke consumers that pass display
+// names — most visibly app.quicktype.io, which stores the chosen language's
+// display name (e.g. "TypeScript") and crashed to a blank page for
+// returning users: see https://github.com/glideapps/quicktype/issues/2769.
+//
+// The fixture harness never calls languageNamed() with anything but exact
+// lowercase names, so it can't catch this; we assert on the API directly.
+
+import { isLanguageName, languageNamed } from "quicktype-core";
+import { describe, expect, test } from "vitest";
+
+// input → expected displayName of the resolved language
+const resolvedCases: Array<[string, string]> = [
+    // exact lowercase names (the only thing 23.1.0 accepted)
+    ["typescript", "TypeScript"],
+    ["c++", "C++"],
+    // display names, as saved by app.quicktype.io
+    ["TypeScript", "TypeScript"],
+    ["C++", "C++"],
+    ["Objective-C", "Objective-C"],
+    ["JSON Schema", "JSON Schema"],
+    // case-insensitive
+    ["Typescript", "TypeScript"],
+    ["PYTHON", "Python"],
+    // extensions (matched only after names and display names, so
+    // Flow's extension "js" must not shadow JavaScript's name "js")
+    ["kt", "Kotlin"],
+    ["js", "JavaScript"],
+];
+
+const unknownNames = ["this-is-not-a-language", ""];
+
+describe("languageNamed", () => {
+    test.each(resolvedCases)("resolves %j to %j", (input, expected) => {
+        expect(languageNamed(input)?.displayName).toBe(expected);
+    });
+
+    test.each(resolvedCases)("isLanguageName(%j) is true", (input) => {
+        expect(isLanguageName(input)).toBe(true);
+    });
+
+    test.each(unknownNames)(
+        "returns undefined for unknown name %j",
+        (input) => {
+            expect(languageNamed(input)).toBeUndefined();
+            expect(isLanguageName(input)).toBe(false);
+        },
+    );
+});