diff --git a/docs/design/specs/js-language-namespace.md b/docs/design/specs/js-language-namespace.md
new file mode 100644
index 0000000..a3647ad
--- /dev/null
+++ b/docs/design/specs/js-language-namespace.md
@@ -0,0 +1,90 @@
+# Per-module language namespace in `can://` ids
+
+- **Status:** accepted, implementation pending
+- **Scope:** `codeanalyzer-typescript`; schema v2 **id-shape change** (breaking for JS modules)
+- **Tracking:** see the work item filed alongside this spec
+
+## Problem
+
+`src/schema/ids.ts:11` hardcodes `const LANGUAGE = "typescript"`, and every code id descends from it:
+
+```
+applicationIdOf(app)            -> can://typescript/<app>
+moduleIdOf(appId, fileKey)      -> <appId>/<fileKey>
+idFromSig(moduleId, prefix, sig)-> <moduleId>/<tail>
+```
+
+A JavaScript file therefore emits `can://typescript/<app>/lib/foo.js/fn`. The first `can://`
+segment is defined as a language namespace, so a `.js` module is currently labelled as TypeScript.
+The analyzer handles both languages — JS discovery landed in #98 — and nothing in the id records
+which one a module actually is.
+
+Measured: on `nodejs/node/lib` (406 modules, pure JavaScript) every id reads `can://typescript/…`.
+
+This also sits against the repository-artifact layer, which deliberately went language-NEUTRAL
+(`can://artifact/<app>/<path>`) so sibling analyzers over one repository mint identical ids for the
+same file. Code nodes make the opposite choice and encode a language that may be wrong.
+
+## Contract-impact triage
+
+| Question | Answer |
+| --- | --- |
+| Schema v2 output | **id shape changes** for `.js`/`.jsx`/`.mjs`/`.cjs` modules and every descendant id |
+| Levels / monotonicity | unaffected — ids change, structure does not |
+| Repos | `codeanalyzer-typescript`; consumers of JS ids (Neo4j stores, caches, saved queries). No python change: python is single-language and has no analogue |
+| schema_version | unchanged by standing decision — the contract re-baselines once every analyzer's schema is stable, not per release |
+| Shared vocabulary | adds `javascript` as a language namespace alongside `typescript`; no new prov token, no new node or edge kind |
+
+## Decision
+
+**Per-module language in the id. The application anchor stays `can://typescript/<app>`.**
+
+```
+can://typescript/<app>                      :Application anchor (unchanged)
+can://typescript/<app>/src/bar.ts/fn        TypeScript module and descendants
+can://javascript/<app>/lib/foo.js/fn        JavaScript module and descendants
+```
+
+Extension mapping:
+
+| Extension | Namespace |
+| --- | --- |
+| `.ts` `.tsx` `.mts` `.cts` `.d.ts` | `typescript` |
+| `.js` `.jsx` `.mjs` `.cjs` | `javascript` |
+
+### Accepted inconsistency
+
+The application id keeps saying `typescript` while owning `javascript` children. This was chosen
+knowingly over the two alternatives:
+
+- A **neutral application anchor** (`can://app/<app>`, mirroring the artifact layer) is the more
+  coherent end state, but changes *every* id in every projection rather than only JS ones.
+- **Two application anchors** would break the single `:Application` invariant that carries analyzer
+  identity (issue #43).
+
+A mixed repository has no single language, so any single-anchor scheme must either name one
+language or name none. Naming the analyzer's own language is the smaller break today; moving to a
+neutral anchor stays open as a follow-up.
+
+## Consequences
+
+- **Breaking for JS ids.** Neo4j `MERGE` keys on id, so a re-projection creates new nodes for JS
+  modules rather than updating existing ones; a store analyzed with an earlier version needs a
+  rebuild. Saved Cypher and any persisted id references to JS nodes must be updated.
+- **`modulePrefixOf` collision risk is unchanged.** It strips `.ts` and `.js` alike, so `a.ts` and
+  `a.js` still share a module prefix. That collision is prevented upstream by discovery's sibling
+  rule (a `.js` beside a real same-prefix `.ts` is treated as compiled output and skipped), not by
+  the id grammar. Splitting the namespace does not fix it and does not worsen it.
+- **The cache is unaffected.** Ids are per-run (`assignIds` stamps them fresh because they embed
+  `--app-name`); the cached tree is id-free.
+- **One construction site.** `assignIds.ts:54` is the only caller of `moduleIdOf`, so the change is
+  contained: `moduleIdOf` takes the app NAME and the file key and derives the namespace itself.
+
+## Definition of done
+
+- A `.js` module and its callables emit `can://javascript/…`; a `.ts` module emits
+  `can://typescript/…`; the `:Application` id is unchanged.
+- The `.d.ts` case is asserted explicitly (it is `typescript`, and `.d.ts` must not be read as a
+  `.ts` suffix on a `.d` file).
+- Neo4j projection and JSON agree, and the conformance and count-parity gates stay green.
+- A fixture with both a `.ts` and a `.js` module asserts both namespaces in one run.
diff --git a/src/schema/assignIds.ts b/src/schema/assignIds.ts
index 1abe2b8..8c26f24 100644
--- a/src/schema/assignIds.ts
+++ b/src/schema/assignIds.ts
@@ -51,7 +51,7 @@ export function assignIds(app: AnalysisInternal, appName: string): AssignedIds {
   };
 
   for (const [fileKey, mod] of Object.entries(app.symbol_table)) {
-    const moduleId = moduleIdOf(appId, fileKey);
+    const moduleId = moduleIdOf(appName, fileKey);
     const modulePrefix = modulePrefixOf(fileKey);
     mod.id = moduleId;
     // Module-scope execution is a call-graph SOURCE (python #131 parity: a call in module scope
diff --git a/src/schema/ids.ts b/src/schema/ids.ts
index 7876a2e..086dc56 100644
--- a/src/schema/ids.ts
+++ b/src/schema/ids.ts
@@ -9,13 +9,32 @@
  */
 
 const LANGUAGE = "typescript";
+const JS_EXTS = /\.(jsx|js|mjs|cjs)$/;
 
+/**
+ * A module's language namespace, from its file key (#114).
+ *
+ * The analyzer owns both languages (JS discovery, #98), so this is per MODULE rather than per run:
+ * a `.js` file is JavaScript and must not be labelled `typescript`. `.d.ts` falls through to the
+ * default rather than matching, so a declaration file stays TypeScript.
+ */
+export function languageOf(fileKey: string): string {
+  return JS_EXTS.test(fileKey) ? "javascript" : LANGUAGE;
+}
+
+/**
+ * The :Application anchor keeps the analyzer's own language even when it owns `javascript`
+ * children. A mixed repository has no single language, and the alternatives were a neutral anchor
+ * (moves every id in every projection) or two anchors (breaks the single-anchor invariant, #43).
+ * See docs/design/specs/js-language-namespace.md.
+ */
 export function applicationIdOf(appName: string): string {
   return `can://${LANGUAGE}/${appName}`;
 }
 
-export function moduleIdOf(appId: string, fileKey: string): string {
-  return `${appId}/${fileKey}`;
+/** Takes the app NAME, not the app id: a module's namespace is its own, not the application's. */
+export function moduleIdOf(appName: string, fileKey: string): string {
+  return `can://${languageOf(fileKey)}/${appName}/${fileKey}`;
 }
 
 /** The module/signature prefix: the file key without its TS/JS extension. */
diff --git a/test/checker-guard.test.ts b/test/checker-guard.test.ts
index aaf4ee0..3fef890 100644
--- a/test/checker-guard.test.ts
+++ b/test/checker-guard.test.ts
@@ -23,7 +23,9 @@ import { checkerFailures, resetCheckerFailures, symbolAt } from "../src/schema/c
 import type { AnalysisOptions } from "../src/options";
 
 const FIXTURE = path.resolve(import.meta.dir, "fixtures/unresolvable-js-app");
-const ID = "can://typescript/unresolvable-js-app";
+// Two namespaces in one run (#114): a .js module is `javascript`, a .ts module is `typescript`.
+const TS = "can://typescript/unresolvable-js-app";
+const JS = "can://javascript/unresolvable-js-app";
 
 function options(over: Partial<AnalysisOptions> = {}): AnalysisOptions {
   return {
@@ -43,8 +45,8 @@ describe("a .js file outside tsconfig's include still resolves (#103)", () => {
     expect(js).toBeDefined();
     expect(Object.keys(js!.types ?? {})).toContain("defaultDpapi");
     const methods = Object.values((js!.types ?? {})["defaultDpapi"]?.callables ?? {}).map((c) => c.id);
-    expect(methods).toContain(`${ID}/mocks/dpapi.js/defaultDpapi/protectData`);
-    expect(methods).toContain(`${ID}/mocks/dpapi.js/defaultDpapi/unprotectData`);
+    expect(methods).toContain(`${JS}/mocks/dpapi.js/defaultDpapi/protectData`);
+    expect(methods).toContain(`${JS}/mocks/dpapi.js/defaultDpapi/unprotectData`);
   });
 
   test("its call edges resolve — with allowJs off, the checker threw before reaching them", async () => {
@@ -53,9 +55,9 @@ describe("a .js file outside tsconfig's include still resolves (#103)", () => {
 
     // Module-scope `new defaultDpapi()`, attributed to the MODULE. This edge does not merely go
     // missing without the fix — resolving it is what threw.
-    expect(edges).toContain(`${ID}/mocks/dpapi.js -> ${ID}/mocks/dpapi.js/defaultDpapi/constructor`);
+    expect(edges).toContain(`${JS}/mocks/dpapi.js -> ${JS}/mocks/dpapi.js/defaultDpapi/constructor`);
     // The healthy TypeScript file is unaffected either way.
-    expect(edges).toContain(`${ID}/src/index.ts/run -> ${ID}/src/index.ts/greet`);
+    expect(edges).toContain(`${TS}/src/index.ts/run -> ${TS}/src/index.ts/greet`);
   });
 
   test("nothing is skipped any more — the checker resolves the file cleanly", async () => {
diff --git a/test/ids.test.ts b/test/ids.test.ts
new file mode 100644
index 0000000..7fbf12c
--- /dev/null
+++ b/test/ids.test.ts
@@ -0,0 +1,38 @@
+/**
+ * The `can://` language namespace is per MODULE (#114).
+ *
+ * The analyzer owns both languages (JS discovery, #98), so a `.js` module must not be labelled
+ * `typescript`. The :Application anchor deliberately keeps the analyzer's own language even when it
+ * owns `javascript` children — a mixed repository has no single language, and the alternatives
+ * moved every id in every projection or broke the single-anchor invariant (#43).
+ * See docs/design/specs/js-language-namespace.md.
+ */
+import { describe, expect, test } from "bun:test";
+import { applicationIdOf, languageOf, moduleIdOf } from "../src/schema/ids";
+
+describe("per-module language namespace (#114)", () => {
+  test.each([["a.ts"], ["a.tsx"], ["a.mts"], ["a.cts"]])("%s is typescript", (f) => {
+    expect(languageOf(f)).toBe("typescript");
+  });
+
+  test.each([["b.js"], ["b.jsx"], ["b.mjs"], ["b.cjs"]])("%s is javascript", (f) => {
+    expect(languageOf(f)).toBe("javascript");
+  });
+
+  // A declaration file is TypeScript. The suffix must not be read as `.ts` on a file named `a.d`.
+  test("a .d.ts declaration file is typescript", () => {
+    expect(languageOf("types/x.d.ts")).toBe("typescript");
+    expect(moduleIdOf("app", "types/x.d.ts")).toBe("can://typescript/app/types/x.d.ts");
+  });
+
+  // The match is anchored: a DIRECTORY named `foo.js` must not make its .ts children javascript.
+  test("a directory named *.js does not change its children's namespace", () => {
+    expect(languageOf("vendor.js/index.ts")).toBe("typescript");
+  });
+
+  test("module ids carry their own namespace; the application anchor keeps typescript", () => {
+    expect(applicationIdOf("app")).toBe("can://typescript/app");
+    expect(moduleIdOf("app", "src/bar.ts")).toBe("can://typescript/app/src/bar.ts");
+    expect(moduleIdOf("app", "lib/foo.js")).toBe("can://javascript/app/lib/foo.js");
+  });
+});