Proposal: Multiple Snapshot Formats

Author: avivkeller

proposals/multiple-snapshot-formats.md

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+# Support Multiple Snapshot Formats in the Node.js Test Runner
+
+The current snapshot implementation in the Node.js Test Runner assumes a single snapshot file format (`.snapshot`).
+
+However, Node.js itself natively supports multiple module and data formats:
+
+* `.cjs` (CommonJS)
+* `.mjs` (ESM)
+* `.js` (Both)
+* `.json`
+
+The test runner already embraces this flexibility for **test files themselves** (it automatically discovers `.js`, `.cjs`, and `.mjs` test files), so one would expect snapshots to be loadable the same way.
+
+## Goal
+
+When resolving a snapshot file, the test runner should support multiple extensions.
+
+Example resolution order:
+
+```text
+<test>.snapshot.mjs
+<test>.snapshot.cjs
+<test>.snapshot.js
+<test>.snapshot.json
+```
+
+Example error:
+
+```
+Multiple snapshot files found for test "foo.test.js":
+- foo.test.snapshot.mjs
+- foo.test.snapshot.json
+```