Fix TypeError with babel-register's cache (#5260)

packages/babel-register/package.json

@@ -15,5 +15,8 @@
     "lodash": "^4.2.0",
     "mkdirp": "^0.5.1",
     "source-map-support": "^0.4.2"
+  },
+  "devDependencies": {
+    "decache": "^4.1.0"
   }
 }

packages/babel-register/src/cache.js

@@ -3,25 +3,28 @@ import fs from "fs";
 import { sync as mkdirpSync } from "mkdirp";
 import homeOrTmp from "home-or-tmp";
 
-const FILENAME = process.env.BABEL_CACHE_PATH || path.join(homeOrTmp, ".babel.json");
-let data = {};
+const FILENAME: string = process.env.BABEL_CACHE_PATH || path.join(homeOrTmp, ".babel.json");
+let data: Object = {};
 
 /**
  * Write stringified cache to disk.
  */
 
 export function save() {
-  let serialised = {};
+  let serialised: string = "{}";
+
   try {
     serialised = JSON.stringify(data, null, "  ");
   } catch (err) {
+
     if (err.message === "Invalid string length") {
       err.message = "Cache too large so it's been cleared.";
       console.error(err.stack);
     } else {
       throw err;
     }
   }
+
   mkdirpSync(path.dirname(FILENAME));
   fs.writeFileSync(FILENAME, serialised);
 }
@@ -49,6 +52,6 @@ export function load() {
  * Retrieve data from cache.
  */
 
-export function get() {
+export function get(): Object {
   return data;
 }

packages/babel-register/test/index.js

@@ -0,0 +1,84 @@
+import { expect } from "chai";
+import fs from "fs";
+import path from "path";
+import decache from "decache";
+
+const testCacheFilename = path.join(__dirname, ".babel");
+const oldBabelDisableCacheValue = process.env.BABEL_DISABLE_CACHE;
+
+process.env.BABEL_CACHE_PATH = testCacheFilename;
+delete process.env.BABEL_DISABLE_CACHE;
+
+function writeCache(data) {
+  if (typeof data === "object") {
+    data = JSON.stringify(data);
+  }
+
+  fs.writeFileSync(testCacheFilename, data);
+}
+
+function cleanCache() {
+
+  try {
+    fs.unlinkSync(testCacheFilename);
+  } catch (e) {
+    // It is convenient to always try to clear
+  }
+}
+
+function resetCache() {
+  process.env.BABEL_CACHE_PATH = null;
+  process.env.BABEL_DISABLE_CACHE = oldBabelDisableCacheValue;
+}
+
+describe("babel register", () => {
+
+  describe("cache", () => {
+    let load, get, save;
+
+    beforeEach(() => {
+      // Since lib/cache is a singleton we need to fully reload it
+      decache("../lib/cache");
+      const cache = require("../lib/cache");
+
+      load = cache.load;
+      get = cache.get;
+      save = cache.save;
+    });
+
+    afterEach(cleanCache);
+    after(resetCache);
+
+    it("should load and get cached data", () => {
+      writeCache({ foo: "bar" });
+
+      load();
+
+      expect(get()).to.be.an("object");
+      expect(get()).to.deep.equal({ foo: "bar" });
+    });
+
+    it("should load and get an object with no cached data", () => {
+      load();
+
+      expect(get()).to.be.an("object");
+      expect(get()).to.deep.equal({});
+    });
+
+    it("should load and get an object with invalid cached data", () => {
+      writeCache("foobar");
+
+      load();
+
+      expect(get()).to.be.an("object");
+      expect(get()).to.deep.equal({});
+    });
+
+    it("should create the cache on save", () => {
+      save();
+
+      expect(fs.existsSync(testCacheFilename)).to.be.true;
+      expect(get()).to.deep.equal({});
+    });
+  });
+});