Add support for browsers which don’t support Promises

src/browser-polyfill.js

@@ -6,7 +6,52 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 "use strict";
 
-if (typeof browser === "undefined" || Object.getPrototypeOf(browser) !== Object.prototype) {
+/**
+ * Returns true if the given object is an object with a `then` method, and can
+ * therefore be assumed to behave as a Promise.
+ *
+ * @param {*} value The value to test.
+ * @returns {boolean} True if the value is thenable.
+ */
+const isThenable = value => {
+  return value && typeof value === "object" && typeof value.then === "function";
+};
+
+/**
+ * Checks if the `browser` namespace object is an actual
+ * `browser` namespace object and not an element with
+ * the `[id]` attribute with a value of `browser`.
+ */
+const HAS_BROWSER_NAMESPACE = (typeof browser !== "undefined" && Object.getPrototypeOf(browser) === Object.prototype);
+
+/**
+ * Checks if the current browser supports
+ * the Promise-based WebExtension APIs.
+ *
+ * @returns {boolean} True if yes.
+ */
+const supportsPromises = () => {
+  if (!HAS_BROWSER_NAMESPACE) {
+    return false;
+  }
+  // If both `browser.runtime.lastError` and `browser.extension.lastError`
+  // don’t exist, assume promises are supported.
+  if (browser.runtime && !browser.runtime.hasOwnProperty("lastError")) {
+    if (!browser.extension || !browser.extension.hasOwnProperty("lastError")) {
+      return true;
+    }
+  }
+  try {
+    return isThenable(browser.runtime.getPlatformInfo());
+  } catch (error) { /* silence ESLint */ }
+  // Content scripts can’t access the platform info
+  try {
+    return isThenable(browser.i18n.getAcceptLanguages());
+  } catch (error) { /* silence ESLint */ }
+  return false;
+};
+
+if (!supportsPromises()) {
   const CHROME_SEND_MESSAGE_CALLBACK_NO_RESPONSE_MESSAGE = "The message port closed before a response was received.";
   const SEND_RESPONSE_DEPRECATION_WARNING = "Returning a Promise is the preferred way to send a reply from an onMessage/onMessageExternal listener, as the sendResponse will be removed from the specs (See https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage)";
 
@@ -50,17 +95,6 @@ if (typeof browser === "undefined" || Object.getPrototypeOf(browser) !== Object.
       }
     }
 
-    /**
-     * Returns true if the given object is an object with a `then` method, and can
-     * therefore be assumed to behave as a Promise.
-     *
-     * @param {*} value The value to test.
-     * @returns {boolean} True if the value is thenable.
-     */
-    const isThenable = value => {
-      return value && typeof value === "object" && typeof value.then === "function";
-    };
-
     /**
      * Creates and returns a function which, when called, will resolve or reject
      * the given promise based on how it is called:
@@ -509,7 +543,7 @@ if (typeof browser === "undefined" || Object.getPrototypeOf(browser) !== Object.
 
   // The build process adds a UMD wrapper around this file, which makes the
   // `module` variable available.
-  module.exports = wrapAPIs(chrome); // eslint-disable-line no-undef
+  module.exports = wrapAPIs(HAS_BROWSER_NAMESPACE ? browser : chrome); // eslint-disable-line no-undef
 } else {
   module.exports = browser; // eslint-disable-line no-undef
 }

test/test-browser-global.js

@@ -1,6 +1,6 @@
 "use strict";
 
-const {deepEqual, equal, ok} = require("chai").assert;
+const {deepEqual, equal, notEqual, ok} = require("chai").assert;
 
 const {setupTestDOMWindow} = require("./setup");
 
@@ -12,7 +12,7 @@ describe("browser-polyfill", () => {
     });
   });
 
-  it("does not override the global browser namespace if it already exists", () => {
+  it("does not override the global browser namespace if it already exists and supports promises", () => {
     const fakeChrome = {
       runtime: {lastError: null},
     };
@@ -26,6 +26,38 @@ describe("browser-polyfill", () => {
     });
   });
 
+  it("wraps the global browser namespace if it doesn’t support promises", () => {
+    const fakePlatformInfo = {
+      os: "test",
+      arch: "test",
+    };
+
+    const fakeBrowser = {
+      runtime: {
+        lastError: null,
+        getPlatformInfo: (cb) => {
+          if (typeof cb !== "function") {
+            throw new Error(cb + " is not a callback");
+          }
+          cb(fakePlatformInfo);
+        },
+      },
+    };
+
+    return setupTestDOMWindow(undefined, fakeBrowser).then(window => {
+      console.log(window.browser.runtime.getPlatformInfo());
+      notEqual(window.browser, fakeBrowser,
+               "The existing browser has been wrapped");
+      return Promise.all([
+        window,
+        window.browser.runtime.getPlatformInfo(),
+      ]);
+    }).then(([window, platformInfo]) => {
+      deepEqual(platformInfo, fakePlatformInfo,
+                "The wrapped browser returns the expected value");
+    });
+  });
+
   describe("browser wrapper", () => {
     it("supports custom properties defined using Object.defineProperty", () => {
       const fakeChrome = {};