Bug 1155877 - ObjectUtils.strict. r=mossop

--HG--
extra : rebase_source : e69178e9208b149fc66c61f15d74750d1d765907

toolkit/modules/AppConstants.jsm

@@ -151,6 +151,13 @@ this.AppConstants = Object.freeze({
   false,
 #endif
 
+  DEBUG:
+#ifdef DEBUG
+  true,
+#else
+  false,
+#endif
+
   MOZ_APP_NAME: "@MOZ_APP_NAME@",
   MOZ_APP_VERSION: "@MOZ_APP_VERSION@",
   MOZ_BUILD_APP: "@MOZ_BUILD_APP@",

toolkit/modules/ObjectUtils.jsm

@@ -14,6 +14,12 @@ this.EXPORTED_SYMBOLS = [
 
 const {classes: Cc, interfaces: Ci, results: Cr, utils: Cu} = Components;
 
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+
+// Used only to cause test failures.
+XPCOMUtils.defineLazyModuleGetter(this, "Promise",
+  "resource://gre/modules/Promise.jsm");
+
 this.ObjectUtils = {
   /**
    * This tests objects & values for deep equality.
@@ -30,6 +36,25 @@ this.ObjectUtils = {
   deepEqual: function(a, b) {
     return _deepEqual(a, b);
   },
+
+  /**
+   * A thin wrapper on an object, designed to prevent client code from
+   * accessing non-existent properties because of typos.
+   *
+   * // Without `strict`
+   * let foo = { myProperty: 1 };
+   * foo.MyProperty; // undefined
+   *
+   * // With `strict`
+   * let strictFoo = ObjectUtils.strict(foo);
+   * strictFoo.myProperty; // 1
+   * strictFoo.MyProperty; // TypeError: No such property "MyProperty"
+   *
+   * Note that `strict` has no effect in non-DEBUG mode.
+   */
+  strict: function(obj) {
+    return _strict(obj);
+  }
 };
 
 // ... Start of previously MIT-licensed code.
@@ -129,3 +154,21 @@ function objEquiv(a, b) {
 }
 
 // ... End of previously MIT-licensed code.
+
+function _strict(obj) {
+  if (typeof obj != "object") {
+    throw new TypeError("Expected an object");
+  }
+
+  return new Proxy(obj, {
+    get: function(target, name) {
+      if (name in obj) {
+        return obj[name];
+      }
+
+      let error = new TypeError(`No such property: "${name}"`);
+      Promise.reject(error); // Cause an xpcshell/mochitest failure.
+      throw error;
+    }
+  });
+}

toolkit/modules/tests/xpcshell/test_ObjectUtils_strict.js

@@ -0,0 +1,32 @@
+"use strict";
+
+let {ObjectUtils} = Components.utils.import("resource://gre/modules/ObjectUtils.jsm", {});
+let {Promise} = Components.utils.import("resource://gre/modules/Promise.jsm", {});
+
+add_task(function* init() {
+  // The code will cause uncaught rejections on purpose.
+  Promise.Debugging.clearUncaughtErrorObservers();
+});
+
+add_task(function* test_strict() {
+  let loose = { a: 1 };
+  let strict = ObjectUtils.strict(loose);
+
+  loose.a; // Should not throw.
+  loose.b || undefined; // Should not throw.
+
+  strict.a; // Should not throw.
+  Assert.throws(() => strict.b, /No such property: "b"/);
+  "b" in strict; // Should not throw.
+  strict.b = 2;
+  strict.b; // Should not throw.
+
+  Assert.throws(() => strict.c, /No such property: "c"/);
+  "c" in strict; // Should not throw.
+  loose.c = 3;
+  strict.c; // Should not throw.
+});
+
+function run_test() {
+  run_next_test();
+}

toolkit/modules/tests/xpcshell/xpcshell.ini

@@ -17,6 +17,7 @@ support-files =
 [test_Log.js]
 [test_NewTabUtils.js]
 [test_ObjectUtils.js]
+[test_ObjectUtils_strict.js]
 [test_PermissionsUtils.js]
 [test_Preferences.js]
 [test_Promise.js]