const<\/code> bindings, unlike their legacy counterparts, are no longer properties on the global object. Instead, they are bindings in the global lexical scope directly below the global object.<\/p>\nFor example,<\/p>\n
const x = 42;\r\n\/\/ Prints false for ES6, true for legacy\r\ndump('x' in this);<\/pre>\nMany add-ons have the expectation that global let<\/code> and const<\/code> introduce properties. For instance, a legacy JSM might define constants and globals:<\/p>\n\/\/ Foo.jsm\r\nconst MY_CONSTANT = 42;\r\nlet gFoo = \"foo\";\r\n\r\n\/\/ addon.js\r\nvar FooModule = Cu.import(\"Foo.jsm\", {});\r\ndump(FooModule.MY_CONSTANT);\r\ndump(FooModule.gFoo);<\/pre>\nWith ES6 const<\/code>, FooModule.MY_CONSTANT<\/code> and FooModule.gFoo<\/code> are both undefined<\/code>, because their bindings are in a separate scope and not properties on the global object. This makes bugs caused by these errors particularly elusive.<\/p>\nFor uses of global legacy let<\/code> bindings that need to be accessed as properties, I recommend declaring them with var<\/code>.<\/p>\nUnfortunately, there is no ES6 syntax with the same semantics as legacy const<\/code>. If the read-only aspect of the property is necessary, I recommend manually defining a property on the global object:<\/p>\nObject.defineProperty(globalObject, \"MY_CONSTANT\", {\r\n value: 42,\r\n enumerable: true,\r\n writable: false\r\n});<\/pre>\nIf your add-on imports XPCOMUtils<\/code>, XPCOMUtils.defineConstant(obj, key, value)<\/code> does exactly that.<\/p>\nES6 global lexical bindings may not be redeclared<\/h3>\n
In ES6, global lexical bindings may not be redeclared in any way: not by let<\/code>, const<\/code>, nor var<\/code>. For example, if the global level has let foo<\/code> or const foo<\/code>, any subsequent occurrences of let foo<\/code>, const foo<\/code>, or var foo<\/code> will throw.<\/p>\nRedeclaration errors are easy to fix: rename the variable or remove the declarator and assign to the already-declared variable directly.<\/p>\n
ES6 global lexical bindings have TDZ<\/h3>\n
In ES6, no lexical binding may be used before its declaration is reached. For example, the following throws:<\/p>\n
dump(x);\r\nlet x;<\/pre>\nThis has long been regarded as poor style in the community, and fortunately, such errors in existing code are rare. If such an error is encountered, it is very likely a bug in the code.<\/p>\n