Does the script element encapsulate the strict mode in any way or is it passed through?

Yes, the strict mode is own per each script element (i.e for each loaded file or evaluated source inside it.

Actually, script element is a separate program, though all these programs share the same global object.

<script>
  "use strict";
  var a = 10; // OK, no error
</script>

<script>
  b = 20; // OK, no error
</script>

<script>
  "use strict";
  b = 20; // Error
</script>

Dmitry.

fantastic article. Though I have a question, how the strict mode is handled throughout different files:

<script src="a.js">
<script src="b.js">
<script>
var mode = 42; // which mode is this here?
</script>

a.js:

var foo = "bar";

b.js:

"use strict";
var bar = "baz";

In which mode is the rest of the script on this demo? Does the script element encapsulate the strict mode in any way or is it passed through?

Thanks
gossi

Yeah, thanks, Allen. I wanted to add this case with indirect eval regarding creation of global bindings, but hadn’t time before.

Now added, and into three sections (small related mentions in every): Strictness scope, eval and arguments restrictions and Indirect eval call itself. So, thanks again, it’s a needed addition.

Dmitry.

Yeah, good points, thanks.

Regarding both — these were bugs in BESEN. The second one (with “use strict” position) was fixed earlier, but after the article was written. However, the spec is clear in this case — it is an initial SourceElement.

The first one is not so easy to catch in spec, but, yeah, obviously, it was also a bug in BESEN, and the spec is also clear here. I’ve talked to BeRo just some minutes ago, it’s fixed now.

Dmitry.

But a couple of important corrections:

1. “eval” and “arguments” not being able to occur as properties — e.g. `({ eval: 1 })` — is a misconception. From what I can see, there’s nothing in the specs that disallows it. The confusion probably comes from the fact that “eval” and “arguments” can NOT occur as /Identifier/ in /PropertySetParameterList/ — `({ set foo(eval, arguments) { } }) /* SyntaxError */` By the way, IIRC, last time I checked, BESEN was incorrectly throwing an error on `({ eval: 1 })`.

2. Use Strict Directive (being an /ExpressionStatement/ in Directive Prologue) can NOT occur anywhere in the scope of a function (or program body), contrary to what you say. Well, technically it can, but then it’s just an /ExpressionStatement/ consisting of a string literal, not a Use Strict Directive. The special nature of /ExpressionStatement/’s in Directive Prologue is that they occur as the _initial /SourceElement/’s_ in function or program. So `alert(1); “use strict”` is not a strict-mode code, but `”use strict”; alert(1);` is.

You can track the status of ES5 implementations in progress

Yeah, thanks Mark, that’s useful.

Dmitry.

here

I’d be useful to know which browsers support this strict mode

Currently, the only complete implementation of ES5 is the BESEN engine. I’ve updated the article, mentioning it. It still has some bugs, several of them I’ve posted during writing the article — and glad to see that the implementer has already fixed them. You can download it and test ES5 there now. Hope, some major implementations will follow soon.

Dmitry.

Надеюсь появится перевод на русский

Пока, к сожалению, не планирую; возможно, позже — когда допишу все части этой новой серии. Но, этим может заняться кто-нибудь другой. Я смогу проверить качество перевода в техническом плане (относительно самого JS имеется в виду); отсюда смогу дать ссылки, как это сделано с китайскими переводами, например.

потому как не совсем все понятно к сожалению. К примеру это:

var get = eval;
get("alert(x)"); // 10 - from the global context

Суть в том, что есть явный (direct) и неявный (indirect) вызовы eval’a. Вариации неявного вызова описаны в статье; одна из них — это использовать другое имя идентификатора. Главный момент, это то, что неявный вызов исполняется всегда в глобальном контексте, поэтому “x” из примера выше берётся именно оттуда, а не из локального контекста функции.

да и вобще это было единственным местом, где про js можно было почитать на русском и подробно.

Ну, все русские (оригинальные) статьи серии ES3 по-прежнему здесь. Возможно, появятся из переводы ES5 серии.

Dmitry.