Да, в случае delete‘a исключение будет, т.к. var создает non-[[Configurable]] свойство (только если это не контекст eval‘a).

По поводу ASI (Automatic Semicolon Insertion) — нет, точка с запятой все еще дополняется автоматом. Хотя, были дебаты, что могли бы убрать этот механизм в строгом режиме. С другой стороны, точки с запятыми тоже можно отнести к “синтаксическому мусору” в языке. Однако все ставят в крупных проектах из-за порой сложных правил ASI.

Не совсем понял про delete локальных переменных:

(function () {
    "use strict";
    var a = 1;
    delete a; // будет ли исключение?
})();

И есть ли в ES5 какие-нибудь изменения про обязательность ; в конце строк?

Seems just an old typo in exactly that sentence (will fix, thanks). Because if you’ll carefully read related section from above, you see I write that eval and arguments are allowed as property names.

[…] eval and arguments identifiers are not allowed for assignment, function argument names or property names […]

Why wouldn’t they be allowed for property names? The spec only mentions the following:

It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code and its Identifier is either eval or arguments.

So, I don’t see why this would throw an error:

(function() {
  'use strict';
  var object = {
    eval: 1,
    arguments: 2
  };
}());

Could you please clarify? Thanks!

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.