什么是序言指令？ [英] What is Prologue Directives?

问题描述

我偶然发现了人们选择称之为Prologue Directives的东西。更常见的是使用JavaScript中的use strict; 字符串文字。我已经知道了。但共同点是Prologue Directive。这是什么？关于这个主题的文献很少。最好的一个是我链接的问题。

I stumbled upon something people choose to call Prologue Directives. More commonly known with the "use strict"; string literal in JavaScript. Which i already know all about. But the common denominator Prologue Directive. What it is? There's very little documentation available on this subject. Best one is the question i linked.

ECMAScript多个序言指令

我的问题是通用的：

它们是什么？

它们可以用于什么？

谁使用它们以及为什么？

Who uses them and why?

我可以制作吗？

我应该吗？

推荐答案

无需文档。只需查看来源。

指令序言是ExpressionStatement
产品中最长的序列，作为
程序或FunctionBody的初始SourceElement产生而发生，并且
序列中的每个ExpressionStatement完全由一个StringLiteral标记后跟一个
分号。分号可以明确显示，也可以通过
自动分号插入插入。指令序言可能是空的
序列。

A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.

使用严格指令是指令
Prologue中的ExpressionStatement，其StringLiteral是确切的字符序列
use strict或use strict。使用严格指令可能不包含
EscapeSequence或LineContinuation。

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact character sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

指令序言可能包含多个使用严格指令。
但是，如果发生这种情况，实施可能会发出警告。

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

换句话说，指令序言是最长的序列在函数或程序的确切开始处的字符串文字+分号（顶级代码）：

In other words, Directive Prologue is the longest sequence of string literal + semicolon at the exact start of function or program (top-level code):

(function(){
  "use strict"; // <-- Directive Prologue
})()

或：

(function() {
  // Directive Prologue start
  "foo bar"
  "baz";
  '123';
  '';
  // Directive Prologue end
})();

或：

'blah'; // <-- Directive Prologue (top-level code)
/* rest of the code here */

请注意，一旦字符串文字不是第一个语句，它就不再是指令序言：

Notice that as soon as the string literal is not the first statement, it's no longer a Directive Prologue:

var x;
"use strict"; // <-- NOT a Directive Prologue

或：

(function() {
  1 + "use magic"; // <-- NOT a Directive Prologue
})();

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