ECMAScript 符号语法的含义？

Question

我不知道如何正确阅读 this syntax line。

为什么 IdentifierName 是用 IdentifierName 定义的？
为什么在定义的中间有一个新行？

这个符号怎么读？

Syntax

IdentifierName ::
       IdentifierStart
       IdentifierName IdentifierPart

Answer 1

这是一个递归定义，这在语法符号中并不少见。

IdentifierName由IdentifierStart或IdentifierName IdentifierPart组成。如果我们再次在第二个备选方案中展开 IdentifierName，我们会得到

IdentifierStart IdentifierPart
IdentifierName IdentifierPart IdentifierPart

等等。

用非正式的方式表达：IdentifierName 由 IdentifierStart 后跟任意数量的 IdentifierPart 组成。

来自specifiction itself:

As another example, the syntactic definition:
ArgumentList:
  AssignmentExpression
  ArgumentList,AssignmentExpression
states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

ECMAScript 符号语法的含义？

Meaning of ECMAScript notation syntax?

ecmascript-6

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