有人能解释一下为什么 for, while, do-while 语句的语法定义 header 中的条件部分是 "Expression"

Question

谁能给我解释一下为什么 for, while, do-while statement's define that the condition part within the header is an "Expression" 的语法，谁能给我解释一下下面的东西是什么意思

Expression[In, Yield, Await] :
      AssignmentExpression[?In, ?Yield, ?Await]
      Expression[?In, ?Yield, ?Await] , AssignmentExpression[?In, ?Yield, ?Await]

如您所见，for-statement, while-statement, do-while statement

的 header 中没有 Literal at the right hand side after the non-terminal Expression[In, Yield, Await] : but still it's possible to write literals

for (var i = 0; null; i ++) {
             /* ^^^^ */
             /* null */   
}

while (null) {
    /* ^^^^ */
    /* null */
}

do {} while (null) ; 
          /* ^^^^ */
          /* null */

也可以写 function expressions within the header of those statement's even though FunctionExpression 不存在于 non-terminal Expression[In, Yield, Await] : 的右侧

for (var i = 0; function () {}; i ++) ;
             /* ^^^^^^^^^^^^^^ */
             /*  functionExpr  */

while (function () {}) ;
    /* ^^^^^^^^^^^^^^ */
    /*  functionExpr  */
    
do ; while (function () {}) ;
         /* ^^^^^^^^^^^^^^ */
         /*  functionExpr  */

Answer 1

我在文档中进行了一些搜索，但我发现的是（总结）：

Expression: 
    AssignmentExpression

然后，除其他外：

AssignmentExpression: 
    LeftHandSideExpression

所以我们可以深入嵌套：

LeftHandSideExpression: 
    NewExpression

NewExpression: 
    MemberExpression

MemberExpression: 
    PrimaryExpression

并发现 PrimaryExpression 包含许多其他项，还有 Literal 和 FunctionExpression:

PrimaryExpression[Yield, Await]:
     this
     IdentifierReference[?Yield, ?Await]
     Literal
     ArrayLiteral[?Yield, ?Await]
     ObjectLiteral[?Yield, ?Await]
     FunctionExpression
     ClassExpression[?Yield, ?Await]
     GeneratorExpression
     AsyncFunctionExpression
     AsyncGeneratorExpression
     RegularExpressionLiteral
     TemplateLiteral[?Yield, ?Await, ~Tagged]
     CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]

希望我的回答够清楚，能解开你的疑惑

有人能解释一下为什么 for, while, do-while 语句的语法定义 header 中的条件部分是 "Expression"

can someone explain me why the grammar of for, while, do-while statement define that the condition part within the header is an "Expression"

javascript

context-free-grammar

ecmascript-2020