JavaScript 的表达式语句和表达式

ECMAScript2015 的上下文无关文法 [1] [2] 如下所示：

Program :
    SourceElements

SouceElements :
    SouceElement
    SouceElements SourceElement

SouceElement :
    Statement
    FunctionDeclaration

Statement :
    Block
    DeclarationsStatement
    VariableStatement
    EmptyStatement
    ExpressionStatement
    IfStatement
    IterationStatement
    ContinueStatement
    BreakStatement
    ReturnStatement
    WithStatement
    LabelledStatement
    SwitchStatement
    ThrowStatement
    TryStatement
    DebuggerStatement
ExpressionStatement :
    [lookahead ∉ \{\{, function, class, let [\}] Expression ;

FunctionDeclaration :
    function Identifier ( FormalParameterList[opt] ) { FunctionBody }

ECMAScript2015 规范 提示说：

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

Statement

《你不知道的JavaScript（中卷）》P92

拿英语语法打个比方：“句子”（sentence）是完整表达某个意思的一组词，由一个或多个“短语”（phrase）组成，它们之间由标点符号或连接词（and 和 or 等）连接起来。短语可以由更小的短语组成。有些短语是不完整的，不能独立表达意思；有些短语则相对完整，并且能够独立表达某个意思。

JavaScript 的语法也是如此。JavaScript 的 statement 相当于 sentence，表达式（expression）相当于短语（phrase），运算符（operator）相当于标点符号和连接词。

Expression

An expression is a sequence of operators and their operands, that specifies a computation. Expression evaluation may produce a result and may generate side-effects.

Expressions and operators - MDN

For Example

var a = 1 + 2       // Declarations Statement
var b = a           // Declarations Statement
b = a               // Assignment Expression
a = 3 * 6           // Assignment Expression
b;                  // Expression Statement

Reference

• JavaScript 匿名函数有哪几种执行方式?