javascript - 在 Ecmascript 5.1 中，哪里提到 if 语句中的左花括号是 Not Acceptable ？

Question

我不记得 if 语句的规则，所以我查看了第 12.5 条，但我仍然不清楚是否需要大括号。那么，我想问一下，在 Ecmascript 5.1 中哪里提到 if 语句中没有左花括号是可以接受的？

Answer 1

这是 12.5 节中的 if 语句的语法:

IfStatement :     if ( Expression ) Statement else Statement     if ( Expression ) Statement

From the looks of it no curly braces are required but we need to clarify what exactly a statement is. This syntax is found in section 12.

Statement :     Block     VariableStatement     EmptyStatement     ExpressionStatement     IfStatement     IterationStatement     ContinueStatement     BreakStatement     ReturnStatement     WithStatement     LabelledStatement     SwitchStatement     ThrowStatement     TryStatement     DebuggerStatement 

If we dig a bit deeper into section 12.1 we'll find the definition of the block statement.

Block :     { StatementListopt } 

We found the curly braces but does this mean they are required? The answer is no. When examining the syntax of a statement each element following the colon is considered a statement and therefore a valid part of the if statement syntax. The following are actual examples of valid if statements.

// if ( Expression ) ReturnStatement
if (true) 
    return false;

// if ( Expression ) Block
if (true) {
    return false;
}

// if ( Expression ) TryStatement
if (true) try {
    return false;
} catch (e) {
    return e;
}

// if ( Expression ) EmptyStatement
if (true) ;

// if ( Expression ) IfStatement
if (true)
    if (false)
        return false;