Question

I couldn’t recall the rules for the if statement, so I looked at clause 12.5, but it wasn’t obvious to me still if the curly brace is necessary. So, I ask, where in Ecmascript 5.1 is it mentioned that no left curly brace in an if statement is acceptable?

Answer 1

Here is the syntax of an if statement found in section 12.5:

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;