Understanding object initializer notation in JavaScript

Question

In the sec 11.1.5 of ECMA-262 we have the object declaration notation:

ObjectLiteral :
    { }
    { PropertyNameAndValueList }
    { PropertyNameAndValueList , }
PropertyNameAndValueList :
    PropertyAssignment
    PropertyNameAndValueList , PropertyAssignment
PropertyAssignment :
    PropertyName : AssignmentExpression
    get PropertyName ( ) { FunctionBody }
    set PropertyName ( PropertySetParameterList ) { FunctionBody }
PropertyName :
    IdentifierName
    StringLiteral
    NumericLiteral
PropertySetParameterList :
    Identifier

Well, consider the following ObjectLiteral: {prop: 'prop'}. Thus we have literal of the form {PropertyName: AssignmentExpression}. Now clearly that 'prop' is AssignmentExpression. By defenition from sec 11.13, AssignmentExpression is

AssignmentExpression :
    ConditionalExpression 
    LeftHandSideExpression = AssignmentExpression 
    LeftHandSideExpression AssignmentOperator AssignmentExpression

Question:

Why 'prop' is AssignmentExpression? There is no AssignmentOperator or = and 'prop' is no ConditionalExpression certanly.

@lonesomeday I'm confused, can you explain how 'prop' is evaluated in my ObjectLiteral? — St.Antario
– St.Antario, Commented Dec 19, 2013 at 21:50

cHao · Accepted Answer · 2013-12-19 22:29:17Z

2

An AssignmentExpression means pretty much any expression in the language, other than ones that use the comma operator/punctuator.

Basically, the production rules have operator precedence and associativity baked in.

 AssignmentExpression 
 => ConditionalExpression                          (11.13)
  => LogicalORExpression                           (11.12)
   => LogicalANDExpression                         (11.11)
    => BitwiseORExpression                         (11.10)
     => BitwiseXORExpression                       (11.10)
      => BitwiseANDExpression                      (11.10)
       => EqualityExpression                       (11.10)
        => RelationalExpression                    (11.9)
         => ShiftExpression                        (11.8)
          => AdditiveExpression                    (11.7)
           => MultiplicativeExpression             (11.6)
            => UnaryExpression                     (11.5)
             => PostfixExpression                  (11.4)
              => LeftHandSideExpression            (11.3)
               => NewExpression                    (11.2)
                => MemberExpression                (11.2)
                 => PrimaryExpression              (11.2)
                  => Literal                       (11.1)
                   => StringLiteral                (7.8)
                    => ' SingleStringChars(opt) '  (7.8.4)

Note how the deeper we go, the higher the precedence. You probably already knew that * has higher precedence than + or -, for example. One very common way of specifying precedence in a grammar is to have a production rule for each tier of operators, and specifically say (for example) that an "additive expression" is one or more "multiplicative expressions" separated by + or -. Given such a rule, the parser would have to handle multiplicative subexpressions before it could even attempt to produce an addition or subtraction expression.

edited Dec 19, 2013 at 22:29

answered Dec 19, 2013 at 21:55

cHao

87k21 gold badges147 silver badges178 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

St.Antario Over a year ago

In the 11.1.5 said that The values need not be literals; they are evaluated each time the object initialiser is evaluated. But as i understood from your post AssignmentExpression can be a Literal

cHao Over a year ago

Right. Can be, but doesn't have to be.

St.Antario Over a year ago

Awesome explanation! I'm understand. Many thanks! :)

Bergi · Accepted Answer · 2013-12-19 21:58:04Z

'prop' is no ConditionalExpression certainly.

Yes it is. The term conditional doesn't mean that it necessarily contains a conditional, but that it is at the grammatical level of expressions that are allowed to contain conditionals - to achieve the operator associativity ordering.

Here's a break down:

ConditionalExpression: LogicalORExpression (§11.12)
LogicalORExpression: LogicalANDExpression (§11.11)
LogicalANDExpression: BitwiseORExpression (§11.11)
BitwiseORExpression: BitwiseXORExpression (§11.10)
BitwiseXORExpression: BitwiseANDExpression (§11.10)
BitwiseANDExpression: EqualityExpression (§11.10)
EqualityExpression: RelationalExpression (§11.9)
RelationalExpression: ShiftExpression (§11.8)
ShiftExpression: AdditiveExpression (§11.7)
AdditiveExpression: MultiplicativeExpression (§11.6)
MultiplicativeExpression: UnaryExpression (§11.5)
UnaryExpression: PostfixExpression (§11.4)
PostfixExpression: LeftHandSideExpression (§11.3)
LeftHandSideExpression: NewExpression (§11.2)
NewExpression: MemberExpression (§11.2)
MemberExpression: PrimaryExpression (§11.2)
PrimaryExpression: Literal (§11.1)
Literal: StringLiteral (§7.8)

Barmar · Accepted Answer · 2013-12-19 21:31:34Z

0

It's an AssignmentExpression because you're allowed to write:

{ prop: a = 'prop' }

or:

{ prop: a == 3 ? 'prop1' : 'prop2' }

answered Dec 19, 2013 at 21:31

Barmar

789k57 gold badges554 silver badges669 bronze badges

Collectives™ on Stack Overflow

Understanding object initializer notation in JavaScript

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related