• It is a Syntax Error if the BoundNames of BindingList contains "let".
• It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

• It is a Syntax Error if Initializer is not present and IsConstantDeclaration of the LexicalDeclaration containing this LexicalBinding is true.

The syntax-directed operation PropertyBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all bound property names. It is defined piecewise over the following productions:

The syntax-directed operation RestBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

The syntax-directed operation KeyedBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion.

It is defined piecewise over the following productions:

The abstract operation LoopContinues takes arguments completion (a Completion Record) and labelSet (a List of Strings) and returns a Boolean. It performs the following steps when called:

The syntax-directed operation LoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

• It is a Syntax Error if IsLabelledFunction(Statement) is true.

The syntax-directed operation DoWhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

• It is a Syntax Error if IsLabelledFunction(Statement) is true.

The syntax-directed operation WhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

• It is a Syntax Error if IsLabelledFunction(Statement) is true.

• It is a Syntax Error if any element of the BoundNames of LexicalDeclaration also occurs in the VarDeclaredNames of Statement.

The syntax-directed operation ForLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

The abstract operation ForBodyEvaluation takes arguments test (an Expression Parse Node or empty), increment (an Expression Parse Node or empty), stmt (a Statement Parse Node), perIterationBindings (a List of Strings), and labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a List of Strings) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

• It is a Syntax Error if IsLabelledFunction(Statement) is true.

• If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, LeftHandSideExpression must cover an AssignmentPattern.
• If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, it is a Syntax Error if the AssignmentTargetType of LeftHandSideExpression is not simple.

• It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
• It is a Syntax Error if any element of the BoundNames of ForDeclaration also occurs in the VarDeclaredNames of Statement.
• It is a Syntax Error if the BoundNames of ForDeclaration contains any duplicate entries.

The syntax-directed operation IsDestructuring takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

The syntax-directed operation ForDeclarationBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

It is defined piecewise over the following productions:

The syntax-directed operation ForDeclarationBindingInstantiation takes argument environment (a Declarative Environment Record) and returns unused. It is defined piecewise over the following productions:

The syntax-directed operation ForInOfLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames (a List of Strings), expr (an Expression Parse Node or an AssignmentExpression Parse Node), and iterationKind (enumerate, iterate, or async-iterate) and returns either a normal completion containing an Iterator Record or an abrupt completion. It performs the following steps when called:

The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse Node), stmt (a Statement Parse Node), iteratorRecord (an Iterator Record), iterationKind (enumerate or iterate), lhsKind (assignment, var-binding, or lexical-binding), and labelSet (a List of Strings) and optional argument iteratorKind (sync or async) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an iterator object. It performs the following steps when called:

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, TypedArray, module namespace exotic object, or implementation provided exotic object, then the iterator must behave as would the iterator given by CreateForInIterator(O) until one of the following occurs:

• the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
• a property is removed from O or an object in its prototype chain,
• a property is added to an object in O's prototype chain, or
• the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.

            function* EnumerateObjectProperties(obj) {
              const visited = new Set();
              for (const key of Reflect.ownKeys(obj)) {
                if (typeof key === "symbol") continue;
                const desc = Reflect.getOwnPropertyDescriptor(obj, key);
                if (desc) {
                  visited.add(key);
                  if (desc.enumerable) yield key;
                }
              }
              const proto = Reflect.getPrototypeOf(obj);
              if (proto === null) return;
              for (const protoKey of EnumerateObjectProperties(proto)) {
                if (!visited.has(protoKey)) yield protoKey;
              }
            }
          

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.