Symbol

The Symbol Constructor

Symbol ( [ description ] )

1. If NewTarget is not undefined, throw a TypeError exception.
2. Note: the Symbol Constructor is not intended to be used with the new operator(not the target for new)
3. If description is undefined, let descString be undefined; Else, let descString be ? ToString(description).
4. Return a new unique Symbol value whose [[Description]] value is descString.
5. Note: the Symbol() function returns a Symbol type instead of a Symbol object

Symbol Prototype Object

abstract operation thisSymbolValue(value)

1. If Type(value) is Symbol, return value.
2. If Type(value) is Object and value has a [[SymbolData]] internal slot
3. then Return value.[[SymbolData]] //a Symbol Object
4. Throw a TypeError exception.

Example

mySymbol = Symbol( "my symbol" )

// Object() performs a type conversion when called as a function rather than as a constructor.
mySymObj = Object(mySymbol)

console.log(mySymObj.toString()) // Symbol(my symbol)

Function.prototype.call

Example

function anInstanceOfFunction(data)
{
    this.data = data;
}

thisArgument = {"data" : null};

// all instances of Function(function definitions or dynamical functions by `new Function`) has
// their [[prototype]] chained to the Function constructor's prototype property(Function.prototype)
anInstanceOfFunction.call(thisArgument, "dat");

console.log(thisArgument.data);

Function.prototype.call ( thisArgument, argumentsList )

note Function.prototype itself is a function object(to ensure compatibility with es2015), which returns undefined when called; but the call here is `Function.prototype.` instead of `Function.prototype(`

1. If IsCallable(F) is false, throw a TypeError exception.//F is anInstanceOfFunction in our example
2. Pop from the execution context stack//the top execution context has no further use and optimized out
3. Let calleeContext be PrepareForOrdinaryCall(F, undefined):

           calleeContext = {
               "Function" : F,
               "LexicalEnvironment" : new NewFunctionEnvironment(F),
           }
           calleeContext.VariableEnvironment = calleeContext.LexicalEnvironment
           Push calleeContext onto the execution context stack
           return calleeContext
       
   calleeContext.LexicalEnvironment.EnvironmentRecord.[[ThisValue]] = thisArgument
   Let result be Evaluate F with argumentsList.
   Remove(exactly, Pop) calleeContext from the execution context stack
   If result.[[Type]] is return, return NormalCompletion(result.[[Value]]).
   ReturnIfAbrupt(result).
   Return NormalCompletion(undefined).
   

Function.prototype.bind

Example

const person = {
    getFullName: function () {
        return this.firstName + " " + this.lastName;
    }
}

const thisPerson = {
	firstName   :   "John",
	lastName    :   "Xu",
}

let getFullName = person.getFullName.bind(thisPerson);

console.log( getFullName() )

Function.prototype.bind( boundThis, ...boundArgs )

1. Let targetFunction be the this value. // for our example, person.getFullName
2. If IsCallable(targetFunction) is false, throw a TypeError exception.
3. Let boundArgs be a new List consisting of all of the argument values provided after boundThis(for our example, empty) .
4. Let F be ?BoundFunctionCreate(targetFunction, boundThis, boundArgs)

           return {
               [[BoundTargetFunction]]: targetFunction,
               [[BoundThis]]: boundThis,
               [[BoundArguments]]: boundArgs,
               [[Prototype]]:targetFunction.[[Prototype]],
           }
       

5. Perform SetFunctionName(F, name = Get(Target, "name"), prefix = "bound").

NewTarget and new.target

example 01

base_obj = {
    "constructor" : my_constructor,
}

function my_constr_stub()
{
    console.log("in constructor stub:new.target=" + new.target);//undefined
}

function my_constructor()
{
    console.log("in constructor:new.target=" + new.target);// function my_constructor(){...
    my_constr_stub();
}

my_constructor.prototype = base_obj;

new my_constructor()

Object [[Construct]](a List of any, newTarget)

The second argument is the object to which the new operator was initially applied. For example, in following code, the new operator is initially applied to the Square object:

class Rectangle {
  constructor(height, width) {
    console.log("in Rectangle:new.target=" + new.target);
    this.name = 'Rectangle';
    this.height = height;
    this.width = width;
  }
}

class Square extends Rectangle {
  constructor(length) {
    console.log("in Square:new.target=" + new.target);
    super(length, length);

    this.name = 'Square';
  }
}

aS = new Square(2);
console.log(
    "name=" + aS.name +
    ";height=" + aS.height
);

function expression

factit = function factorial(n) {
    console.log(n)
    if (n < 2) {
      return 1;
    }
    return n * factorial(n - 1);
}

non_recursive = function(){}

/*
can't visit factorial here:
as is indicated in `Runtime Semantics: Evaluation` of NAMED function expression,
we will NewDeclarativeEnvironment(scope) as a middle env to recv factorial

from ecma:
The BindingIdentifier in a FunctionExpression can be referenced
 from inside the FunctionExpression's
FunctionBody to allow the function to call itself recursively.

However, unlike in a FunctionDeclaration, the BindingIdentifier in
a FunctionExpression cannot be referenced from and does
not affect the scope enclosing the FunctionExpression.
*/

TopLevelVarScopedDeclarations

Block

1. Block : { }

       Return a new empty List.
   

2. StatementList : StatementList StatementListItem //StatementListItem can be Statement or Declaration

       Let declarations be TopLevelVarScopedDeclarations of StatementList.
       Append to declarations the elements of the TopLevelVarScopedDeclarations of StatementListItem.
       Return declarations.
   

3. StatementListItem : Declaration

       if is function Declaration, return a list contains the Declaration// function is not taken as Lex but top level Var!
   

Label Statement

LabelledStatement : LabelIdentifier : LabelledItem

    Return the TopLevelVarScopedDeclarations of LabelledItem.

LabelledItem : Statement

    // recursive to the last non-labelled Statement
    If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement
    Return VarScopedDeclarations of Statement.

Others

not documented in ecma documents

TopLevelLexicallyScopedDeclarations

Block

1. Block : { }

       Return a new empty List.
   

2. StatementList : StatementList StatementListItem //StatementListItem can be Statement or Declaration

       Let declarations be TopLevelLexicallyScopedDeclarations of StatementList.
       Append to declarations the elements of the TopLevelLexicallyScopedDeclarations of StatementListItem.
       Return declarations.
   

3. StatementListItem : Statement

       Return a new empty List.//VariableStatement contribute to Var instead of Lex
   

4. StatementListItem : Declaration

       if Declaration is `let ` or `const `, return a new List containing Declaration;
       else Return a new empty List// TopLevel functions are taken as Var instead
   

Labelled Statements

Return a new empty List

Others

not documented in ecma documents