Develop Reference

How come instance is not undefined [duplicate]

If I return some value or object in constructor function, what will the var get?
function MyConstroctor()
{
//what in case when return 5;
//what in case when return someObject;
}
var n = new MyConstroctor();
what n will get in both cases?
Actually its a quiz question, what will be the answer?
What is returned from a custom object constructor?
a)The newly-instantiated object
b)undefined - constructors do not return values
c)Whatever is the return statement
d)Whatever is the return statement; the newly-instantiated object if no return statement

Short Answer
The constructor returns the this object.
function Car() {
this.num_wheels = 4;
}
// car = { num_wheels:4 };
var car = new Car();
Long Answer
By the Javascript spec, when a function is invoked with new, Javascript creates a new object, then sets the "constructor" property of that object to the function invoked, and finally assigns that object to the name this. You then have access to the this object from the body of the function.
Once the function body is executed, Javascript will return:
ANY object if the type of the returned value is object:
function Car(){
this.num_wheels = 4;
return { num_wheels:37 };
}
var car = new Car();
alert(car.num_wheels); // 37
The this object if the function has no return statement OR if the function returns a value of a type other than object:
function Car() {
this.num_wheels = 4;
return 'VROOM';
}
var car = new Car();
alert(car.num_wheels); // 4
alert(Car()); // No 'new', so the alert will show 'VROOM'

Basically if your constructor returns a primitive value, such as a string, number, boolean, null or undefined, (or you don't return anything which is equivalent to returning undefined), a newly created object that inherits from the constructor's prototype will be returned.
That's the object you have access with the this keyword inside the constructor when called with the new keyword.
For example:
function Test() {
return 5; // returning a primitive
}
var obj = new Test();
obj == 5; // false
obj instanceof Test; // true, it inherits from Test.prototype
Test.prototype.isPrototypeOf(obj); // true
But if the returned value is an object reference, that will be the returned value, e.g.:
function Test2() {
this.foo = ""; // the object referred by `this` will be lost...
return {foo: 'bar'};
}
var obj = new Test2();
obj.foo; // "bar"
If you are interested on the internals of the new operator, you can check the algorithm of the [[Construct]] internal operation, is the one responsible of creating the new object that inherits from the constructor's prototype, and to decide what to return:
13.2.2 [[Construct]]
When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:
Let obj be a newly created native ECMAScript object.
Set all the internal methods of obj as specified in 8.12.
Set the [[Class]] internal property of obj to "Object".
Set the [[Extensible]] internal property of obj to true.
Let proto be the value of calling the [[Get]] internal property of F with argument "prototype".
If Type(proto) is Object, set the[[Prototype]]` internal property of obj to proto.
If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in Object prototype object as described in 15.2.4.
Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
If Type(result) is Object then return result.
Return obj.

I found this great link:
JavaScript: Constructor Return Value
The second piece of magic eluded to above is the ability for a
constructor to return a specific, possibly pre-existing object, rather
than a reference to a new instance. This would allow you to manage the
number of actual instances yourself if needed; possibly for reasons of
limited resources or whatnot.
var g_deebee = new Deebee();
function Deebee() { return g_deebee; }
var db1 = new Deebee();
var db2 = new Deebee();
if (db1 != db2)
throw Error("JS constructor returned wrong object!");
else console.log('Equal');

It's as easy as it said in documentation (new operator) :
The object returned by the constructor function becomes the result of the whole new expression. If the constructor function doesn't explicitly return an object, the object created in step 1 is used instead. (Normally constructors don't return a value, but they can choose to do so if they want to override the normal object creation process.)

Trying to simplify the existing answers by providing discrete examples proving that:
Only constructors that return primitive or undefined implicitly create a new instance of themselves. Otherwise the exact object returned by the constructor is used as is.
Consider the following constructor that returns exactly what we pass to it. Check the usages:
//A constructor returning the passed value, or not returning at all.
function MyConstructor(obj){
if(obj!==undefined){
return obj;
}
//else do not call return
}
//no value passed (no value is returned from constructor)
console.log((new MyConstructor()) instanceof MyConstructor)
//true
//Primitive passed:
console.log((new MyConstructor(1)) instanceof MyConstructor)
//true
console.log((new MyConstructor(false)) instanceof MyConstructor)
//true
console.log((new MyConstructor("1")) instanceof MyConstructor)
//true
console.log((new MyConstructor(1.0)) instanceof MyConstructor)
//true
//Object passed
console.log((new MyConstructor(new Number(1))) instanceof MyConstructor)
//false
console.log((new MyConstructor({num:1})) instanceof MyConstructor)
//false
console.log((new MyConstructor([1])) instanceof MyConstructor)
//false
console.log((new MyConstructor(MyConstructor)) instanceof MyConstructor)
//false
//Same results if we use: MyConstructor.prototype.isPrototypeOf(new MyConstructor()) e.t.c..
The same rules as above apply also for class constructors. This means that, if the constructor does not return undefined or primitive, we can have the following, which might feel weird to people coming from java:
(new MyClass()) instanceof MyClass //false
Using the same constructor, check in practice how the instance is different when undefined or primitive is returned:
//As above
function MyConstructor(obj){
if(obj!==undefined){
return obj;
}
//else do not call return
}
//Same object passed (same instance to both variables)
let obj = {};
let a1 = new MyConstructor(obj)
let a2 = new MyConstructor(obj)
a1.x=1
a2.x=2
console.log(a1.x === a2.x, a1.x, a2.x)
//true 2 2
//undefined passed (different instance to each variable)
let b1 = new MyConstructor()
let b2 = new MyConstructor()
b1.x=1
b2.x=2
console.log(b1.x === b2.x, b1.x, b2.x)
//false 1 2
//Primitive passed (different instance to each variable)
let c1 = new MyConstructor(5)
let c2 = new MyConstructor(5)
c1.x=1
c2.x=2
console.log(c1.x === c2.x, c1.x, c2.x)
//false 1 2
Additional note: Sometimes a function could act as a constructor even if it is not called as a constructor:
function F(){
//If not called as a constructor, call as a constructor and return the result
if(!new.target){
return new F();
}
}
console.log(F() instanceof F)
//true
console.log(new F() instanceof F)
//true

You shouldn't return anything in a constructor. A constructor is used to initialize the object. In case you want to know what happens is that if you return 5 then n will simply be an empty object and if you return for example { a: 5 }, then n will have a property a=5.

To answer your specific question:
function MyConstructor()
{
return 5;
}
var n = new MyConstructor();
n is an object instance of MyConstructor.
function SomeObject(name) {
this.name = name;
this.shout = function() {
alert("Hello " + this.name)
}
}
function MyConstructor()
{
return new SomeObject("coure06");
}
var n = new MyConstructor();
n.shout();
n is an instance of SomeObject (call n.shout() to prove it)
To make this all absolutely clear...
1) If you return a primitive type, like a number or a string, it will be ignored.
2) Otherwise, you will pass back the object
Functions and constructors are exactly the same in JavaScript, but how you call them changes their behaviour. A quick example of this is below...
function AddTwoNumbers(first, second) {
return first + second;
}
var functionCall = AddTwoNumbers(5, 3);
alert(functionCall);// 8
var constructorCall = new AddTwoNumbers(5, 3);
alert(constructorCall);// object

The new Operator

Is it possible to write a javascript function that follows this (valid) typescript interface:
interface Foo{
// constructor:
new (): string;
}
i.e. Something that when called with a new operator returns a string. e.g. the following will not work.
function foo(){
return "something";
}
var x = new foo();
// x is now foo (and not string) whether you like it or not :)

You should be able to do:
function foo(){
return new String("something");
}
var x = new foo();
console.log(x);
You can return any object, but literals don't work. See here: What values can a constructor return to avoid returning this?

ECMAScript 5's Section 13.2.2 (on the [[Construct]] internal property) has this to say about the return value of a constructor:
1) Let obj be a newly created native ECMAScript object.
...
8) Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
9) If Type(result) is Object then return result.
10) Return obj.
Thus, the return value of a constructor can only be an object. A string primitive like "foo" has a Type result of String rather than Object. This means that step 9 is false, so step 10 returns the constructed object, instead of the return value of the constructor function.
Instead, you must return an object (new String("foo")), as detailed in RobH's answer.

What does "this" refer to

what is the this (inside inner functions) referring to in the following code context? Does it point to TimeSpan?
var TimeSpan = function (days, hours, minutes, seconds, milliseconds) {
var attrs = "days hours minutes seconds milliseconds".split(/\s+/);
var gFn = function (attr) {
return function () {
return this[attr];
};
};
var sFn = function (attr) {
return function (val) {
this[attr] = val;
return this;
};
};
}
Thanks

The this value is set implicitly depending on how the function is invoked, there are three cases where this happens:
When a reference with no base-object, or a non-reference is invoked:
myFn(); // the myFn reference has no base object
(function () {})(); // non-reference
The this value will point to the global object 1
When a reference contains a base object, for example:
myObj.method();
The this value inside method will point to myObj.
When the new operator is used:
var obj = new Foo();
The this value inside the Foo function, will point to a newly created object that inherits from Foo.prototype.
The this value can be set also explicitly, by using the call and apply methods, for example, with call:
function test(a) {
return alert(this + a);
}
test.call("hello", " world"); // alerts "hello world"
Or with apply if we need to "apply" a set of arguments from an array to a function:
function test(a, b) {
return alert(this + a + b);
}
var args = ["my ", "world "];
test.apply("hello ", args); // alerts "hello my world"
[1] This has changed on the new ECMAScript 5th Strict Mode, now when a function reference with no base object, or a non-reference is invoked (as the first case), the this value will contain undefined.
This was made because when working with constructor functions, people often forgot to use the new operator when calling the constructor.
When that happened, the this value pointed to the global object, and that ended up adding unwanted global properties.
Now on strict mode, this will contain undefined, and if property lookup is made on it (this.foo = 'foo') we will have a nice TypeError exception, instead of having a global foo property.

this refers to the current object, in this case the function you are inside of. Since everything in JavaScript is an object you can modify the attributes of a function object using the this keyword:
var f = function() {
this.key = "someValue";
}
console.log(f.key); // prints "someValue"
So in this case this should point to the function at the deepest scope level, and not TimeSpan.

What is returned from a constructor?

If I return some value or object in constructor function, what will the var get?
function MyConstroctor()
{
//what in case when return 5;
//what in case when return someObject;
}
var n = new MyConstroctor();
what n will get in both cases?
Actually its a quiz question, what will be the answer?
What is returned from a custom object constructor?
a)The newly-instantiated object
b)undefined - constructors do not return values
c)Whatever is the return statement
d)Whatever is the return statement; the newly-instantiated object if no return statement

Short Answer
The constructor returns the this object.
function Car() {
this.num_wheels = 4;
}
// car = { num_wheels:4 };
var car = new Car();
Long Answer
By the Javascript spec, when a function is invoked with new, Javascript creates a new object, then sets the "constructor" property of that object to the function invoked, and finally assigns that object to the name this. You then have access to the this object from the body of the function.
Once the function body is executed, Javascript will return:
ANY object if the type of the returned value is object:
function Car(){
this.num_wheels = 4;
return { num_wheels:37 };
}
var car = new Car();
alert(car.num_wheels); // 37
The this object if the function has no return statement OR if the function returns a value of a type other than object:
function Car() {
this.num_wheels = 4;
return 'VROOM';
}
var car = new Car();
alert(car.num_wheels); // 4
alert(Car()); // No 'new', so the alert will show 'VROOM'

Basically if your constructor returns a primitive value, such as a string, number, boolean, null or undefined, (or you don't return anything which is equivalent to returning undefined), a newly created object that inherits from the constructor's prototype will be returned.
That's the object you have access with the this keyword inside the constructor when called with the new keyword.
For example:
function Test() {
return 5; // returning a primitive
}
var obj = new Test();
obj == 5; // false
obj instanceof Test; // true, it inherits from Test.prototype
Test.prototype.isPrototypeOf(obj); // true
But if the returned value is an object reference, that will be the returned value, e.g.:
function Test2() {
this.foo = ""; // the object referred by `this` will be lost...
return {foo: 'bar'};
}
var obj = new Test2();
obj.foo; // "bar"
If you are interested on the internals of the new operator, you can check the algorithm of the [[Construct]] internal operation, is the one responsible of creating the new object that inherits from the constructor's prototype, and to decide what to return:
13.2.2 [[Construct]]
When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments, the following steps are taken:
Let obj be a newly created native ECMAScript object.
Set all the internal methods of obj as specified in 8.12.
Set the [[Class]] internal property of obj to "Object".
Set the [[Extensible]] internal property of obj to true.
Let proto be the value of calling the [[Get]] internal property of F with argument "prototype".
If Type(proto) is Object, set the[[Prototype]]` internal property of obj to proto.
If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in Object prototype object as described in 15.2.4.
Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list passed into [[Construct]] as args.
If Type(result) is Object then return result.
Return obj.

I found this great link:
JavaScript: Constructor Return Value
The second piece of magic eluded to above is the ability for a
constructor to return a specific, possibly pre-existing object, rather
than a reference to a new instance. This would allow you to manage the
number of actual instances yourself if needed; possibly for reasons of
limited resources or whatnot.
var g_deebee = new Deebee();
function Deebee() { return g_deebee; }
var db1 = new Deebee();
var db2 = new Deebee();
if (db1 != db2)
throw Error("JS constructor returned wrong object!");
else console.log('Equal');

It's as easy as it said in documentation (new operator) :
The object returned by the constructor function becomes the result of the whole new expression. If the constructor function doesn't explicitly return an object, the object created in step 1 is used instead. (Normally constructors don't return a value, but they can choose to do so if they want to override the normal object creation process.)

Trying to simplify the existing answers by providing discrete examples proving that:
Only constructors that return primitive or undefined implicitly create a new instance of themselves. Otherwise the exact object returned by the constructor is used as is.
Consider the following constructor that returns exactly what we pass to it. Check the usages:
//A constructor returning the passed value, or not returning at all.
function MyConstructor(obj){
if(obj!==undefined){
return obj;
}
//else do not call return
}
//no value passed (no value is returned from constructor)
console.log((new MyConstructor()) instanceof MyConstructor)
//true
//Primitive passed:
console.log((new MyConstructor(1)) instanceof MyConstructor)
//true
console.log((new MyConstructor(false)) instanceof MyConstructor)
//true
console.log((new MyConstructor("1")) instanceof MyConstructor)
//true
console.log((new MyConstructor(1.0)) instanceof MyConstructor)
//true
//Object passed
console.log((new MyConstructor(new Number(1))) instanceof MyConstructor)
//false
console.log((new MyConstructor({num:1})) instanceof MyConstructor)
//false
console.log((new MyConstructor([1])) instanceof MyConstructor)
//false
console.log((new MyConstructor(MyConstructor)) instanceof MyConstructor)
//false
//Same results if we use: MyConstructor.prototype.isPrototypeOf(new MyConstructor()) e.t.c..
The same rules as above apply also for class constructors. This means that, if the constructor does not return undefined or primitive, we can have the following, which might feel weird to people coming from java:
(new MyClass()) instanceof MyClass //false
Using the same constructor, check in practice how the instance is different when undefined or primitive is returned:
//As above
function MyConstructor(obj){
if(obj!==undefined){
return obj;
}
//else do not call return
}
//Same object passed (same instance to both variables)
let obj = {};
let a1 = new MyConstructor(obj)
let a2 = new MyConstructor(obj)
a1.x=1
a2.x=2
console.log(a1.x === a2.x, a1.x, a2.x)
//true 2 2
//undefined passed (different instance to each variable)
let b1 = new MyConstructor()
let b2 = new MyConstructor()
b1.x=1
b2.x=2
console.log(b1.x === b2.x, b1.x, b2.x)
//false 1 2
//Primitive passed (different instance to each variable)
let c1 = new MyConstructor(5)
let c2 = new MyConstructor(5)
c1.x=1
c2.x=2
console.log(c1.x === c2.x, c1.x, c2.x)
//false 1 2
Additional note: Sometimes a function could act as a constructor even if it is not called as a constructor:
function F(){
//If not called as a constructor, call as a constructor and return the result
if(!new.target){
return new F();
}
}
console.log(F() instanceof F)
//true
console.log(new F() instanceof F)
//true

You shouldn't return anything in a constructor. A constructor is used to initialize the object. In case you want to know what happens is that if you return 5 then n will simply be an empty object and if you return for example { a: 5 }, then n will have a property a=5.

To answer your specific question:
function MyConstructor()
{
return 5;
}
var n = new MyConstructor();
n is an object instance of MyConstructor.
function SomeObject(name) {
this.name = name;
this.shout = function() {
alert("Hello " + this.name)
}
}
function MyConstructor()
{
return new SomeObject("coure06");
}
var n = new MyConstructor();
n.shout();
n is an instance of SomeObject (call n.shout() to prove it)
To make this all absolutely clear...
1) If you return a primitive type, like a number or a string, it will be ignored.
2) Otherwise, you will pass back the object
Functions and constructors are exactly the same in JavaScript, but how you call them changes their behaviour. A quick example of this is below...
function AddTwoNumbers(first, second) {
return first + second;
}
var functionCall = AddTwoNumbers(5, 3);
alert(functionCall);// 8
var constructorCall = new AddTwoNumbers(5, 3);
alert(constructorCall);// object

What values can a constructor return to avoid returning this?

What are the exact circumstances for which a return statement in Javascript can return a value other than this when a constructor is invoked using the new keyword?
Example:
function Foo () {
return something;
}
var foo = new Foo ();
If I'm not mistaken, if something is a non-function primitive, this will be returned. Otherwise something is returned. Is this correct?
In other words, what values can something take to cause (new Foo () instanceof Foo) === false?

The exact condition is described on the [[Construct]] internal property, which is used by the new operator:
From the ECMA-262 3rd. Edition Specification:
13.2.2 [[Construct]]
When the [[Construct]] property for a Function object F is
called, the following steps are taken:
Create a new native ECMAScript object.
Set the [[Class]] property of Result(1) to "Object".
Get the value of the prototype property of F.
If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as
described in 15.2.3.1.
Invoke the [[Call]] property of F, providing Result(1) as the this value and
providing the argument list passed into [[Construct]] as the
argument values.
If Type(Result(6)) is
Object then return Result(6).
Return Result(1).
Look at steps 7 and 8, the new object will be returned only if the
type of Result(6) (the value returned from the F constructor
function) is not an Object.

Concrete examples
/*
ECMA 262 v 5
http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf
"4.3.2
primitive value
member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause 6"
*/
var Person = function(x){
return x;
};
console.log(Person.constructor);
console.log(Person.prototype.constructor);
console.log(typeof(Person));
console.log(typeof(Person.prototype));
function log(x){
console.log(x instanceof Person);
console.log(typeof x);
console.log(typeof x.prototype);
}
log(new Person(undefined));
log(new Person(null));
log(new Person(true));
log(new Person(2));
log(new Person(""));
//returns a function not an object
log(new Person(function(){}));
//implementation?
//log(new Person(Symbol('%')));

I couldn't find any documentation on the matter, but I think you're correct. For example, you can return new Number(5) from a constructor, but not the literal 5 (which is ignored and this is returned instead).

As a side note, the return value or this is just part of the equation.
For example, consider this:
function Two() { return new Number(2); }
var two = new Two;
two + 2; // 4
two.valueOf = function() { return 3; }
two + 2; // 5
two.valueOf = function() { return '2'; }
two + 2; // '22'
As you can see, .valueOf() is internally used and can be exploited for fun and profit. You can even create side effects, for example:
function AutoIncrementingNumber(start) {
var n = new Number, val = start || 0;
n.valueOf = function() { return val++; };
return n;
}
var auto = new AutoIncrementingNumber(42);
auto + 1; // 43
auto + 1; // 44
auto + 1; // 45
I can imagine this must have some sort of practical application. And it doesn't have to be explicitly a Number either, if you add .valueOf to any object it can behave as a number:
({valueOf: function() { return Math.random(); }}) + 1; // 1.6451723610516638
You can exploit this to make an object that always returns a new GUID, for instance.

Trying to put a few points in simpler words.
In javascript, when you use a new keyword on a function and if,
function does not return anything, it will return an intended object
function User() {
this.name = 'Virat'
}
var user = new User();
console.log(user.name); //=> 'Virat'
function returns any truthy complex object [object, array, function etc], that complex object takes priority and user variable will hold the returned complex object
function User() {
this.name = 'Virat';
return function(){};
}
var user = new User();
console.log(user.name); //=> undefined
console.log(user); //=> function
function returns any literal, constructor takes priority and it will return an intended object
function User() {
this.name = 'Virat';
return 10;
}
var user = new User();
console.log(user.name); //=> 'Virat'

When you are using the new keyword, an object is created. Then the function is called to initialise the object.
There is nothing that the function can do to prevent the object being created, as that is done before the function is called.