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How do recursive functions have access to info stored in a local memo object? [duplicate]

This question already has answers here:
Is JavaScript a pass-by-reference or pass-by-value language?
(33 answers)
Closed 8 years ago.
I'm looking for some good comprehensive reading material on when JavaScript passes something by value and when by reference and when modifying a passed item affects the value outside a function and when not. I'm also interested in when assigning to another variable is by reference vs. by value and whether that follows any different rules than passing as a function parameter.
I've done a lot of searching and find lots of specific examples (many of them here on SO) from which I can start to piece together pieces of the real rules, but I haven't yet found a single, well written document that describes it all.
Also, are there ways in the language to control whether something is passed by reference or by value?
Here are some of the types of questions I want to understand. These are just examples - I'm actually looking to understand the rules the language goes by, not just the answers to specific examples. But, here are some examples:
function f(a,b,c) {
a = 3;
b.push("foo");
c.first = false;
}
var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);
When are the contents of x, y and z changed outside the scope of f for all the different types?
function f() {
var a = ["1", "2", "3"];
var b = a[1];
a[1] = "4";
// what is the value of b now for all possible data types that the array in "a" might hold?
}
function f() {
var a = [{yellow: "blue"}, {red: "cyan"}, {green: "magenta"}];
var b = a[1];
a[1].red = "tan";
// what is the value of b now and why?
b.red = "black";
// did the value of a[1].red change when I assigned to b.red?
}
If I want to make a fully independent copy of an object (no references whatsoever), what's the best practice way to do that?

My understanding is that this is actually very simple:
Javascript is always pass by value, but when a variable refers to an object (including arrays), the "value" is a reference to the object.
Changing the value of a variable never changes the underlying primitive or object, it just points the variable to a new primitive or object.
However, changing a property of an object referenced by a variable does change the underlying object.
So, to work through some of your examples:
function f(a,b,c) {
// Argument a is re-assigned to a new value.
// The object or primitive referenced by the original a is unchanged.
a = 3;
// Calling b.push changes its properties - it adds
// a new property b[b.length] with the value "foo".
// So the object referenced by b has been changed.
b.push("foo");
// The "first" property of argument c has been changed.
// So the object referenced by c has been changed (unless c is a primitive)
c.first = false;
}
var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);
console.log(x, y, z.first); // 4, ["eeny", "miny", "mo", "foo"], false
Example 2:
var a = ["1", "2", {foo:"bar"}];
var b = a[1]; // b is now "2";
var c = a[2]; // c now references {foo:"bar"}
a[1] = "4"; // a is now ["1", "4", {foo:"bar"}]; b still has the value
// it had at the time of assignment
a[2] = "5"; // a is now ["1", "4", "5"]; c still has the value
// it had at the time of assignment, i.e. a reference to
// the object {foo:"bar"}
console.log(b, c.foo); // "2" "bar"

Javascript always passes by value. However, if you pass an object to a function, the "value" is really a reference to that object, so the function can modify that object's properties but not cause the variable outside the function to point to some other object.
An example:
function changeParam(x, y, z) {
x = 3;
y = "new string";
z["key2"] = "new";
z["key3"] = "newer";
z = {"new" : "object"};
}
var a = 1,
b = "something",
c = {"key1" : "whatever", "key2" : "original value"};
changeParam(a, b, c);
// at this point a is still 1
// b is still "something"
// c still points to the same object but its properties have been updated
// so it is now {"key1" : "whatever", "key2" : "new", "key3" : "newer"}
// c definitely doesn't point to the new object created as the last line
// of the function with z = ...

Yes, Javascript always passes by value, but in an array or object, the value is a reference to it, so you can 'change' the contents.
But, I think you already read it on SO; here you have the documentation you want:
http://snook.ca/archives/javascript/javascript_pass

Primitive type variable like string,number are always pass as pass
by value.
Array and Object is passed as pass by reference or pass by value based on these two condition.
if you are changing value of that Object or array with new Object or Array then it is pass by Value.
object1 = {item: "car"};
array1=[1,2,3];
here you are assigning new object or array to old one.you are not changing the value of property
of old object.so it is pass by value.
if you are changing a property value of an object or array then it is pass by Reference.
object1.item= "car";
array1[0]=9;
here you are changing a property value of old object.you are not assigning new object or array to old one.so it is pass by reference.
Code
function passVar(object1, object2, number1) {
object1.key1= "laptop";
object2 = {
key2: "computer"
};
number1 = number1 + 1;
}
var object1 = {
key1: "car"
};
var object2 = {
key2: "bike"
};
var number1 = 10;
passVar(object1, object2, number1);
console.log(object1.key1);
console.log(object2.key2);
console.log(number1);
Output: -
laptop
bike
10

How is this variable scope resolving performed exactly? [duplicate]

This question already has answers here:
variable binding inside an object
(2 answers)
Closed 2 years ago.
While trying to understand how javaScript resolve variable, i wrote this code:
const a = 0;
var obj = {a: 1, foo: function(){console.log(a);}};
obj.foo();
I know that javaScript, like most of the programming languages, use the lexical/static scope to resolving variables.
But in this example, the lexical scope outside of foo function is obj scope and not global scope.
So why do i get output 0 instead of 1 ?

the lexical scope outside of foo function is obj scope and not global scope.
There is no such thing as "obj scope" or "object scope". In Javascript, every variable name (or identifier) is scoped to either a block (such as inside a loop, or inside a function block), or is global.
An object isn't a block. Putting a property on an object does not create a new identifier; referencing such a property name as if it were a standalone variable does not work, because it's not an identifier in scope.
The only time in which something like this would be possible would be if you used with and referenced the property as if it was a standalone variable inside the with block, but with should never be used.
const a = 0;
var obj = {
a: 1,
foo: function() {
console.log(a);
}
};
with (obj) {
console.log(a);
}

In an object with {a: 1} the 'a' is a string, not a variable. It does not assign 1 to 'a'. Instead it sets a key/value pair equivalent to { "a": 1 }.
It has nothing to do with scope. The variable a defined in the first line is the only variable of that name and never has its value changed in the code you gave.
Note also that you defined it as const meaning it cannot be assigned a different value.

If you need reference the a object property, is necessary use this.a, because a without this is the global variable.
const a = 0;
var obj = {a: 1, foo: function(){console.log(this.a);}};
obj.foo();

Because there is only one a, and it's declared on your first line.
In order to use this object's variables, you have to use this. For example:
const a = 0
var obj = { a: 1, foo: function() { console.log(this.a) } }
obj.foo(); // 1
When you use this.a, that's the same thing, if you use obj.a outside of this object.
To understand this better, you can check the example below:
const mark = 10
const obj = {
name: 'Sardor',
mark: mark,
setMark: function(num) { this.mark = num },
getMark: function() { console.log(this.mark) }
}
obj.getMark() // 10
obj.setMark(8)
obj.getMark() // 8
console.log(mark) // 10
It means, that you filled mark by the date of num. Then you called .setMark() function of obj and changed its value to 8. But you didn't change the variable mark, that is outside of obj. That's why finnaly you have 8 in obj.mark and 10 in mark.
Advice: read more about this

JavaScript closure and pointers situation [duplicate]

This question already has answers here:
Is JavaScript a pass-by-reference or pass-by-value language?
(33 answers)
Closed 8 years ago.
I'm looking for some good comprehensive reading material on when JavaScript passes something by value and when by reference and when modifying a passed item affects the value outside a function and when not. I'm also interested in when assigning to another variable is by reference vs. by value and whether that follows any different rules than passing as a function parameter.
I've done a lot of searching and find lots of specific examples (many of them here on SO) from which I can start to piece together pieces of the real rules, but I haven't yet found a single, well written document that describes it all.
Also, are there ways in the language to control whether something is passed by reference or by value?
Here are some of the types of questions I want to understand. These are just examples - I'm actually looking to understand the rules the language goes by, not just the answers to specific examples. But, here are some examples:
function f(a,b,c) {
a = 3;
b.push("foo");
c.first = false;
}
var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);
When are the contents of x, y and z changed outside the scope of f for all the different types?
function f() {
var a = ["1", "2", "3"];
var b = a[1];
a[1] = "4";
// what is the value of b now for all possible data types that the array in "a" might hold?
}
function f() {
var a = [{yellow: "blue"}, {red: "cyan"}, {green: "magenta"}];
var b = a[1];
a[1].red = "tan";
// what is the value of b now and why?
b.red = "black";
// did the value of a[1].red change when I assigned to b.red?
}
If I want to make a fully independent copy of an object (no references whatsoever), what's the best practice way to do that?

My understanding is that this is actually very simple:
Javascript is always pass by value, but when a variable refers to an object (including arrays), the "value" is a reference to the object.
Changing the value of a variable never changes the underlying primitive or object, it just points the variable to a new primitive or object.
However, changing a property of an object referenced by a variable does change the underlying object.
So, to work through some of your examples:
function f(a,b,c) {
// Argument a is re-assigned to a new value.
// The object or primitive referenced by the original a is unchanged.
a = 3;
// Calling b.push changes its properties - it adds
// a new property b[b.length] with the value "foo".
// So the object referenced by b has been changed.
b.push("foo");
// The "first" property of argument c has been changed.
// So the object referenced by c has been changed (unless c is a primitive)
c.first = false;
}
var x = 4;
var y = ["eeny", "miny", "mo"];
var z = {first: true};
f(x,y,z);
console.log(x, y, z.first); // 4, ["eeny", "miny", "mo", "foo"], false
Example 2:
var a = ["1", "2", {foo:"bar"}];
var b = a[1]; // b is now "2";
var c = a[2]; // c now references {foo:"bar"}
a[1] = "4"; // a is now ["1", "4", {foo:"bar"}]; b still has the value
// it had at the time of assignment
a[2] = "5"; // a is now ["1", "4", "5"]; c still has the value
// it had at the time of assignment, i.e. a reference to
// the object {foo:"bar"}
console.log(b, c.foo); // "2" "bar"

Javascript always passes by value. However, if you pass an object to a function, the "value" is really a reference to that object, so the function can modify that object's properties but not cause the variable outside the function to point to some other object.
An example:
function changeParam(x, y, z) {
x = 3;
y = "new string";
z["key2"] = "new";
z["key3"] = "newer";
z = {"new" : "object"};
}
var a = 1,
b = "something",
c = {"key1" : "whatever", "key2" : "original value"};
changeParam(a, b, c);
// at this point a is still 1
// b is still "something"
// c still points to the same object but its properties have been updated
// so it is now {"key1" : "whatever", "key2" : "new", "key3" : "newer"}
// c definitely doesn't point to the new object created as the last line
// of the function with z = ...

Yes, Javascript always passes by value, but in an array or object, the value is a reference to it, so you can 'change' the contents.
But, I think you already read it on SO; here you have the documentation you want:
http://snook.ca/archives/javascript/javascript_pass

Primitive type variable like string,number are always pass as pass
by value.
Array and Object is passed as pass by reference or pass by value based on these two condition.
if you are changing value of that Object or array with new Object or Array then it is pass by Value.
object1 = {item: "car"};
array1=[1,2,3];
here you are assigning new object or array to old one.you are not changing the value of property
of old object.so it is pass by value.
if you are changing a property value of an object or array then it is pass by Reference.
object1.item= "car";
array1[0]=9;
here you are changing a property value of old object.you are not assigning new object or array to old one.so it is pass by reference.
Code
function passVar(object1, object2, number1) {
object1.key1= "laptop";
object2 = {
key2: "computer"
};
number1 = number1 + 1;
}
var object1 = {
key1: "car"
};
var object2 = {
key2: "bike"
};
var number1 = 10;
passVar(object1, object2, number1);
console.log(object1.key1);
console.log(object2.key2);
console.log(number1);
Output: -
laptop
bike
10

Is there a form of C++ references in Javascript? [duplicate]

This question already has answers here:
Is JavaScript a pass-by-reference or pass-by-value language?
(33 answers)
Closed 3 years ago.
Does JavaScript pass by references or pass by values?
Here is an example from JavaScript: The Good Parts. I am very confused about the my parameter for the rectangle function. It is actually undefined, and redefined inside the function. There are no original reference. If I remove it from the function parameter, the inside area function is not able to access it.
Is it a closure? But no function is returned.
var shape = function (config) {
var that = {};
that.name = config.name || "";
that.area = function () {
return 0;
};
return that;
};
var rectangle = function (config, my) {
my = my || {};
my.l = config.length || 1;
my.w = config.width || 1;
var that = shape(config);
that.area = function () {
return my.l * my.w;
};
return that;
};
myShape = shape({
name: "Unhnown"
});
myRec = rectangle({
name: "Rectangle",
length: 4,
width: 6
});
console.log(myShape.name + " area is " + myShape.area() + " " + myRec.name + " area is " + myRec.area());

Primitives are passed by value, and Objects are passed by "copy of a reference".
Specifically, when you pass an object (or array) you are (invisibly) passing a reference to that object, and it is possible to modify the contents of that object, but if you attempt to overwrite the reference it will not affect the copy of the reference held by the caller - i.e. the reference itself is passed by value:
function replace(ref) {
ref = {}; // this code does _not_ affect the object passed
}
function update(ref) {
ref.key = 'newvalue'; // this code _does_ affect the _contents_ of the object
}
var a = { key: 'value' };
replace(a); // a still has its original value - it's unmodfied
update(a); // the _contents_ of 'a' are changed

Think of it like this:
Whenever you create an object in ECMAscript, this object is formed in a mystique ECMAscript universal place where no man will ever be able to get. All you get back is a reference to that object in this mystique place.
var obj = { };
Even obj is only a reference to the object (which is located in that special wonderful place) and hence, you can only pass this reference around. Effectively, any piece of code which accesses obj will modify the object which is far, far away.

My two cents.... It's irrelevant whether JavaScript passes parameters by reference or value. What really matters is assignment vs. mutation.
I wrote a longer, more detailed explanation in this link.
When you pass anything (whether that be an object or a primitive), all JavaScript does is assign a new variable while inside the function... just like using the equal sign (=).
How that parameter behaves inside the function is exactly the same as it would behave if you just assigned a new variable using the equal sign... Take these simple examples.
var myString = 'Test string 1';
// Assignment - A link to the same place as myString
var sameString = myString;
// If I change sameString, it will not modify myString,
// it just re-assigns it to a whole new string
sameString = 'New string';
console.log(myString); // Logs 'Test string 1';
console.log(sameString); // Logs 'New string';
If I were to pass myString as a parameter to a function, it behaves as if I simply assigned it to a new variable. Now, let's do the same thing, but with a function instead of a simple assignment
function myFunc(sameString) {
// Reassignment... Again, it will not modify myString
sameString = 'New string';
}
var myString = 'Test string 1';
// This behaves the same as if we said sameString = myString
myFunc(myString);
console.log(myString); // Again, logs 'Test string 1';
The only reason that you can modify objects when you pass them to a function is because you are not reassigning... Instead, objects can be changed or mutated.... Again, it works the same way.
var myObject = { name: 'Joe'; }
// Assignment - We simply link to the same object
var sameObject = myObject;
// This time, we can mutate it. So a change to myObject affects sameObject and visa versa
myObject.name = 'Jack';
console.log(sameObject.name); // Logs 'Jack'
sameObject.name = 'Jill';
console.log(myObject.name); // Logs 'Jill'
// If we re-assign it, the link is lost
sameObject = { name: 'Howard' };
console.log(myObject.name); // Logs 'Jill'
If I were to pass myObject as a parameter to a function, it behaves as if I simply assigned it to a new variable. Again, the same thing with the exact same behavior but with a function.
function myFunc(sameObject) {
// We mutate the object, so the myObject gets the change too... just like before.
sameObject.name = 'Jill';
// But, if we re-assign it, the link is lost
sameObject = {
name: 'Howard'
};
}
var myObject = {
name: 'Joe'
};
// This behaves the same as if we said sameObject = myObject;
myFunc(myObject);
console.log(myObject.name); // Logs 'Jill'
Every time you pass a variable to a function, you are "assigning" to whatever the name of the parameter is, just like if you used the equal = sign.
Always remember that the equals sign = means assignment.
And passing a parameter to a function also means assignment.
They are the same and the two variables are connected in exactly the same way.
The only time that modifying a variable affects a different variable is when the underlying object is mutated.
There is no point in making a distinction between objects and primitives, because it works the same exact way as if you didn't have a function and just used the equal sign to assign to a new variable.

Function arguments are passed either by-value or by-sharing, but never ever by reference in JavaScript!
Call-by-Value
Primitive types are passed by-value:
var num = 123, str = "foo";
function f(num, str) {
num += 1;
str += "bar";
console.log("inside of f:", num, str);
}
f(num, str);
console.log("outside of f:", num, str);
Reassignments inside a function scope are not visible in the surrounding scope.
This also applies to Strings, which are a composite data type and yet immutable:
var str = "foo";
function f(str) {
str[0] = "b"; // doesn't work, because strings are immutable
console.log("inside of f:", str);
}
f(str);
console.log("outside of f:", str);
Call-by-Sharing
Objects, that is to say all types that are not primitives, are passed by-sharing. A variable that holds a reference to an object actually holds merely a copy of this reference. If JavaScript would pursue a call-by-reference evaluation strategy, the variable would hold the original reference. This is the crucial difference between by-sharing and by-reference.
What are the practical consequences of this distinction?
var o = {x: "foo"}, p = {y: 123};
function f(o, p) {
o.x = "bar"; // Mutation
p = {x: 456}; // Reassignment
console.log("o inside of f:", o);
console.log("p inside of f:", p);
}
f(o, p);
console.log("o outside of f:", o);
console.log("p outside of f:", p);
Mutating means to modify certain properties of an existing Object. The reference copy that a variable is bound to and that refers to this object remains the same. Mutations are thus visible in the caller's scope.
Reassigning means to replace the reference copy bound to a variable. Since it is only a copy, other variables holding a copy of the same reference remain unaffected. Reassignments are thus not visible in the caller's scope like they would be with a call-by-reference evaluation strategy.
Further information on evaluation strategies in ECMAScript.

As with C, ultimately, everything is passed by value. Unlike C, you can't actually back up and pass the location of a variable, because it doesn't have pointers, just references.
And the references it has are all to objects, not variables. There are several ways of achieving the same result, but they have to be done by hand, not just adding a keyword at either the call or declaration site.

JavaScript is pass by value.
For primitives, primitive's value is passed. For Objects, Object's reference "value" is passed.
Example with Object:
var f1 = function(inputObject){
inputObject.a = 2;
}
var f2 = function(){
var inputObject = {"a": 1};
f1(inputObject);
console.log(inputObject.a);
}
Calling f2 results in printing out "a" value as 2 instead of 1, as the reference is passed and the "a" value in reference is updated.
Example with primitive:
var f1 = function(a){
a = 2;
}
var f2 = function(){
var a = 1;
f1(a);
console.log(a);
}
Calling f2 results in printing out "a" value as 1.

In the interest of creating a simple example that uses const...
const myRef = { foo: 'bar' };
const myVal = true;
function passes(r, v) {
r.foo = 'baz';
v = false;
}
passes(myRef, myVal);
console.log(myRef, myVal); // Object {foo: "baz"} true

In practical terms, Alnitak is correct and makes it easy to understand, but ultimately in JavaScript, everything is passed by value.
What is the "value" of an object? It is the object reference.
When you pass in an object, you get a copy of this value (hence the 'copy of a reference' that Alnitak described). If you change this value, you do not change the original object; you are changing your copy of that reference.

"Global" JavaScript variables are members of the window object. You could access the reference as a member of the window object.
var v = "initialized";
function byref(ref) {
window[ref] = "changed by ref";
}
byref((function(){for(r in window){if(window[r]===v){return(r);}}})());
// It could also be called like... byref('v');
console.log(v); // outputs changed by ref
Note, the above example will not work for variables declared within a function.

Without purisms, I think that the best way to emulate scalar argument by reference in JavaScript is using object, like previous an answer tells.
However, I do a little bit different:
I've made the object assignment inside function call, so one can see the reference parameters near the function call. It increases the source readability.
In function declaration, I put the properties like a comment, for the very same reason: readability.
var r;
funcWithRefScalars(r = {amount:200, message:null} );
console.log(r.amount + " - " + r.message);
function funcWithRefScalars(o) { // o(amount, message)
o.amount *= 1.2;
o.message = "20% increase";
}
In the above example, null indicates clearly an output reference parameter.
The exit:
240 - 20% Increase
On the client-side, console.log should be replaced by alert.
★ ★ ★
Another method that can be even more readable:
var amount, message;
funcWithRefScalars(amount = [200], message = [null] );
console.log(amount[0] + " - " + message[0]);
function funcWithRefScalars(amount, message) { // o(amount, message)
amount[0] *= 1.2;
message[0] = "20% increase";
}
Here you don't even need to create new dummy names, like r above.

I can't see pass-by-reference in the examples where people try to demonstrate such. I only see pass-by-value.
In the case of variables that hold a reference to an object, the reference is the value of those variables, and therefore the reference is passed, which is then pass-by-value.
In a statement like this,
var a = {
b: "foo",
c: "bar"
};
the value of the 'a' is not the Object, but the (so far only) reference to it. In other words, the object is not in the variable a - a reference to it is. I think this is something that seems difficult for programmers who are mainly only familiar with JavaScript. But it is easy for people who know also e.g. Java, C#, and C.

Objects are always pass by reference and primitives by value. Just keep that parameter at the same address for objects.
Here's some code to illustrate what I mean (try it in a JavaScript sandbox such as https://js.do/).
Unfortunately you can't only retain the address of the parameter; you retain all the original member values as well.
a = { key: 'bevmo' };
testRetain(a);
document.write(' after function ');
document.write(a.key);
function testRetain (b)
{
document.write(' arg0 is ');
document.write(arguments[0].key);
b.key = 'passed by reference';
var retain = b; // Retaining the original address of the parameter
// Address of left set to address of right, changes address of parameter
b = {key: 'vons'}; // Right is a new object with a new address
document.write(' arg0 is ');
document.write(arguments[0].key);
// Now retrieve the original address of the parameter for pass by reference
b = retain;
document.write(' arg0 is ');
document.write(arguments[0].key);
}
Result:
arg0 is bevmo arg0 is vons arg0 is passed by reference after function passed by reference

Primitives are passed by value. But in case you only need to read the value of a primitve (and value is not known at the time when function is called) you can pass function which retrieves the value at the moment you need it.
function test(value) {
console.log('retrieve value');
console.log(value());
}
// call the function like this
var value = 1;
test(() => value);

Understanding pass by reference vs value with functions

As I understand objects are passed by reference in JavaScript (and primitives are passed by value?).
var a, b;
a = {
Foo: "Bar"
}
b = a;
a.Foo = "Other";
console.log(b.Foo); // "Other"
This worked similarly with arrays but did not work like I expect with functions:
var a, b;
a = function(){ return 20; }
b = a;
a = function(){ return 40; }
console.log(b()); // returns 20 ?
I'm confused because I thought functions are objects. Shouldn't the above example return 40?

In the first case, a.Foo = ..., You are changing the value of a property in the object, referred by both a and b. This is called mutating an object.
But in the second case, you are making a refer a new function object. Now, a and b are referring to different function objects.
That is why you are getting 20 in the second case.

First regarding the question in the title (which is actually different from what you demonstrate with the code examples):
"As I understand objects are passed by reference in JavaScript"
No, Javascript doesn't support passing parameter by reference at all. All parameters are passed by value, but the value can be a reference.
Example:
var a = { answer: 42 };
function f(x) {
alert(x.answer); // shows 42
x = { answer: 0 };
alert(x.answer); // shows 0
}
f(a);
alert(a.answer); // shows 42
As the parameter is passed by value, the variable x is separate from the variable a, so assigning a new object to x doesn't change the value of a.
Back to your code:
When you assign an object reference from a variable to another, it's the reference that is copied, not the object. You get two variables that reference the same object.
Either of the variables can be used to access members in the object, which your first code example demonstrates.
If you assign a new object to the first variable, you will replace the reference in the variable, you will not overwrite the object that it currently points to. The object is still intact, and the second variable still points to it. You get two variables that point to one object each, which your second code example demonstrates.

Further to #thefoutheye's answer, consider the following proof of your statement functions are objects:
var a, b;
a = function() { return 20; }
a.Foo = "Bar";
b = a;
a.Foo = "Other";
console.log(b.Foo); // "Other"

You are reassigning the variable a to a new function. That's not the same as changing a property value.
Try:
var a, b;
a = {Foo: function() {return 20;}};
b = a;
a.Foo = function() {return 40;};
console.log(b()); // returns 40