I just started learning about Dart.
Before Dart, I worked with Javascript and have some experience.
Now, as I was going through the documentation from Tutorial Point. They have mentioned something like this
All variables in dart store a reference to the value rather than
containing the value. The variable called name contains a reference to
a String object with a value of “Smith”.
In Javascript, I guess arrays and objects are reference types.
Meaning, If we do something like this
[Update:] This code snippet is incorrect
let a = ["apple", "orange"]
let b = a
a = ["banana"]
console.log(b) //["banana"]
but that is probably only for objects and arrays in JS (and not for const and let)
let a = 5
let b = a
a = 7
console.log(b) //5
From the quote,
All variables in Dart store a reference to the value
[Question:] Does this mean that even things like int, string.. and every variable we create in Dart are references? and the equivalence of the above code will print 7 in Dart or I am getting something wrong (in general)?
let a = 5
let b = a
a = 7
console.log(b) //7
Everything is an Object in Dart. Some objects are mutable - that is they can be modified, and some are immutable, that is they will always be the same value.
When you assign with var b = a; both b and a will reference the same Object, but there is no further association between the names b and a. If you mutate that Object by calling methods on it or assigning to fields on it (things like List.add for example) then you will be able to observe the mutated Object through either name b or a. If you assign to a then the variable b is unaffected. This is true in javascript as well.
The reason some types, like numbers or Strings, appear special is that they cannot be mutated, so the only way to "change" a is to reassign it, which won't impact b. Other types, like collections, are mutable and so a.add("Banana") would be a mutation visible through either variable referencing that list.
For example, with assignment:
var a = ['Apple', 'Orange'];
var b = a;
a = ['Banana']; // Assignment, no impact to b
print(a); // [Banana]
print(b); // [Apple, Orange]
With mutation:
var a = ['Apple', 'Orange'];
var b = a;
a.clear(); // Mutation, the _list instance_ is changed
a.add('Banana') // Another mutation
print(a); // [Banana]
print(b); // [Banana]
Related
I know that ES6 is not standardized yet, but a lot of browsers currently support const keyword in JS.
In spec, it is written that:
The value of a constant cannot change through re-assignment, and a
constant cannot be re-declared. Because of this, although it is
possible to declare a constant without initializing it, it would be
useless to do so.
and when I do something like this:
const xxx = 6;
xxx = 999;
xxx++;
const yyy = [];
yyy = 'string';
yyy = [15, 'a'];
I see that everything is ok: xxx is still 6 and yyy is [].
But if I do yyy.push(6); yyy.push(1); , my constant array has been changed. Right now it is [6, 1] and by the way I still can not change it with yyy = 1;.
Is this a bug, or am I missing something? I tried it in the latest chrome and FF29
The documentation states:
...constant cannot change through re-assignment
...constant cannot be re-declared
When you're adding to an array or object you're not re-assigning or re-declaring the constant, it's already declared and assigned, you're just adding to the "list" that the constant points to.
So this works fine:
const x = {};
x.foo = 'bar';
console.log(x); // {foo : 'bar'}
x.foo = 'bar2';
console.log(x); // {foo : 'bar2'}
and this:
const y = [];
y.push('foo');
console.log(y); // ['foo']
y.unshift("foo2");
console.log(y); // ['foo2', 'foo']
y.pop();
console.log(y); // ['foo2']
but neither of these:
const x = {};
x = {foo: 'bar'}; // error - re-assigning
const y = ['foo'];
const y = ['bar']; // error - re-declaring
const foo = 'bar';
foo = 'bar2'; // error - can not re-assign
var foo = 'bar3'; // error - already declared
function foo() {}; // error - already declared
This happens because your constant is actually storing a reference to the array. When you join something into your array you are not modifying your constant value, but the array it points to. The same would happen if you assigned an object to a constant and tried to modify any property of it.
If you want to freeze an array or object so it can't be modified, you can use the Object.freeze method, which is already part of ECMAScript 5.
const x = Object.freeze(['a'])
x.push('b')
console.log(x) // ["a"]
Came through this article while searching on why I was able to update an Object even after defining it as const. So the point here is that it is not the Object directly but the attributes it contains which can be updated.
For example, my Object looks like:
const number = {
id:5,
name:'Bob'
};
The above answers correctly pointed out that it's the Object which is const and not its attribute. Hence, I will be able to update the id or name by doing:
number.name = 'John';
But, I will not be able to update the Object itself like:
number = {
id:5,
name:'John'
};
TypeError: Assignment to constant variable.
This is consistent behavior with every programming language I can think of.
Consider C - arrays are just glorified pointers. A constant array only means that the value of the pointer will not change - but in fact the data contained at that address is free to.
In javascript, you are allowed to call methods of constant objects (of course - otherwise constant objects would not serve much purpose!) These methods might have the side effect of modifying the object. Since arrays in javascript are objects, this behavior applies to them as well.
All you are assured of is that the constant will always point to the same object. The properties of the object itself are free to change.
The keyword const is a little misleading.
It does not define a constant value. It defines a constant reference to a value.
Because of this you can NOT:
Reassign a constant value
Reassign a constant array
Reassign a constant object
But you CAN:
Change a constant array
Change a constant object
I think this would give you more clarity on the issue : https://codeburst.io/explaining-value-vs-reference-in-javascript-647a975e12a0 .
Basically it boils down to the const always pointing to the same address in memory. You can change the value stored in that address but cannot change the address the const is pointing too.
The definition of const you mentioned will hold true when the const is pointing to an address that holds a primitive value . This is because you cannot assign a value to this const without changing its address (because this is how assigning primitive values work) and changing the address of a const is not allowed.
Where as if the const is pointing to non-primitive value , it is possible to edit the value of the address.
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in the case where the content is an object, this means the object's contents (e.g., its parameters) can be altered.
In addition, an also important note:
Global constants do not become properties of the window object ...
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const
The value of a const can't be changed through reassignment, and it can't be redeclared.
const testData = { name:"Sandeep",lastName:"Mukherjee",company:"XYZ"}
First case
testData = {name:"hello"}
console.log(testData);//throws an Error:Assignment to constant variable
Here we are reassigning testData again
Second case
const testData = {name:"Sandeep",lastName:"Mukherjee",company:"ABC"}
console.log(testData); //throws an Error: Identifier 'testData' has already been declared
Here we are redeclaring testData again
When a variable is declared using const it means it points to some memory location the
behaviour of const is we can manipulate the value stored in that memory location but not
the memory location,when we reassign/redeclare the const variable it
does not allow to change the memory location
We can change the value of a specific key
testData.company = "Google"
console.log(testData);
//{ name: 'Sandeep', lastName: 'Mukherjee', company: 'Google' }
We can add any new key value pair to it
testData.homeTown = "NewYork"
console.log(testData)
//{name: 'Sandeep',lastName:'Mukherjee',company:'Google',homeTown: 'NewYork'}
Because in const you can change the values of an object, so the object does not actually store the assignment data but instead, it points to it.
so there is a difference between primitives and objects in Javascript.
const variable stores the address (memory address such as 0xFF2DFC) that is constant.
The constant is NOT the content at the memory.
constant is memory address ONLY
Thank you for reading.
const MY_OBJECT = {'key': 'value'};
// Attempting to overwrite the object throws an error
// Uncaught TypeError: Assignment to constant variable.
MY_OBJECT = {'OTHER_KEY': 'value'};
// However, object keys are not protected,
// so the following statement is executed without problem
MY_OBJECT.key = 'otherValue';
// Use Object.freeze() to make object immutable
// The same applies to arrays
const MY_ARRAY = [];
// It's possible to push items into the array
MY_ARRAY.push('A'); // ["A"]
// However, assigning a new array to the variable throws an error
// Uncaught TypeError: Assignment to constant variable.
MY_ARRAY = ['B'];
In your constant is saved not the object, but link to the object.
You can't change this link, because it is constant.
But object you can change.
So I was playing around the other day just to see exactly how mass assignment works in JavaScript.
First I tried this example in the console:
a = b = {};
a.foo = 'bar';
console.log(b.foo);
The result was "bar" being displayed in an alert. That is fair enough, a and b are really just aliases to the same object. Then I thought, how could I make this example simpler.
a = b = 'foo';
a = 'bar';
console.log(b);
That is pretty much the same thing, isn't it? Well this time, it returns foo not bar as I would expect from the behaviour of the first example.
Why does this happen?
N.B. This example could be simplified even more with the following code:
a = {};
b = a;
a.foo = 'bar';
console.log(b.foo);
a = 'foo';
b = a;
a = 'bar';
console.log(b);
(I suspect that JavaScript treats primitives such as strings and integers differently to hashes. Hashes return a pointer while "core" primitives return a copy of themselves)
In the first example, you are setting a property of an existing object. In the second example, you are assigning a brand new object.
a = b = {};
a and b are now pointers to the same object. So when you do:
a.foo = 'bar';
It sets b.foo as well since a and b point to the same object.
However!
If you do this instead:
a = 'bar';
you are saying that a points to a different object now. This has no effect on what a pointed to before.
In JavaScript, assigning a variable and assigning a property are 2 different operations. It's best to think of variables as pointers to objects, and when you assign directly to a variable, you are not modifying any objects, merely repointing your variable to a different object.
But assigning a property, like a.foo, will modify the object that a points to. This, of course, also modifies all other references that point to this object simply because they all point to the same object.
Your question has already been satisfyingly answered by Squeegy - it has nothing to do with objects vs. primitives, but with reassignment of variables vs. setting properties in the same referenced object.
There seems to be a lot of confusion about JavaScript types in the answers and comments, so here's a small introduction to JavaScript's type system:
In JavaScript, there are two fundamentally different kinds of values: primitives and objects (and there is no thing like a 'hash').
Strings, numbers and booleans as well as null and undefined are primitives, objects are everything which can have properties. Even arrays and functions are regular objects and therefore can hold arbitrary properties. They just differ in the internal [[Class]] property (functions additionally have a property called [[Call]] and [[Construct]], but hey, that's details).
The reason that primitive values may behave like objects is because of autoboxing, but the primitives themselves can't hold any properties.
Here is an example:
var a = 'quux';
a.foo = 'bar';
document.writeln(a.foo);
This will output undefined: a holds a primitive value, which gets promoted to an object when assigning the property foo. But this new object is immediately discarded, so the value of foo is lost.
Think of it like this:
var a = 'quux';
new String(a).foo = 'bar'; // we never save this new object anywhere!
document.writeln(new String(a).foo); // a completly new object gets created
You're more or less correct except that what you're referring to as a "hash" is actually just shorthand syntax for an Object.
In the first example, a and b both refer to the same object. In the second example, you change a to refer to something else.
here is my version of the answer:
obj = {a:"hello",b:"goodbye"}
x = obj
x.a = "bonjour"
// now obj.a is equal to "bonjour"
// because x has the same reference in memory as obj
// but if I write:
x = {}
x.a = obj.a
x.b = obj.b
x.a = "bonjour"
// now x = {a:"bonjour", b:"goodbye"} and obj = {a:"hello", b:"goodbye"}
// because x points to another place in the memory
You are setting a to point to a new string object, while b keeps pointing to the old string object.
In the first case you change some property of the object contained in the variable, in the second case you assign a new value to the variable. That are fundamentally different things. The variables a and b are not somehow magically linked by the first assignment, they just contain the same object. That's also the case in the second example, until you assign a new value to the b variable.
The difference is between simple types and objects.
Anything that's an object (like an array or a function) is passed by reference.
Anything that's a simple type (like a string or a number) is copied.
I always have a copyArray function handy so I can be sure I'm not creating a bunch of aliases to the same array.
I would like to understand well something i observe more and more.
In some circonstances, different instances from a same model change their attributes the same way (if i have 2 UserModel A and B, if i change A, B will be affected the same way).
I observed some practical cases:
When i send an instance of a model to a view constructor, they are linked, if i change the model in my view, the outside one will be affected the same way. I guess sometime we just send a pointer and not a copy of the instance to the view.
More specific with some code;
A = new UserModel();
B = new UserModel();
var Data = A.get('info'); //where info = {some: "thing"};
Data.some = 'other';
B.set('info', Data); //A.get('info') == B.get('info')
Because i got the object info and not only the attributes separately (i tested it and there is no repercution between the values this way).
So my question is, are we always using pointers with objects in javascript ? Is it specific to backbone ? I would like to understand what is behind this behavior.
Thanks.
Objects and Arrays are passed or assigned as references in javascript, not copies. If you want a copy of an object or an array, you have to explicity make a copy.
Simpler types such as numbers, boolean are copied when assigned or passed.
Strings are a bit of special case. They are passed as references, but since strings are immutable (can't be changed), you can't really use a string reference for anything because any attempt to modify the string creates a new string.
A couple examples:
// arrays assigned by reference
var a = [1,2,3];
var b = a;
a[0] = 0;
alert(b[0]); // alerts 0 because b and a are the same array
// objects assigned by reference
var c = {greeting: "hello"};
var d = c;
c.greeting = "bye";
alert(d.greeting); // alerts "bye" because c and d are the same object
// numbers assigned as copies
var e = 3.414;
var f = e;
e = 999;
alert(f); // alerts 3.414 because f is its own copy of the original number
// make a copy of an array
var g = [1,2,3];
var h = g.slice(0); // h is now a copy
h[0] = 9;
alert(g); // shows [1,2,3]
alert(h); // shows [9,2,3]
The same is true for passing arguments to a function or returning values from a function. Unless an explicit copy is created, arrays and objects are passed or returned by reference.
A shallow copy of an array can be made with the .slice() method.
var arr1 = [1,2,3];
var arr2 = arr1.slice(0); // make independent copy of first array
A shallow copy of an object can be made by copying each property from the original object to a new object.
Deep copies involve testing the type of each item being copied and recursing on the object if it is an object or array so nested objects and arrays are copied too.
Why can I do this:
Array.prototype.foo = function() {
this.splice(0, this.length);
return this.concat([1,2,3]);
}
But I can't do this:
Array.prototype.foo = function() {
return this = [1,2,3];
}
Both functions destroy the value of this and change it to [1,2,3] but the second one throws the following error: Uncaught ReferenceError: Invalid left-hand side in assignment
I suspect it's because allowing assignment means I could potentially change the array to something else (like a string), but I'm hoping someone out there knows for sure and/or has a more detailed explanation.
You're confusing objects with references.
An array is an object, when you use a literal like [1,2,3] you're making a new array.
A variable name like this or a is a reference to an object. If it helps, imagine an object as a person, and the reference as their nickname. You can have more than one reference to the same object, for example:
var a = [1,2];
var b = a;
b.push(3);
alert(a.length); // Gives "3"
Imagine if you had a friend named Sarah. You also sometimes call her "Ace". If Sarah gets a haircut one day, Ace has a haircut too, because "Sarah" and "Ace" are both different names for the same person.
If you use a mutating array method like a.push or a.splice (concat however is not one!), you change the existing Array object, and a still refers to the same object. This is like getting a hair cut - you may look different, but you're still the same person.
When you assign a reference to a new value, with a = [1,2,3], you're creating a new array, and changing a to refer to it. This is like finding a new friend with different hair, and deciding to call her Ace instead.
Now this is a special name generated by Javascript. It's not a given name like Sarah, but more of a title, like "mother". Your mother can get a new haircut, but you can't get a new mother. Likewise, you can't change what this refers to from inside a function.
It's not permitted to assign a value to this within a function. Suppose that you could do this, and your code looked something like:
Array.prototype.foo = function() {
return this = [1, 2, 3];
}
var a = ["beans", "rice"];
a.foo();
// a now points to an object containing [1, 2, 3]
Now, what if you did this:
var a = ["beans", "rice"];
var b = a; // b refers to the same object as a
b.foo();
// what does b refer to now? how about a?
The act of calling a function .foo() on an object should not change the identity of the object. This would be very confusing for the caller if b suddenly started referring to a different object than a simply because some method was called.
You are not allowed to re-assign this. Because the this value associated with an execution context is immutable.
Is it possible to change the value of this? Not by assignment, you cannot assign a value to this
Your case:
Array.prototype.foo = function() {
this.splice(0, this.length);
return this.concat([1,2,3]);
}
But I can't do this:
Array.prototype.foo = function() {
return this = [1,2,3];
}
When you modify the prototype adding method foo(), the first part makes the array become empty, equals to reassign its instance.
For example to var a = []
Then in the return value:
return this.concat([1,2,3]);
Remember that concat does not destroys an array, it creates a new array and returns it
Given a = ['x'] and b = ['y'], c = a.concat(b) results in c = ['x','y'], and a and b remain the same as they were
Now since you are doing this inside the array
a=['a','b','c']
then you call a.foo();, inside a will get equal a=[], with this.splice(0, this.length); then will concatenate [ ] with [1,2,3]
So if I say just a.foo(), in fact nothing happens outside if I do not assign the result to something, just a remains empty.
If a is assigned to something
c = a.foo()
then c will be [1,2,3], and a still empty [ ]
In the second part, this = [1,2,3]
It is possible to change the value of this?, not by assignment, you cannot assign a value to this
So I was playing around the other day just to see exactly how mass assignment works in JavaScript.
First I tried this example in the console:
a = b = {};
a.foo = 'bar';
console.log(b.foo);
The result was "bar" being displayed in an alert. That is fair enough, a and b are really just aliases to the same object. Then I thought, how could I make this example simpler.
a = b = 'foo';
a = 'bar';
console.log(b);
That is pretty much the same thing, isn't it? Well this time, it returns foo not bar as I would expect from the behaviour of the first example.
Why does this happen?
N.B. This example could be simplified even more with the following code:
a = {};
b = a;
a.foo = 'bar';
console.log(b.foo);
a = 'foo';
b = a;
a = 'bar';
console.log(b);
(I suspect that JavaScript treats primitives such as strings and integers differently to hashes. Hashes return a pointer while "core" primitives return a copy of themselves)
In the first example, you are setting a property of an existing object. In the second example, you are assigning a brand new object.
a = b = {};
a and b are now pointers to the same object. So when you do:
a.foo = 'bar';
It sets b.foo as well since a and b point to the same object.
However!
If you do this instead:
a = 'bar';
you are saying that a points to a different object now. This has no effect on what a pointed to before.
In JavaScript, assigning a variable and assigning a property are 2 different operations. It's best to think of variables as pointers to objects, and when you assign directly to a variable, you are not modifying any objects, merely repointing your variable to a different object.
But assigning a property, like a.foo, will modify the object that a points to. This, of course, also modifies all other references that point to this object simply because they all point to the same object.
Your question has already been satisfyingly answered by Squeegy - it has nothing to do with objects vs. primitives, but with reassignment of variables vs. setting properties in the same referenced object.
There seems to be a lot of confusion about JavaScript types in the answers and comments, so here's a small introduction to JavaScript's type system:
In JavaScript, there are two fundamentally different kinds of values: primitives and objects (and there is no thing like a 'hash').
Strings, numbers and booleans as well as null and undefined are primitives, objects are everything which can have properties. Even arrays and functions are regular objects and therefore can hold arbitrary properties. They just differ in the internal [[Class]] property (functions additionally have a property called [[Call]] and [[Construct]], but hey, that's details).
The reason that primitive values may behave like objects is because of autoboxing, but the primitives themselves can't hold any properties.
Here is an example:
var a = 'quux';
a.foo = 'bar';
document.writeln(a.foo);
This will output undefined: a holds a primitive value, which gets promoted to an object when assigning the property foo. But this new object is immediately discarded, so the value of foo is lost.
Think of it like this:
var a = 'quux';
new String(a).foo = 'bar'; // we never save this new object anywhere!
document.writeln(new String(a).foo); // a completly new object gets created
You're more or less correct except that what you're referring to as a "hash" is actually just shorthand syntax for an Object.
In the first example, a and b both refer to the same object. In the second example, you change a to refer to something else.
here is my version of the answer:
obj = {a:"hello",b:"goodbye"}
x = obj
x.a = "bonjour"
// now obj.a is equal to "bonjour"
// because x has the same reference in memory as obj
// but if I write:
x = {}
x.a = obj.a
x.b = obj.b
x.a = "bonjour"
// now x = {a:"bonjour", b:"goodbye"} and obj = {a:"hello", b:"goodbye"}
// because x points to another place in the memory
You are setting a to point to a new string object, while b keeps pointing to the old string object.
In the first case you change some property of the object contained in the variable, in the second case you assign a new value to the variable. That are fundamentally different things. The variables a and b are not somehow magically linked by the first assignment, they just contain the same object. That's also the case in the second example, until you assign a new value to the b variable.
The difference is between simple types and objects.
Anything that's an object (like an array or a function) is passed by reference.
Anything that's a simple type (like a string or a number) is copied.
I always have a copyArray function handy so I can be sure I'm not creating a bunch of aliases to the same array.