This question already has answers here:
Why can I add named properties to an array as if it were an object?
(8 answers)
Closed 3 months ago.
I noticed that Javascript allows you to add a name/value (or key/value) pair inside already-made regular arrays (not objects). Why is this allowed? Is there an intention behind this? Is this another data type? I understand this is neither an object nor an associative array (which is really an object). I know arrays are objects too, and in this case JS is allowing me to use a string instead of a number as a key, but this would put the data in an odd category which is neither a regular array nor an object.
array1 = ['a', 'b', 'c']
array1.propertyName = 'property value'
console.log(array1); // [ 'a', 'b', 'c', propertyName: 'property value' ]
This question was already addressed in How to create an associative array in JavaScript literal notation, but no real answer was given rather than "You need to use an object". I already use objects in JS, I am just curious about this behaviour in arrays.
Thank you!
You can assume pair is just a value. Value inside a value is normal unless if you are using static typed languange.
Why is that allowed? computer take any argument u pass as a value despite it having a pair, array or an object. Doesn't matter.
Related
This question already has answers here:
How to compare arrays in JavaScript?
(61 answers)
Why are two identical objects not equal to each other?
(9 answers)
Closed 2 years ago.
I started with:
"1:2".split(':') == ["1","2"];
// false
Then tried:
[1,2] == [1,2];
// false
and ultimately:
[] == [];
// false
I've since found that:
"1:2".split(':').toString() == [1,2].toString();
// true
So I've solved my initial issue (kind of) but why can't arrays match each other?
Javascript arrays are objects and you can't simply use the equality operator == to understand if the content of those objects is the same. The equality operator will only test if two object are actually exactly the same instance (e.g. myObjVariable==myObjVariable, works for null and undefined too).
If you need to check if two array are equals i'd recommend to just traverse both arrays and verify that all the elements have the same value (and that the two array have the same length).
Regarding custom objects equality i'd build instead a specific equals function and i'd add it to the prototype of your class.
Considering that in the end you converted both arrays to a String and tested equality of the resulting strings, you could one day consider using a similar but more generic technique you'll find described in more than a few places:
JSON.stringify(OBJ1) === JSON.stringify(OBJ2)
Well, don't.
While this could work if the order of the properties will always the same for those object instances, this leaves the door open for extremely nasty bugs that could be hard to track down. Always favor a more explicit approach and just write a clean and readable function that will test for equality checking all the required fields.
The == operator for Objects in Javascript only checks to see if the objects are the same actual object reference, not if they are two separate object that contain the same contents. There is no built in operator for checking if they contain the same contents. You would have to write a function to do that sort of comparison yourself.
Your string conversion is one way of comparing two arrays as long as the array elements only contain primitive values (not other objects). If the array elements could contain other elements, then you would have to make sure those objects were themselves converted into representative strings too.
And, converting to a string would not discern between an array element that contains "4" versus one that contains 4 since both convert to "4" in the string representation.
FYI, you can read in this answer about records and tuples, two new data types coming to future Javascript that will allow comparing an immutable version of arrays and objects by value instead of by only comparing to see if they are the same object.
Equality for objects will tell you if the two objects are the same one.
var a = [];
var b = a;
a === b; // True, a and b refer to the same object
[] === []; // False, two separate objects
You will have to loop through the arrays to see if they have the same elements.
See: How to check if two arrays are equal with JavaScript?
In javascript each [] is an instance of window.Array class. So you are basically trying to compare two different objects. Since array's can have any no. and any type of elements including Objects and Custom Objects and those nested arrays can again have numerous properties and arrays and so on.
It becomes ambiguous when it comes to comparison, you will never be sure what do you want to do with those objects and nested properties. So what you are trying to achieve by comparing can be done in so many other ways. You just have to figure out the right way for your case.
One way is to make your own Array checking function:
How to compare arrays in JavaScript?!
Another way is to convert the Array to a String with .join(), and compare the strings. Then convert them back into Arrays with .split().
If I relate this problem with that in Python:
Input:
a="1 2 3 4"
case I:
a=input.split(' ')
output: ['1', '2', '3', '4']
case II:
a=map(int,input.split(' '))
output: [1, 2, 3, 4]
So, the fault is that of type, as it could come-out aa 'true' for:
"1:2".split(':').toString() == [1,2].toString(); //true
This question already has answers here:
JavaScript property access: dot notation vs. brackets?
(17 answers)
Are Javascript arrays primitives? Strings? Objects?
(7 answers)
Closed 2 years ago.
I'm looking for an elegant way of understanding JavaScript array and objects.
I came to an anomaly in which I got stuck.
Since in PHP or other languages, when we make an array e.g
$a = [
admin => 1,
staff => 2
];
so if we want to access its element we can do so by for e.g $a[admin] and we will get 1.
similarly if its an object e.g
$a = (object) [];
$a->sadd = 'sas';
we can access it with arrow
$a->sadd
and if we try to access object elements in the style of array i.e like this $a['sadd'] it will throw error that you can not use it as array style.
But I was surprised by the anomaly in JavaScript.
I have observed that in JavaScript no matter what I am making, an array or object, the elements of both can be accessed via dot or via array style and i found no difference in there accessing style.
for e.g
var a = {sadd : 1}
I can access its element via a['sadd'] or a.sadd both will give 1
So I am confused by this anomaly and wondering whether array and object both datatypes are considered same in JavaScript?
An array is indeed an object.
Javascript is a dynamic language and accepts mixed types of entities. Also while accessing, dot notation seems to be more clearer (atleast imo) and is preferred. Bracket notation is used for dyanamic keys.
The difference between array and objects boils down to their usecase:
Array -> Contiguous block of memory
Object -> key-value pair (like a dictionary)
Your php example is actually creating what we'd call an object in JS, not an array. In JS an array is a list of items, which you can find items by array[i], or by looping.
An object is a collection of fields, which you can go into by object.fieldName or object[fieldName].
This can be confusing in JS though, because theoretically everything is an "object", including arrays, due to the way things are handled lower down..
I would recommend following along with the https://justjavascript.com/ course for good mental models on how objects work in JS.
This question already has answers here:
How to compare arrays in JavaScript?
(55 answers)
Why are two identical objects not equal to each other?
(9 answers)
Closed 2 years ago.
I started with:
"1:2".split(':') == ["1","2"];
// false
Then tried:
[1,2] == [1,2];
// false
and ultimately:
[] == [];
// false
I've since found that:
"1:2".split(':').toString() == [1,2].toString();
// true
So I've solved my initial issue (kind of) but why can't arrays match each other?
Javascript arrays are objects and you can't simply use the equality operator == to understand if the content of those objects is the same. The equality operator will only test if two object are actually exactly the same instance (e.g. myObjVariable==myObjVariable, works for null and undefined too).
If you need to check if two array are equals i'd recommend to just traverse both arrays and verify that all the elements have the same value (and that the two array have the same length).
Regarding custom objects equality i'd build instead a specific equals function and i'd add it to the prototype of your class.
Considering that in the end you converted both arrays to a String and tested equality of the resulting strings, you could one day consider using a similar but more generic technique you'll find described in more than a few places:
JSON.stringify(OBJ1) === JSON.stringify(OBJ2)
Well, don't.
While this could work if the order of the properties will always the same for those object instances, this leaves the door open for extremely nasty bugs that could be hard to track down. Always favor a more explicit approach and just write a clean and readable function that will test for equality checking all the required fields.
The == operator for Objects in Javascript only checks to see if the objects are the same actual object reference, not if they are two separate object that contain the same contents. There is no built in operator for checking if they contain the same contents. You would have to write a function to do that sort of comparison yourself.
Your string conversion is one way of comparing two arrays as long as the array elements only contain primitive values (not other objects). If the array elements could contain other elements, then you would have to make sure those objects were themselves converted into representative strings too.
And, converting to a string would not discern between an array element that contains "4" versus one that contains 4 since both convert to "4" in the string representation.
FYI, you can read in this answer about records and tuples, two new data types coming to future Javascript that will allow comparing an immutable version of arrays and objects by value instead of by only comparing to see if they are the same object.
Equality for objects will tell you if the two objects are the same one.
var a = [];
var b = a;
a === b; // True, a and b refer to the same object
[] === []; // False, two separate objects
You will have to loop through the arrays to see if they have the same elements.
See: How to check if two arrays are equal with JavaScript?
In javascript each [] is an instance of window.Array class. So you are basically trying to compare two different objects. Since array's can have any no. and any type of elements including Objects and Custom Objects and those nested arrays can again have numerous properties and arrays and so on.
It becomes ambiguous when it comes to comparison, you will never be sure what do you want to do with those objects and nested properties. So what you are trying to achieve by comparing can be done in so many other ways. You just have to figure out the right way for your case.
One way is to make your own Array checking function:
How to compare arrays in JavaScript?!
Another way is to convert the Array to a String with .join(), and compare the strings. Then convert them back into Arrays with .split().
If I relate this problem with that in Python:
Input:
a="1 2 3 4"
case I:
a=input.split(' ')
output: ['1', '2', '3', '4']
case II:
a=map(int,input.split(' '))
output: [1, 2, 3, 4]
So, the fault is that of type, as it could come-out aa 'true' for:
"1:2".split(':').toString() == [1,2].toString(); //true
This question already has answers here:
Keys in Javascript objects can only be strings?
(8 answers)
Closed 4 years ago.
const obj = {
15: 100
};
for(let key in obj)
console.log(key, typeof(key), typeof(+key))
The result is 15 string number. I'm trying to iterate over object values and put some of them into Map object but types compatibility seems unable to achieve. Am I doing something wrong here or object keys are always strings?
Object.keys(obj)
also returns ["15"]
Object keys are always strings. You can see more about it here:
Property names must be strings. This means that non-string objects cannot be used as keys in the object. Any non-string object, including a number, is typecasted into a string via the toString method.
For you to be able to achieve what you want you will need to cast the keys back to integers.
This question already has answers here:
Does JavaScript guarantee object property order?
(13 answers)
Closed 4 years ago.
I was trying out different objects and I noticed a little strange behavior in Javascript JSON objects. Would be great if someone can elaborate.
Example:
var bat={ 3:"FA", 7:"WER", 1:"JWRT",d:"EWR",a:"bA",8:"ADB"};
For this Json object, when it has integers as keys (works even if you enclose the integers in double quotes), and when you try to get the keys using Object.keys() or by simply printing bat the following order is seen.
{1: "JWRT", 3: "FA", 7: "WER", 8: "ADB", d: "EWR", a: "bA"}
The ordering does not happen automatically for Json Objects when the keys are strings (even if it is a Json Object containing string keys exclusively, or has integer and string keys interspersed).
In the case where there are string and integer keys in the same Json Object the integers get to the beginning of the array of keys.
Is there some reason for this behavior? Thanks in advance...
The order of object keys is never guaranteed. Even when you use numeric keys, they are internally strings.
From the ECMA Specification:
4.3.3 Object
An object is a member of the type Object. It is an unordered collection of properties each of which contains a primitive
value, object, or function. A function stored in a property of an
object is called a method.