creating new array in jQuery with specified length - javascript

How can I create a blank array of length 4 in jQuery. I want to create the array first and later want to punch values in it.

Use Array Constructor, new Array(arrayLength)
var array = new Array(4);
Note: This array is not essentially blank, it contains undefined but it's length is 4.
If feasible,
var arr = [1, 2, 3, 4];

Related

What is the different vs array.push() and array = [...array, newItem]?

I have seen some YouTubers not use array.push() to add items to an array but rather do something like:
let item = 3
let array = [1, 2];
array = [...array, item]; // Why not use array.push()?
Is there any difference between the two methods and which one should i use?
Push: Use push when you want to add data in the existing array and don't want to cra
When you use the push method, you are adding the element to the existing one i.e not creating a new array.
The push() method adds one or more elements to the end of an array and
returns the new length of the array. - MDN
const arr = [1, 2, 3, 4, 5];
const returnValue = arr.push(6);
console.log(arr);
console.log(returnValue)
Spread Syntax: Use spread syntax if you want to add elements in new array(it can be from spread or can add more new elements also) but will create a new array []
This is called spread syntax
Spread syntax (...) allows an iterable such as an array expression or
string to be expanded in places where zero or more arguments (for
function calls) or elements (for array literals) are expected, or an
object expression to be expanded in places where zero or more
key-value pairs (for object literals) are expected.
let array = [1, 2];
let item = 3
result = [...array, item];
console.log(result === array);
In the above snippet you are creating a new array and assigning values 1, 2 then you are using spread syntax to spread it into a new array. You can also add new elements also like item whose value is 3.
array.push manipulates the existing array. The latter makes a new copy with the new value.
Array push is used to push items kn exisiting array but if we want a copy of an element of an array as a new array variable- you can use […array] operator.

How can store an array all values less than the first 3?

I have this example:
var array=[0.1,2,3,4,6,7,8,9]
What I want is to store it in a variable only those values
var newArray=[3,4,6,7,8,9]
Of course, this must be dynamic ... regardless of the number of elements to store more than the first 3 items.
You need to use JavaScript's slice:
var array = [0.1,2,3,4,6,7,8,9];
var newArray = array.slice(2);
// [3, 4, 6, 7, 8, 9]
Read more here: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/slice
Usage: arr.slice([begin[, end]])
slice does not alter. It returns a shallow copy of elements from the original array. Elements of the original array are copied into the returned array.

when push an element in sized array the array size is dynamically grow why?

i'm trying to push a string elements to sized array but when pushing an element the sized array start insert the elements in the last index of the array and grow but i want to insert the first element in the index [0] and second on [1] and so on till index [3] but i don't know why it start to insert elements after the last index and grow when add more element my code :
var users = new Array(3);
users.push(someData);
console.log(users);
console.log(users.length)
and the result like this:
[ , , , 'd', 'dd' ]
5
Avoid using new Array(count). Just create an empty array and then push to it like that:
var users = [];
users.push(someData);
Or, if you have some initial static content, this will be better:
var users = [ someData ];
The reason why I advise you to avoid new Array(...) is because it can be confusing and error-prone:
new Array(3) will create an array with 3 items: [undefined, undefined, undefined].
new Array(3, 4) will create an array containing the items 3 and 4: [3, 4].
Push adds a new element to the end of the array. You create the array with three empty entries then push somethign onto the end. Javascript does not have limited sized arrays. In your case I would guess you want to create an empty arraya nd just push onto it.
The argument you pass to the new Array call sets the initial size, not the maximum size. There is no max size. Just create the array like this:
var users = new Array()
or
var users = []
When you create an array with the constructor and specify a size, that will only set the current length of the array, it won't limit the size of the array.
Doing this:
var users = new Array(3);
does exactly the same thing as this:
var users = new Array();
users.length = 3;
Growing the array by setting the length doesn't actually put anything in the array, it will only set the length property.
The push method looks at the length property when it adds an item, so doing this:
users.push(someData);
does exactly the same thing as:
users[users.length] = someData;
If you want your array to be empty from the start, just create it without setting the length:
var users = new Array();
You can also use the array literal syntax to create an empty array:
var users = [];
Arrays doesn't support limiting the size, so if you want a collection with a limited size, you have to create it yourself (or find one that someone else has created).
array.push() method of array insert element from last index of array. use array.unshift() method to push from starting index of array
Because in javascript "var users = new Array(3);" means creation of array with 3 undefined elements.
For basic Array initialization details you can refer to : http://www.w3schools.com/js/js_arrays.asp

How do array sizes work in Javascript

In JavaScript, if you set an array to be of size 5 ( var foo = new Array(5); ), is this just an initial size? Can you expand the number of elements after it is created. Is it possible to do something like this as well - arr = new Array() and then just assign elements one by one? Thanks in advance :-)
Yes it is just an initial size, and it is not required. If you don't use a single number, you can immediately populate.
It is also more common to use the simpler [] syntax.
var arr = ['something', 34, 'hello'];
You can set (or replace) a specific index by using brackets:
arr[0] = "I'm here replacing whatever your first item was";
You can add to the end by using push:
arr.push('add me');
There may be faster performance in some browsers if you do it like this instead:
arr[arr.length] = 'add me';
You can add something at any index.
You can remove an item completely using splice:
arr.splice(0, 1); // remove first item in the array (start at index 0, with 1 being removed)
When you give a new array an explicit size in javascript (using new Array(5)), you populate each index with value of undefined. It is generally considered better practice to instantiate using the array literal [] expression:
var arr = [];
Then, you can push new elements using push()
var arr = [];
arr.push('first value'):
arr.push('second value'):
Check out the MDC Array documentation for more info: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array
Theres a few ways to declare and put values in an array.
First like what you want to do,
var myarr = new Array();
myarr[0] = 'element1';
myarr[1] = 'element2';
myarr[2] = 'element3';
Second way is to define them
var myarr =new Array("element1","element2","element3");
and third is similar to the second
var myarr =["element1","element2","element3"];
You can also check out https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array for a little more information about using the arrays as well. You could use push and pop if you wanted to as well.
If you use jquery or mootools they also have built-in functions to perform on arrays,
http://api.jquery.com/jQuery.each/ for instance.
Have a look at http://www.w3schools.com/js/js_obj_array.asp
var myCars=new Array(); // regular array (add an optional integer
myCars[0]="Saab"; // argument to control array's size)
myCars[1]="Volvo";
myCars[2]="BMW";
Check the documentation for Array, but the simple answer to your question is yes.
var arr5 = new Array(1, 2, 3, 4, 5); // an array with initial 5 elements
var arr = new Array(); // an array without initial
You can also use array literals:
var arr5 = [1, 2, 3, 4, 5];
var arr = [];
Arrays are dynamic in JavaScript. You don't have to initialize them with a certain length. In fact you should use the literal notation [] because of the Array constructor's ambiguity:
If you pass only one parameter to Array, it will set the array length to this parameter. If you pass more than one parameter, these elements are added to the array.
How is the size of the array determined?
The size of an array is the highest index + 1. This can be quite confusing. Consider this:
var arr = [];
arr[41] = 'The answer?';
console.log(arr); // [undefined, undefined, ..., 'The answer?']
console.log(arr.length) // 42
You can even set the length yourself by assigning a number to .length:
arr.length = 99;
If you now add a new element using arr.push(), it will get the index 100 and the length will increase. Whenever you add an element to the array via an index, it is tested whether arr.length is smaller than the index and updated accordingly. But it does not get smaller.
So in fact what var arr = new Array(5) is doing is setting the length of the array to 5. Nothing else.
For more information about creating and populating arrays, I suggest to read about it in the MDC JavaScript Guide.

Array construction and negative numbers

I was implementing a routing algorithm in javascript, but when I assign a negative one variable in the array gives me this error: invalid array length.
var node = new Array()
node[0] = new Array(6,7)
node[1] = new Array(5,-4,8)
node[2] = new Array(-2) //Here, invalid array length
I do not know how to resolve this error.
If you are trying to initialize an array that contains only a negative number, use the literal syntax:
var a = [-2];
The problem with the Array constructor is that when it is invoked only with only one argument, this number is used as the length of the new array, e.g.:
var b = new Array(5);
b.length; // 5
I recommend you to stick with the literal syntax to avoid those ambiguities.
Don't declare arrays that way!
var node = [6, 7];
It's because one integer argument sets the size of new Array.
The array constructor documentation shows the following
var arr1 = new Array(arrayLength);
var arr2 = new Array(element0, element1, ..., elementN);
So, if you use only one parameter, it creates an array of arrayLength; otherwise, if you use more than one, it will populate the array with those values.
However, as others have pointed out, it is best use the literal notation *
var node = [
[6, 7],
[5, -4 8],
[-2]
];
* Array literal notation is slightly slightly faster than new Array(), but that's a micro optimization and not very important in most cases.

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