Develop Reference

push value in map of arrays at a specific postition [duplicate]

This question already has answers here:
How can I create a two dimensional array in JavaScript?
(56 answers)
Closed 5 years ago.
I am creating a map of array using this:
var m = new Map(Array(40).fill(new Array()).entries());
Now, I want to push values in those array. But when I do this:
m.get(1).push(10)
The value 10 gets pushed into all the arrays instead of the one at 1st position.

You could take another pattern to build independent arrays.
var m = new Map(Array.from({ length: 40 }, _=> []).entries());
m.get(1).push(10);
console.log([...m]);

fill gets single array an uses it to fill all rows of the given array, it doesn't create a new array for each row. This means that your single array reference is shared between all rows. Because array is a reference type, you use the single reference to manipulate it, so the actual object is changed. You can check this by comparing the references of each row.
const arr = new Array(2).fill(new Array());
console.log(arr[0] === arr[1]);
For creating separate arrays, you can see #Nina's answer above

Splitting an array into more than one array [duplicate]

This question already has answers here:
Split array into chunks
(73 answers)
Closed 7 years ago.
I have a dynamic created array:
var myArray = ['content_1','content_2','content_3','content_4'];
Other times my array could have more items in as such:
var myArray = ['content_4','content_4','content_new','content_new','content_new','content_new','content_new'];
My if statement looks like:
if (myArray.length > 8) { //then do something }
If myArray has more then 8 items in the array, create a new one, each array can only hold 8 items though. So my first array could have 40 items in, 20 items in, or more... I never know, how could I split these into dynamic arrays, is there a way I can do this?
After the 8th item in myArray push those 8 into a new array, then the next 8 (if applicable) and so forth

You'd either have to store your arrays in another array or create a new variable for each array. Personally I'd use an array to store all the arrays so you can still look through all of them when you need to.
var arrays = [['item', 'item'], ['item','item']];
if(arrays[arrays.length].length === 8) {
arrays.push(['new array']);
}
Obviously this won't work for every scenario but it would have to be something similar.

Delete random objects from array [duplicate]

This question already has answers here:
Picking 2 random elements from array
(10 answers)
Closed 7 years ago.
I have an array such as:
array=['a','b','c','d','e','f'];
I want to delete a random 2 elements. How can I do this?

To get two unique items from the array, and if you don't mind mutating the original array, you can use splice() to remove the selected item from the array so it won't be picked when you run it a second time:
var firstRandomChoice = array.splice(Math.floor(Math.random()*array.length), 1);
var secondRandomChoice = array.splice(Math.floor(Math.random()*array.length), 1);
If you use a utility library such as lodash, you may already have a function available to do this for you. For example, lodash provides sample(). So if you were using lodash, you could just do something like this to get an array of two random items:
var results = _.sample(array, 2);

Javascript arrays // reordering an array starting at a specified index while keeping the original order [duplicate]

This question already has answers here:
Rotate the elements in an array in JavaScript
(42 answers)
Closed 8 years ago.
Having the following array :
var arr = [a,b,c,d,e];
I'm struggling to get a clean function that sorts the array based on a specified index, keeping at the same time the original order
For example, sorting the array from index 3 (so here, from "d"), would give the following :
[d,e,a,b,c]
From index 2 :
[c,d,e,a,b]
etc...
It might be obvious for some but I can't make it in my mind
Help appreciated, thx in advance
* Edit *
Is a duplicate. Here is a good one.
https://stackoverflow.com/a/7861200/102133

var arr = ['a','b','c','d','e'];
function reorder(data, index) {
return data.slice(index).concat(data.slice(0, index))
};
console.log(reorder(arr, 3));
console.log(reorder(arr, 2));

This function will do your work
var reorder = function(arr,index){
var start = arr.slice(index); // This will return me elements from a given index
var end = arr.slice(0,index); // This will return me elements before a given index
return start.concat(end); // Concat 2nd array to first and return the result.
}

Counting Object Length based on key value [duplicate]

This question already has answers here:
How can I only keep items of an array that match a certain condition?
(3 answers)
Closed 8 years ago.
I have a JS object/ associative array with some values:
var array =[
{val:1,ref:10,rule:100},
{val:1,ref:12,rule:120},
{val:2,ref:13,rule:165},
];
And I want to perform a .length, but want to be able to slice based on one of the keys (for instance val == 1). I want the length of values with val 1 rather than the length of the entire object. I have looked through the references material and could not find a satisfactory answer and I am unsure if this is feasible.
array.val==1.length = 2
Something like that...

You want to .filter the array for elements that match some predicate:
var filteredArray = array.filter(function(element) { return element.val == 1 })
filteredArray.length // = 2
filter applies the callback function to each element in the array, and the new filtered array contains all elements from original array for which the filter callback function returned true. Here, the function returns true for all elements with a val property of 1.

You need to use a .filter():
array.filter(function(v){return v.val==1}).length