JS Sort an array into three occurrences [closed]

Question

I need to arrange the array so that three are only instances left or less, for example

let tab = [ 3, 3, 4, 4, 5, 4, 5, 6, 6, 4, 6, 3, 7, 7, 3]

I would like to get something like this initially;

let newTab = [3,3,3,4,4,4,5,5,6,6,6,7,7]

I am asking for help because I have no idea anymore.

Answer 1

You could group the data by thie value and take only the max length of three items for each group.

As result take the values in a flat array.

const
    tab = [3, 3, 4, 4, 5, 4, 5, 6, 6, 4, 6, 3, 7, 7, 3],
    result = Object
        .values(tab.reduce((r, v) => {
            r[v] ??= [];
            if (r[v].length < 3) r[v].push(v);
            return r;
        }, {}))
        .flat();

console.log(...result);

Answer 2

You could reduce() to an intermediate count object, and then flatMap() arrays of the correct size:

const tab = [ 3, 3, 4, 4, 5, 4, 5, 6, 6, 4, 6, 3, 7, 7, 3];

const result = Object.entries(
  tab.reduce((a, v) => ({...a, [v]: (a[v] || 0) + 1}), {})
).flatMap(([v, count]) => new Array(Math.min(count, 3)).fill(+v));

console.log(result);

Sorting is achieved by virtue of integer object keys being numerically sorted by default.