add to add a number of duplicate letter to an array javaScript code example

Example 1: counting duplicate values javascript

var counts = {};
your_array.forEach(function(x) { counts[x] = (counts[x] || 0)+1; });

Example 2: how to count duplicates in an array javascript

arr.reduce((b,c)=>((b[b.findIndex(d=>d.el===c)]||b[b.push({el:c,count:0})-1]).count++,b),[]);