js efficient duplicate count code example

Example 1: find duplicates and their count in an array javascript

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

Example 2: duplicate numbers in an array javascript

[1, 1, 2, 2, 3].filter((element, index, array) => array.indexOf(element) !== index) // [1, 2]