For example: I have array that looks like this:
let arr = [{ name: "Peter" }, { name: "Peter" }, { name: "John" }, { name: "Peter" }, { name: "Sarah" }, { name: "John" }] I need to create array, that will be look like this:
let dupArray = [ [{ name: "Peter" }, { name: "Peter" }, { name: "Peter" }], [{ name: "John" }, { name: "John" }], [{ name: "Sarah" }] ] Again, it's not about duplicating, it's about creating new array with duplicates grouped.
Reduce them to a Map and get the values():
let arr = [{ name: "Peter" }, { name: "Peter" }, { name: "John" }, { name: "Peter" }, { name: "Sarah" }, { name: "John" }]; let dupArr = [...arr.reduce((a,b) => a.set(b.name , (a.get(b.name) || []).concat(b)), new Map()).values()]; console.log(dupArr);
...arr means?
var arr = [{name: "Peter"},{name: "Peter"},{name: "John"},{name: "Peter"},{name: "Sarah"},{name: "John"}] var group = function(ar){ // counts the element per group var swap = {}; for(a of ar){ if(swap[a.name] !== undefined){ swap[a.name] = swap[a.name] + 1; } else { swap[a.name] = 1; } } // here you could sort alphabetically // creates your wanted format var target = []; for(s in swap){ var len = swap[s]; var subArray = []; for(var i = 0; i < len; i++){ subArray.push({ "name" : s }); } target.push(subArray); } return target; } document.getElementById("content").innerHTML = JSON.stringify(group(arr));<pre id="content"></pre>In the first part of the group function the whole thing will be grouped by counting. And afterwards your wished representation will be generated. By just adding {"name" : } to an array according to the counts inside the swap array (the naming is arbitrary).
dupArray = arr.sort();it sorts it alphabetically which in turn will group the duplicates together.