如何计算和记录对象数组中的重复项

Question

I have an array and I need to count and record how many times each item is duplicated. For example the following array:

let arr = [
  {item:"pen"},
  {item:"book"},
  {item:"pen"}
];

This should be returned:

let arr = [
  {item:"pen", found: 2},
  {item:"book", found: 1},
  {item:"pen", found: 2}
]

The attribute "found" should indicate how many times an item appears in the array.

Answer 1

you can use two loop one for iteration one for filter ie

arr.forEach(item => {
  item.found = arr.filter(filterObject => filterObject.item == item.item).length;
})

Answer 2

Try like this.

I have looped the array 2 times and found the duplicate elements, then incremented the new found value.

 let arr = [ {item:"pen"}, {item:"book"}, {item:"pen"} ]; var newArr = []; arr.forEach(function(res){ res.found = 0; arr.forEach(function(data){ if(res.item === data.item){ res.found++; } }); newArr.push(res); }); console.log(newArr); 

Answer 3

There are many ways to skin this cat, some more efficient and some more maintainable. Here's a little bit of both (not the most efficient nor maintainable), which should be easy to follow:

 let arr = [ {item:"pen"}, {item:"book"}, {item:"pen"} ]; let found = {}; // Find repeated objects arr.forEach((obj,i)=>{ found[obj.item]=found[obj.item]||[]; found[obj.item].push(i) }) // Apply totals to original objects Object.values(found).forEach(indexes=> indexes.forEach(index=>arr[index].found = indexes.length) ); console.log(arr);