Efficient way to regroup objects in array by key value

Question

I have an array of objects. These objects have two props: property 'label' of type String and a prop 'detections' which is an Array. I need a functions that can group the objects of same label merging the relative arrays.

For instance:

const list = [
    { label: 'cat', detections: ['a','b'] },
    { label: 'horse', detections: ['c','d'] },
    { label: 'cat', detections: ['e','f'] }
]

Would become:

const result = groupMergeByLabel(list)
// value logged would be => [
    { label: 'cat', detections: ['a','b','e','f'] },
    { label: 'horse', detections: ['c','d'] }
]

Does it have to be in an Array? Can the overall List be in an object? — JimiA
– JimiA, Commented Dec 16, 2020 at 18:14

blex · Accepted Answer · 2020-12-16 17:39:21Z

3

You could use reduce:

const list = [
    { label: 'cat', detections: ['a','b'] },
    { label: 'horse', detections: ['c','d'] },
    { label: 'cat', detections: ['e','f'] }
];

const result = list.reduce((res, {label, detections}) => {
  const existing = res.find(x => x.label === label);
  if (existing) {
    existing.detections.push(...detections);
  } else {
    res.push({label, detections});
  }
  return res;
}, []);

console.log(result);

answered Dec 16, 2020 at 17:39

blex

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1 Comment

Pauline Nemchak Over a year ago

hey, idk if it's okay to ask, do you have github?

Md Sabbir Alam · Accepted Answer · 2020-12-16 17:51:46Z

If you want the fastest method you can do that as for loops are faster than map,reduce etc.

const listA = [
    { label: 'cat', detections: ['a','b'] },
    { label: 'horse', detections: ['c','d'] },
    { label: 'cat', detections: ['e','f'] }
]

const groupMergeByLabel = (list) => {
  let res = [];
  for(let i = 0;i<list.length ;i++) {
      const index = res.findIndex(item => item.label === list[i].label);
    if(index > -1) {
      res[index].detections = [...res[index].detections, ...list[i].detections];
    } else {
      res.push(list[i]);
     
    }
  }
  return res;
};

console.log(groupMergeByLabel(listA))

Or you can use reduce as well,

const listA = [
    { label: 'cat', detections: ['a','b'] },
    { label: 'horse', detections: ['c','d'] },
    { label: 'cat', detections: ['e','f'] }
]

const groupMergeByLabel = (list) => {
  const res = list.reduce((acc, curr) => {
    const index = acc.findIndex(item => item.label === curr.label);
    if(index> -1) {
      acc[index].detections = [...acc[index].detections, ...curr.detections];
    } else {
      acc.push(curr);
    }
    return acc;
  }, []);
  return res;
};

console.log(groupMergeByLabel(listA))

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