Create a Map from an array of objects with a condition on array elements

July 5, 2022

I have an array of objects which i receive from a db:

[{
    key: 'PRODUCT',
    item_key: 'ITEM_01',
    sequence: 1
  },
  {
    key: 'STORE',
    item_key: 'STORE_01',
    sequence: 2
  }
]

I want to create a Map from it but the condition is, i want the item_key to be the map key, and only the corresponding array element which matches the item_key will be the value of that map key. So the map looks like

{
  'ITEM_01' => [{
    key: 'PRODUCT',
    item_key: 'ITEM_01',
    sequence: 1
  }],
  'STORE_01' => [{
    key: 'STORE',
    item_key: 'STORE_01',
    sequence: 2
  }]
}

I’ve kept the value of map keys as an array because there can be more values matching the map key in future. How can i do this?

P.S. by Map i don’t mean the array function map, but the JS/TS Map

>Solution :

Use reduce to create an object, if the key exists in the object then push to the array, otherwise create an array and push the item:

const items = [{
    key: 'PRODUCT',
    item_key: 'ITEM_01',
    sequence: 1
  },
  {
    key: 'STORE',
    item_key: 'STORE_01',
    sequence: 2
  }
]

const result = items.reduce((obj, item) => {
  if (!obj.hasOwnProperty(item.item_key)) {
    obj[item.item_key] = [];
  }
  
  obj[item.item_key].push(item);

  return obj;
}, {});

console.log(result);

Here is an exmaple with Map:

const map = new Map();

const items = [{
    key: 'PRODUCT',
    item_key: 'ITEM_01',
    sequence: 1
  },
  {
    key: 'STORE',
    item_key: 'STORE_01',
    sequence: 2
  }
]

items.forEach(item => {
  if (map.has(item.item_key)) {
    map.get(item.item_key).push(item);
  } else {
    map.set(item.item_key, [item])
  }
});

console.log(Object.fromEntries(map))