flatten nested array of objects by a key

_flattenNestedMap.js

function flattenMap(itemList, key) {
  return itemList.flatMap(item => {
    if (typeof item[key] === 'undefined' || item[key].length === 0) {
      return item;
    }
    let childs = item[key];
    delete item[key];
    return [item, ...flattenMap(childs, key)];
  });
}

example.js

// example
let myMap = [
    {a:1,lists:[{a:2},{a:3}]},
    {a:4,lists:[{a:5},{a:6,lists:[{a:61,lists:[{a:611},{a:612}]}]}]},
    {a:7}
];
console.log(flattenMap(myMap,'lists'));
// Out 
[{a: 1},
{a: 2},
{a: 3},
{a: 4},
{a: 5},
{a: 6},
{a: 61},
{a: 611},
{a: 612},
{a: 7}]