Как удалить элемент из массива по условию?

const arr = [
  {
    id: '1',
    firstName: '1',
    lastName: '1'
  },
  {
    id: '2',
    firstName: '2',
    lastName: '2'
  },
  {
    id: '3',
    firstName: null,
    lastName: '3'
  },
  {
    id: '4',
    firstName: '4',
    lastName: '4'
  },
  {
    id: '5',
    firstName: null,
    lastName: '5'
  }
];

const filtered = arr.filter(({ firstName }) => firstName != null);

for(let i = 0; i < arr.length; i++) {
    if(arr[i].firstName === null) arr.splice(i, 1);
}
 
// или...
arr.forEach(({ firstName }, i, a) => firstName === null ? a.splice(i, 1) : null);
 
console.log(arr);
/*
    (3) [{…}, {…}, {…}]
    0: {id: '1', firstName: '1', lastName: '1'}
    1: {id: '2', firstName: '2', lastName: '2'}
    2: {id: '4', firstName: '4', lastName: '4'}
*/

for (let i = arr.length - 1; i >= 0; i--) {
  if (!arr[i].firstName) arr.splice(i, 1);
}

arr.filter(({firstName}) => firstName)

const arr = [
  {
    id: '1',
    firstName: NaN,
    lastName: '1'
  },
  {
    id: '2',
    firstName: '',
    lastName: '2'
  },
  {
    id: '3',
    firstName: null,
    lastName: '3'
  },
  {
    id: '4',
    firstName: 0,
    lastName: '4'
  },
  {
    id: '5',
    firstName: false,
    lastName: '5'
  },
  {
    id: '6',
    firstName: undefined,
    lastName: '6'
  },
];