Converting lodash _.uniqBy() to native javascript

Question

Here in this snippet i am stuck as in _.uniqBy(array,iteratee),this

• iteratee can be a function or a string at the same time

Answer 1

An ES6 uniqBy using Map with a complexity of O(n):

const uniqBy = (arr, predicate) => {
  const cb = typeof predicate === 'function' ? predicate : (o) => o[predicate];
  
  return [...arr.reduce((map, item) => {
    const key = (item === null || item === undefined) ? 
      item : cb(item);
    
    map.has(key) || map.set(key, item);
    
    return map;
  }, new Map()).values()];
};

const sourceArray = [ 
  { id: 1, name: 'bob' },
  { id: 1, name: 'bill' },
  null,
  { id: 1, name: 'bill' } ,
  { id: 2,name: 'silly'},
  { id: 2,name: 'billy'},
  null,
  undefined
];

console.log('id string: ', uniqBy(sourceArray, 'id'));

console.log('name func: ', uniqBy(sourceArray, (o) => o.name));