_.uniqBy(data, 'a');

const sortBy = (key) => {

    return (a, b) => (a[key] > b[key]) ? 1 : (b[key] > a[key] ? -1 : 0);

};

​

const uniqBy = (arr, a) => {

   arr.sort(sortBy(a));

   const arr1 = [];

   arr1.push(arr[0]);

   for (let index = 1; index < arr.length; index++) {

       if (arr[index-1] && arr[index -1].a !== arr[index].a) {

           arr1.push(arr[index]);

       }

   }

   return arr1;

}

uniqBy(data, 'a')

function uniqueBy(arr, prop){

  const record = []

  const seen = {}

  for (let i = 0, len = arr.length; i < len; ++i) { // Notice the len = arr.length

    const item = arr[i]

    const val = item[prop]

    if (!seen[val]) {

      seen[val] = 1

      record.push(item)

    }

  }

}

​

uniqueBy(data, 'a')