Sort an Array by property, then find the duplicates by that property

findDuplicatesInArrayOfObjects.js

const users = [
  {
    firstName: 'Sarah',
    lastName: 'Zulu',
    email: 'sarah.zulu@test.com',
  },
  {
    firstName: 'Josh',
    lastName: 'Able',
    email: 'sarah.zulu@test.com',
  },
  {
    firstName: 'Bob',
    lastName: 'Somebody',
    email: 'bob.somebody@test.com',
  },
  
];

// this is the property name we will use when checking for duplicates
const duplicateProperty = 'email';

// Sort by email so we can use the loop below
users.sort((a, b) => {
    if(a[duplicateProperty] < b[duplicateProperty]) { return -1; }
    if(a[duplicateProperty] > b[duplicateProperty]) { return 1; }
    return 0;
});

var results = [];

// loop through and check the array item before the current one
// since the items are ordered by email in the sort function above
// we should find duplicates
// then we'll push those into the results array
for (var i = 0; i < users.length - 1; i++) {
    if (users[i + 1][duplicateProperty] == users[i][duplicateProperty]) {
        // Push this object
        results.push(users[i]);
        // Push the matching one so we can see both duplicates
        results.push(users[i + 1]);
    }
}

console.log(results);