1var initialData = [{
2    'ID': 1,
3    'FirstName': 'Sally'
4  },
5  {
6    'ID': 2,
7    'FirstName': 'Jim'
8  },
9  {
10    'ID': 3,
11    'FirstName': 'Bob'
12  }
13];
14
15var newData = [{
16    'ID': 2,
17    'FirstName': 'Jim'
18  },
19  {
20    'ID': 4,
21    'FirstName': 'Tom'
22  },
23  {
24    'ID': 5,
25    'FirstName': 'George'
26  }
27];
28
29var ids = new Set(initialData.map(d => d.ID));
30var merged = [...initialData, ...newData.filter(d => !ids.has(d.ID))];

1var merged = [...initialData, ...newData.filter(d => !ids.has(d.ID))];

1// Join Without Dupes.
2const joinWithoutDupes = (A, B) => {
3  const a = new Set(A.map(x => x.item))
4  const b = new Set(B.map(x => x.item))
5  return [...A.filter(x => !b.has(x.item)), ...B.filter(x => !a.has(x.item))]
6}
7
8// Proof.
9const output = joinWithoutDupes([{item:"apple",description: "lorem"},{item:"peach",description: "impsum"}], [{item:"apple", description: "dolor"},{item:"grape", description: "enum"}])
10console.log(output)