Merge two arrays while ignoring duplicates in the second array based on a predicate.

merge-no-duplicates.ts

interface MyObject {
  key: number;
  value: string;
}

const array1: MyObject[] = [
  {
    key: 1,
    value: 'one'
  },
  {
    key: 2,
    value: 'two'
  },
  {
    key: 3,
    value: 'three'
  },
  {
    key: 5,
    value: 'five'
  },
];

const array2: MyObject[] = [
  {
    key: 0,
    value: 'zero'
  },
  {
    key: 2,
    value: 'NOPE'
  },
  {
    key: 4,
    value: 'four'
  },
];

const concatenateArrays = (array1: MyObject[], array2: MyObject[]): MyObject[] => {
  if (!array2 || array2.length === 0) return [...array1];

  return array1
    .concat(array2)
    .filter(function(object: MyObject) {
      return (<Set<number>> this).has(object.key)
        ? false
        : (<Set<number>> this).add(object.key);
    }, new Set<number>());
  
}

const array3 = concatenateArrays(array1, array2);
console.log(array3);

  /*
  [
    { key: 1, value: 'one' },
    { key: 2, value: 'two' },
    { key: 3, value: 'three' },
    { key: 5, value: 'five' },
    { key: 0, value: 'zero' },
    { key: 4, value: 'four' }
  ]
*/