Get Pairs that makes target number

getPairs.ts

function getPairs (array: number[], target: number): number {
  const needed = {};
  let result = 0;

  for (let i = 0; i < array.length; i++) {
    // store pair that makes target with element from original array and index
    needed[array[i] - target] = i;
  }

  for (let i = 0; i < array.length; i++) {
    // check that pair that makes target exists and it's not same element in original array
    if (needed[array[i]] !== undefined && needed[array[i]] !== i) {
      result++
      // element used in pair so deleted from needed array
      needed[array[i]] = undefined;
    }
  }
  return result;
}

// Time complexity is O(N+N) which means O(N)
// https://stackoverflow.com/questions/5872120/time-complexity-of-two-for-loops


test('passing array [1, 2, 3, 4] with target 1', () => {
  const r = getPairs([1,2,3,4], 1);
  expect(r).toBe(3);
});

test('passing array [-2, 2, 0, 4] with target 2', () => {
  const r = getPairs([-2, 2, 0, 4], 2);
  expect(r).toBe(3);
});

test('passing array [-2, 2, 2, 0, 4] with target 2', () => {
  const r = getPairs([-2, 2, 2, 0, 4], 2);
  expect(r).toBe(3);
});

test('passing array [2, 4, 6, 8] with target 2', () => {
  const r = getPairs([2, 4, 6, 8], 2);
  expect(r).toBe(3);
});

test('passing array [8, 5, 3, 0] with target 3', () => {
  const r = getPairs([8, 5, 3, 0], 3);
  expect(r).toBe(2);
});

test('with empty array', () => {
  const r = getPairs([], 10);
  expect(r).toBe(0);
});

test('passing array [-2,2] with target 0', () => {
  const r = getPairs([-2,2],0);
  expect(r).toBe(0);
});