two_sum.js

const twoSum = (arr, num) => {
  const result = []
  const hash = []

  for (let i in arr) {
    const currentNumber = arr[i] 
    const expectedNumber = num - currentNumber 
    
    if (hash.indexOf(expectedNumber) !== -1) {
      result.push([currentNumber, expectedNumber])
    } 
    
    if (hash.indexOf(currentNumber) < 0) hash.push(currentNumber)
  }

  return result
}

const num = 7
const arr = [1,9,9,5,6,5,4,5,3,3] // [[6,1],[3,4],[3,4]]
const result = twoSum(arr, num)

console.log(result)