findMatchingPair.js

function twoDiff(arr, target) {

  let map = {}

  for (let i = 0; i < arr.length; i++) {
    let sub = arr[i] - target
    let add = arr[i] + target
    if ( map[sub] || map[add] ) {
      return true
    } else {
      map[arr[i]] = true
    }
  }

  return false
}

console.log( twoDiff([1,2,3,4,5], 3) ) // true
console.log( twoDiff([5,4,3,2,1], 7) ) // false
console.log( twoDiff([5,4,3,2,1], 2) ) // true