coding exercise 1 created by misterpoloy - https://repl.it/@misterpoloy/coding-exercise-1

index.js

function solution(given, T) {
  let target = []
  // generate target array
  for (let x = 0; x < T.length; x++) {
    target.push(T.charAt(x))
  }
  // target vs given indexes to compare
  let target_index = 0;
  let given_index = 0;

  let error_counter = 0;
  let max_errors = 1; // instead of magic numbers
  let output = "";

  while (target.length > target_index) { // prevent overflow
    if (target[target_index] === given.charAt(given_index)) {
      // both values are correct, next
      given_index++
      target_index++
    } else {
      // check if less than one error
      if (error_counter < max_errors) {
        // validate what kind of error based on lenght
        if (given.length === T.length) {
          // Check if missing letter is given, else by default is REPLACE
          let isSwap = false;

          for (let i = 0; i < given.length; i++) {
            // Check if swipping letters are the same value in the "given" and "T"
            if (target[target_index] === given.charAt(i) && target[i] === given.charAt(target_index)) {
              isSwap = true;
              output = `SWAP ${given.charAt(given_index)} ${target[target_index]}`;
              break;
            }
          }
          if (!isSwap) {
            // Then is Replace by default
            output = `REPLACE ${given.charAt(given_index)} ${target[target_index]}`;
            given_index++ // Then proceed with the next letter
            error_counter++;
          } else {
            // Stop becouse only a swap is necessary
            break;
          }

        } else {
          output = `INSERT ${target[target_index]}`;
          error_counter++;
        }
      } else {
        // too much errors, "Impossible" && STOP program
        output = "IMPOSSIBLE";
        break
      }
      target_index++
    }
  }
  return output
}

console.log("1. ", solution("nice", "niece"))
console.log("2. ", solution("test", "tent"))
console.log("3. ", solution("form", "from"))
console.log("4. ", solution("o", "odd"))

Live version can be found here:
https://repl.it/@misterpoloy/coding-exercise-1