zipObject.js

function zipObject(keys, values) { 
  // often simple edge cases are taken care of at the top so they aren't hidden inside the middle of the function
  if (keys === undefined && values === undefined) {
    return {}; // for test 3
  }
  
  // category of problem: convert [] -> {}
    // simple strategy for conversion problems is to create a blank object at the beginning, build it up in the middle, return it at the end
    
  // create blank starter obj
  const obj = {}
  
  // build up the obj
  if (values !== undefined) {
    // for test 1
    keys.forEach((key, i) => { // second parameter, i, is rarely used, but is useful in this problem to pick the value
      // need to assign this key to a new property on obj dynamically by adding the property with square brackets
      obj[key] = values[i];
    })
  } else if (typeof keys[0] === 'string') {
    // for test 4
    keys.forEach(key => {
      // key is actually the property name
      // value is always undefined
      obj[key] = undefined;
    })
  } else {
    // for test 2
    keys.forEach(key => {
      // key is actually an array
      // key[0] is actually the property name
      // key[1] is actually the value
      obj[key[0]] = key[1];
    })
  }
  
  // return the obj
  return obj
}