a helper function to generate a new array starting from the index next to the lastItem

putAtTheEnd.js

/**
 * a helper function to generate a new array starting from the index next to the *lastItem* 
 * Eg: insertAtTheEnd(['a', 'b', 'c', 'd'], c) => ['d', 'a', 'b', 'c']
*/

function putAtTheEnd(arr, lastItem) {
  const lastIndex = arr.findIndex(n => n === lastItem);
  return arr.map((_, i) => {
    const startIdx = (lastIndex + i + 1) % arr.length;  
    return arr[startIdx]; 
  });
}