blotter.js

// Problem: Given a string "line" and a number "xCount", output all versions of the string
// with exactly xCount locations replaced by "X".
// e.g. "abcd", 2 => ["XXcd", "XbcX", "abXX", etc...]

// For each line in lines, replace the character at index with 'X'
function joinLine(index, lines) {
  function blot(line) {
    return (
      line.substring(0, index - 1) + 'X' + line.substring(index, line.length)
    );
  }
  return lines.map(blot);
}

/* Returns a list of all (length CHOOSE xCount) */
function recurseLine(line, xCount, firstIndex) {
  // Base case
  if (xCount == 0) {
    return [line];
  }

  // Just look at that DP!!!
  let lists = [];
  for (let i = firstIndex; i <= line.length; i++) {
    lists = lists.concat(joinLine(i, recurseLine(line, xCount - 1, i + 1)));
  }
  return lists;
}

// Test:
recurseLine('abcd', 2, 1);