Is it possible to map only a portion of an array? (Array.map())

Question

I am building a project using React.js as a front-end framework. On one particular page I am displaying a full data set to the user. I have an Array which contains this full

Answer 1

Using slice() is better than adding a condition to your map or reduce function, but it still creates an additional, unused copy of that segment of the array. Depending on what you're doing, that might not be desired. Instead, just use a custom map function:

function sliceMap(fn, from, toExclusive, array) {
  const len = toExclusive - from;
  const mapped = Array(len);

  for (let i = 0; i < len; i++) {
    mapped[i] = fn(array[i + from], i);
  }

  return mapped;
};

Note that fn receives the array value and the (now) zero-based index. You might want to pass the original index (i + from). You might also want to pass the full array as a third parameter, which is what Array.map does.