Working with arrays in JS

arrays.js

// The most perfomance way to populate a new array
[...Array(1000)].map((_, i) => i); // [0,...,999]

// Grouping array elements by condition
function groupBy(list, groupingCondition) {
  const groups = [];
  let currentGroup = [];

  for (const item of list) {
    if (currentGroup.length) {
      const lastGroupItem = currentGroup[currentGroup.length - 1];

      if (groupingCondition(item, lastGroupItem)) {
        currentGroup.push(item);
      } else {
        groups.push(currentGroup);
        currentGroup = [item];
      }
    } else {
      currentGroup = [item];
    }
  }

  if (currentGroup.length) {
    groups.push(currentGroup);
  }

  return groups;
}

const sample = [0, 1, 2, 3, 40, 50, 51, 52];
groupBy(sample, (a, b) => b === a - 1) 
// [[0, 1, 2, 3], [40], [50, 51, 52]]
groupBy(sample, (a, b) => b === a - 1).map(l => l[0] + (l.length > 1 ? `-${l[l.length - 1]}` : '')).join(', ')
// "0-3, 40, 50-52"