let arr = [[1,3], [2,6], [8,10], [15,18]]
for(let i = 1; i < arr.length; i++) {
    let prev = arr[i - 1], cur = arr[i]
    if(prev[1] >= cur[0]) {
        arr.splice(--i, 2, [ prev[0], cur[1] ])
    }
}
console.log(arr)

const arr = [
  [1, 3],
  [2, 6],
  [8, 10],
  [15, 18]
];
const result = arr.reduce((acc, v) => {
  if (acc.length === 0) return [v];
  const [start, end] = v;
  const [prevStart, prevEnd] = acc[acc.length - 1];
  if (prevEnd >= start) {
    acc[acc.length - 1][1] = end;
  } else {
    acc.push(v);
  }
  return acc;
}, []);
console.log(result);

"use strict";
const arr = [[1, 3], [2, 6], [8, 10], [15, 18]];
const result = arr.reduce((acc, v) => {
    if (acc.length === 0)
        return [v];
    const [start, end] = v;
    const [prevStart, prevEnd] = acc[acc.length - 1];
    if (prevStart > start)
        acc[acc.length - 1][0] = start;
    if (prevEnd >= start) {
        acc[acc.length - 1][1] = end;
    }
    else {
        acc.push(v);
    }
    return acc;
}, []);
console.log(result);