Javascript group the numbers from an array with series of consecutive numbers

Question

Given a sorted array of ints for example

a = [0,1,2,5,6,9];

I would like to identify the ranges like

[
    [0,1,2],
    [5,6],
    [9]
]

So far I have tried a double/triple loop but it nests to really nasty code. Maybe this problem can be solved using recursion or other smart tricks?

---

Additional example:

input

b = [0,1,5,6,7,9];

output

[
    [0,1],
    [5,6,7],
    [9]
]

Answer 1

Iterate with Array#reduce, and whenever the last number is not equal to the new number - 1, add another sub array. Add the current number to the last sub array:

const a = [0,1,2,5,6,9];

const result = a.reduce((r, n) => {
  const lastSubArray = r[r.length - 1];
  
  if(!lastSubArray || lastSubArray[lastSubArray.length - 1] !== n - 1) {
    r.push([]);
  } 
  
  r[r.length - 1].push(n);
  
  return r;  
}, []);

console.log(result);

Answer 2

Array.reduce like @OriDrori has done works well.

Another idea is just using a simple for loop, and slice.

function groupArray(a) {
  const ret = [];
  if (!a.length) return ret;
  let ixf = 0;
  for (let ixc = 1; ixc < a.length; ixc += 1) {
    if (a[ixc] !== a[ixc-1] + 1) {
      ret.push(a.slice(ixf, ixc));
      ixf = ixc;  
    }
  }
  ret.push(a.slice(ixf, a.length));
  return ret;
}

console.log(JSON.stringify(groupArray([0,1,2,5,6,9])));