Create new array and push into another in forEach() (JS)

I have an object and i want to sort the items inside via forEach().

Each fifth element should be stored in a new array.

let content = []
let index = 0
Obj.forEach(function(item) {
    if(item.includes('<h2>') && index !== 0) {
        index++
    }
    content[index].push(item)
})

Unfortunately, i cannot push into content[index].push(item)

How i can create a new array, push new values and start a new array when my if is true?

>Solution :

You can use nullish coalescing assignment to create an array if it doesn’t already exist.

(content[index] ??= []).push(item);