As The Code Do:

arr.join(',').split(','); 

Create a new array, traverse the current array, and determine the length attribute of each item. If it is not greater than 1, put it into the array. If it is greater than 1, split('') and then concat it into the new array. Then just return the new array.

const arr = ["0", "1", "2", "3,4,5,6", "7,8", "9,10"];
const arr2 = [];

arr.forEach(v => Array.prototype.push.apply(arr2, v.split(',')));

console.log(arr2);

var a = [];
var oldarr = ["0", "1", "2", "3,4,5,6", "7,8", "9,10"];
oldarr.forEach(function(val){
    a = a.concat(val.split(','));
})

// ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]
console.log(a);

let arr = ["0", "1", "2", "3,4,5,6", "7,8", "9,10"];
console.log( arr.join().split(/,|''/) )

https://jsfiddle.net/1yqzea2f/

Another reference

arr.toString().split(',')