How to divide an array?

You could reduce the array and push a new array to the result if you have the wanted value. Then push all values to the last array in the result set.

var array = ["a", "b", "c", "d", "a", "b", "a", "b", "a"],
    result = array.reduce((r, v, i) => {
        if (!i || v === 'a') r.push([]);
        r[r.length - 1].push(v);
        return r;
    }, []);

console.log(result);

You can convert array to string and then use match() and RegExp. Then then use map() to get array of arrays

let arr = ["a","b","c","d","a","b","a","b","a"]

let res = arr.join('').match((/a[^a]+/g)).map(x => [...x]);
console.log(res)

You can do following.

First convert array to string:- "abcdababa" (using .join(""))

Now split string with "a" as seprator:- ["","bcd","b","b",""] (using .split("a"))

Now prepend "a" to all the element:- ["a","abcd","ab","ab","a"] (using .map(x=>"a"+x))

Now split all element with ""(empty string) as seprator:- [["a"],["a","b","c","d"],["a","b"],["a","b"],["a"]] (using .split("")) (this step will be inside map function)

Now remove first element(as it is not part of your string):- [["a","b","c","d"],["a","b"],["a","b"],["a"]] (using .shift())

var arr = ["a","b","c","d","a","b","a","b","a"];
var x1 = arr.join("").split("a").map(x => {
  x = "a" + x;
  return x.split("");
});
x1.shift();
console.log(x1);

A succinct method is to join your characters into a string, then split on each a using a positive lookahead regex to keep the a in the split parts, and finally split the resulting parts into arrays:

const arr = ["a","b","c","d","a","b","a","b","a"];

const result = arr.join('').split(/(?=a)/).map(x => x.split(''));

console.log(JSON.stringify(result));

using simple forEach

var arr = ["a", "b", "c", "d", "a", "b", "a", "b", "a"];
var result = [];
var i = -1;

arr.forEach(function(val) {
    if (val == 'a') {
        result.push([]);
        i++;
    }
    result[i].push(val);
})

console.log(result);

You could do this with split()/join() methods like:

let arr = ["a", "b", "c", "d", "a", "b", "a", "b", "a"];
let r = arr.join('').split('a').map(x => (["a", ...x.split('')]));
r.shift();

console.log(r);