m1=[1,2,3,4,5,6,7,8,9];

​

// array = input array

// part = size of the chunk

function splitArray(array, part) {

    var tmp = [];

    for(var i = 0; i < array.length; i += part) {

        tmp.push(array.slice(i, i + part));

    }

    return tmp;

}

​

console.log(splitArray(m1, 3)); // [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ]