JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://underscorejs.org/underscore.js"></script>

JavaScript

var a = [1,2,3,4,5,6,7,8,9];
function partition(items, size) {
    var result = _.groupBy(items, function(item, i) {
        return Math.floor(i/size);
    });
    return _.map(result, function(val, key) {
        return val;
    });
}
var p = partition(a, 3);
console.log(p);