JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.reshape = function(rows, cols) {
    var copy = this.slice(0); // Copy all elements.
    this.length = 0; // Clear out existing array.
    
    for (var r = 0; r < rows; r++) {
        var row = [];
        for (var c = 0; c < cols; c++) {
            var i = r * cols + c;
            if (i < copy.length) {
                row.push(copy[i]);
            } 
        }
        this.push(row);
    }
};

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

m1.reshape(3, 3); // Reshape array in-place.

console.log(m1);