Prior ES6 (using push):
Array1.length = 0; // Clear contents
Array1.push.apply(Array1, Array2); // Append new contents
Post ES6 (using splice):
Array1.splice(0, Array1.length, ...Array2);
Array1.length = 0; // Clear contents
Array1.push.apply(Array1, Array2); // Append new contents
Array1.splice(0, Array1.length, ...Array2);