mutable array

let array1 = [1,2,3];
let array2 = array1;
console.log(array2); //[ 1, 2, 3 ]
array2.push(4,5,6);
console.log(array2); //[ 1, 2, 3, 4, 5, 6 ]
console.log(array1); //[ 1, 2, 3, 4, 5, 6 ]