Array.prototype.slice is O(n), where n is the number of elements in the slice.
String.prototype.slice is O(1) time complexity
//example
let nested = {name: "test"};
var a = [nested];
console.log(a)
var b = a.slice(0, 1);
console.log(b)
a[0].name = "abc";
console.log(a === b); // false, `b` is a copy
console.log(a[0] === b[0]); // true, `nested` was not copied
console.log(b[0] === nested); // true
console.log(b[0].name);