Answer by Ja͢ck for Create Array reference with element index instead of value copy

Primitive values, such as numbers, booleans, and strings are copied by value, because they're immutable; conversely, objects, arrays, and functions are copied by reference.

See also: Primitive

This can be demonstrated with a simple example:

let a = [{foo: 0},{bar: 1}];let b = [a[0], 2];a[0].foo = 3;console.log(b);

If you want to copy by reference, you need to copy the whole array, instead of individual values.