PHP, for example, has explicit references. If you have an `$arr1=array(1,2,3)` and an `$arr2 = $arr1`, that second array is a full copy of the first array, and updating $arr1 does nothing to $arr2. Similarly, `function update_array($arr) { $arr[0] = 'cake'; }` called with `$arr1` will create a copy of $arr1 for use inside that function. Any changes to `$arr` inside the function only apply to that copy, not to $arr1. Unless you explicitly tell PHP you want to work with references, by using `function update_array(&$arr) { ... }` instead. PHP uses value semantics.

JS, on the other hand, uses implicit reference semantics. If you have a `const arr1 = [1,2,3];` then `arr1` is an alias, simply pointing to a memory location, and declaring a `const arr2 = arr1`, makes arr2 also be an alias for that same memory location. They both reference the same thing, but neither "is" the thing. Similarly, if you have a `function updateArray(arr) { arr[0] = 'cake'; }` then that `arr` is also just an alias pointing to the memory location of whatever you passed in, and changes to `arr` change the underlying data, so whether you try to access that through `arr1`, `arr2` and `arr`, it's the same thing. JS uses implicit reference semantics.

(But note that many languages with implicit reference semantics make exceptions for immutable primitives like numbers or strings)