This method uses a little helper function that stores the previous value for you and takes a callback and the initial value as arguments.
<script>
function memory(fn, initial) {
let oldValue = initial
return function (newValue) {
fn(newValue, oldValue)
oldValue = newValue
}
}
const compare = memory((n, o) => {
console.log(n, o)
})
let value = 0
$: compare(value)
</script>