How to compare Prop changes in Svelte 3

I've actually written a package that uses Svelte Stores to give you a simple interface to reference as many previous values as you need.

Svelte Previous.

<script>
  export let color;
  const [currentColor, previousColor] = usePrevious(color);
  $: $currentColor = color;
</script>

{$previousColor} to {$currentColor}

If you want to execute a piece of code only when color changes, you can use the reactive declaration:

<script>
  export let color;

  $: {
     console.log('color changed', color);
     // will only get called when the `color` changed.
  }
</script>

But if you want to execute that piece of code with both the current value and previous value, you can do the following:

<script>
  export let color;
  let prevColor;

  $: {
     console.log('currentColor:', color, 'prevColor:', prevColor);
     prevColor = color;
  }
</script>