История изменений

<script>
  const options = {
    lake: [1, 2],
    forest: [3, 4]
  }
	
  let first;
  let second;
</script>

<select bind:value={first}>
  <option>?</option>
  {#each Object.keys(options) as option}
    <option value={option}>
      {option}
    </option>
  {/each}
</select>

{#if options[first]}
  <select bind:value={second}>
    <option>?</option>
    {#each options[first] as option}
      <option value={option}>
        {option}
      </option>
    {/each}
  </select>
{/if}

<p>
  {first || "?"}/{second || "?"}
</p>

<script>
  const options = {
    lake: [1, 2],
    forest: [3, 4]
  }
	
  let first;
  let second;
</script>

<select bind:value={first}>
  <option>?</option>
  {#each Object.keys(options) as option}
    <option value={option}>
      {option}
    </option>
  {/each}
</select>

<select bind:value={second}>
  <option>?</option>
  {#each options[first] || [] as option}
    <option value={option}>
      {option}
    </option>
  {/each}
</select>

<p>
  {first || "?"}/{second || "?"}
</p>