當你需要驗證資料時，類別特別有用。以這個 Box 類別為例，它不應該能超出滑桿允許的最大值來持續增大，但這正是發生的情況。
我們可以通過將 width 和 height 替換為 *getter* 和 *setter*（也稱為 *accessor*）來修復此問題。首先，將它們轉換為私有屬性
App
class Box {
#width = $state(0);
#height = $state(0);
area = $derived(this.#width * this.#height);
constructor(width, height) {
this.#width = width;
this.#height = height;
}
// ...
}然後，建立一些 getter 和 setter
App
class Box {
// ...
get width() {
return this.#width;
}
get height() {
return this.#height;
}
set width(value) {
this.#width = value;
}
set height(value) {
this.#height = value;
}
embiggen(amount) {
this.width += amount;
this.height += amount;
}
}最後，將驗證新增到 setter
App
set width(value) {
this.#width = Math.max(0, Math.min(MAX_SIZE, value));
}
set height(value) {
this.#height = Math.max(0, Math.min(MAX_SIZE, value));
}現在，無論是透過範圍輸入上的 bind:value，還是 embiggen 方法，都無法將方塊尺寸增大到安全限制之外，無論你多用力按按鈕都一樣。
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<script>
const MAX_SIZE = 200;
class Box {width = $state(0);
height = $state(0);
area = $derived(this.width * this.height);
constructor(width, height) {this.width = width;
this.height = height;
}
embiggen(amount) {this.width += amount;
this.height += amount;
}
}
const box = new Box(100, 100);
</script>
<label>
<input type="range" bind:value={box.width} min={0} max={MAX_SIZE} /> {box.width}</label>
<label>
<input type="range" bind:value={box.height} min={0} max={MAX_SIZE} /> {box.height}</label>
<button onclick={() => box.embiggen(10)}>embiggen</button><hr>
<div
class="box"
style:width="{box.width}px" style:height="{box.height}px">
{box.area}</div>
<style>
label {display: flex;
align-items: center;
}
hr {margin: 1em 0;
border: none;
border-bottom: 1px solid #888;
}
.box {background: radial-gradient(at 25% 25%, hsl(15 100 60), hsl(15 100 50)) ;
border-radius: 2px;
filter: drop-shadow(0 0 10px hsl(15 100 50 / 0.3));
display: flex;
align-items: center;
justify-content: center;
overflow: hidden;
}
</style>