人非圣贤孰能无过？！谁还不偶尔犯点小错误？！犯了错要尽量减少损失，就像贴心的Svelte提供的<svelte:boundary>一样把错误控制在特定范围内，不要由着它蔓延。
举个例子🌰: <FlakyComponent>里就有Bug——当点击按钮的时候会把mouse的值设置为null, 那{mouse.x}和{mouse.y}还不报错。当然如果能修复Bug最好，但有些时候我们还不能改人家的代码，这时候可以把<FlakyComponent>放到<svelte:boundary>里：

To prevent errors from leaving your app in a broken state, you can contain them inside an error boundary using the <svelte:boundary> element. In this example, <FlakyComponent> contains a bug — clicking the button will set mouse to null, meaning that the {mouse.x} and {mouse.y} expressions in the template will fail to render. In an ideal world we’d simply fix the bug. But that’s not always an option — sometimes the component belongs to someone else, and sometimes you just need to guard against the unexpected. Begin by wrapping <FlakyComponent /> with <svelte:boundary>:

<svelte:boundary>
	<FlakyComponent />
</svelte:boundary>

还看不到什么效果是不？别急!添加个failed代码片段为错误信息展示个UI：

So far, nothing has changed, because the boundary doesn’t specify a handler. Add a failed snippet to provide some UI to show when an error occurs:

<svelte:boundary>
	<FlakyComponent />

	{#snippet failed(error)}
		<p>Oops! {error.message}</p>
	{/snippet}
</svelte:boundary>

现在当点击按钮的时候boundary里的内容就会被代码片段取代。我们还可以通过failed的第二个参数重置：

Now, when we click the button, the contents of the boundary are replaced with the snippet. We can attempt to reset things by making use of the second argument passed to failed:

<svelte:boundary>
	<FlakyComponent />

	{#snippet failed(error, reset)}
		<p>Oops! {error.message}</p>
		<button onclick={reset}>Reset</button>
	{/snippet}
</svelte:boundary>

我们还可以添加一个onerror处理函数，它的参数跟failed一样:

We can also specify an onerror handler, which is called with the same arguments passed to the failed snippet:

<svelte:boundary onerror={(e) => console.error(e)}>
	<FlakyComponent />

	{#snippet failed(error, reset)}
		<p>Oops! {error.message}</p>
		<button onclick={reset}>Reset</button>
	{/snippet}
</svelte:boundary>

可以通过它报告下错误信息或者显示些其它的UI。

This is useful for sending information about the error to a reporting service, or adding UI outside the error boundary itself.