Vue Composition API - 使用 TypeScript 获取引用

Question

Vetur 在下面这一行下划线 null：

const firstRef = ref<HTMLElement>(null)

没有重载匹配这个调用。
 Overload 1 of 3, '(raw: HTMLElement): Ref', 给出了以下错误。
  “null”类型的参数不能分配给“HTMLElement”类型的参数。
 重载 2 of 3, '(raw: HTMLElement): Ref'，出现以下错误。
  “null”类型的参数不能分配给“HTMLElement”类型的参数。Vetur(2769)

这是一个浓缩的上下文。任何想法我做错了什么？

<template>
  <input id="first" ref="firstRef">
  <button type="button" @click.prevent="focusFirst">Focus</button>
</template>

<script lang="ts">
import { defineComponent, ref } from "@vue/composition-api"
export default defineComponent({
  name: "Test",
  setup() {
    const firstRef = ref<HTMLElement>(null)
    const focusFirst = () => {
      const theField = firstRef.value
      theField.focus()
    }

    return { focusFirst }
  }
</script>

Answer 1

As given back by Vetur, you cannot convert a null type to an HTMLELement type. A possible way to fix this would be to write:

const firstRef = ref<HTMLElement | null>(null)

However, keep in mind that you'll have to check if firstRef is of type null every time you want to use it. You could do something like this as well:

if (firstRef.value) {
  // do stuff with firstRef
  // typescript knows that it must be of type HTMLElement here.
}