typescript - 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)

No overload matches this call.
 Overload 1 of 3, '(raw: HTMLElement): Ref', gave the following error.
  Argument of type 'null' is not assignable to parameter of type 'HTMLElement'.
 Overload 2 of 3, '(raw: HTMLElement): Ref', gave the following error.
  Argument of type 'null' is not assignable to parameter of type 'HTMLElement'.Vetur(2769)

Here's a condensed context. Any ideas what I did wrong?

<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

正如 Vetur 给出的，您不能转换 null输入 HTMLELement类型。解决此问题的一种可能方法是编写:

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

但是，请记住，您必须检查 firstRef 的类型是否为 null每次你想使用它。你也可以做这样的事情:

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