Property 'xx' does not exist on

Vue2.6 + TypeScript3.4 项目中使用 this.$refs.fileInput.files.length 报错，提示 files 类型不存在 Property 'files' does not exist on type 'Element | Element[] | Vue | Vue[]'

如下一个 fileChange 函数

  private fileChange() {
    if (this.$refs.fileInput.files.length > 0) {
      this.file = this.$refs.fileInput.files[0]
      const reader = new FileReader()
      const self = this
      reader.readAsDataURL(this.file)
      reader.onload = (e: any) => {
        const dataBase64: any = reader.result
        self.userInfo.avatar = dataBase64
      }
    }
  }

template

<input ref="fileInput" type="file" @change="fileChange()" />

在不使用TypeScript时上面的写法可以正常运行的，但是这样的写法在TypeScript 中会报错，终端报错如下

534:40 Property 'files' does not exist on type 'Element | Element[] | Vue | Vue[]'.
  Property 'files' does not exist on type 'Element'.
    532 |   private fileChange() {
    533 |     if (this.$refs.fileInput.files.length > 0) {
  > 534 |       this.file = this.$refs.fileInput.files[0]
        |                                        ^
    535 |       const reader = new FileReader()
    536 |       const self = this
    537 |       reader.readAsDataURL(this.file)
No lint errors found
Version: typescript 3.8.3, tslint 5.20.1
Time: 1413ms

  App running at:
  - Local:   http://localhost:8080/ 
  - Network: http://192.168.199.172:8080/

screenshot_1.png screenshot_2.png

解决方式

因为TypeScript需要知道属性是什么类型，这里需要强制类型转换

把 this.$refs.fileInput.files.length 换成 (this.$refs.fileInput as Vue & { files: () => any }).files.length

把 this.file = this.$refs.fileInput.files[0] 换成
const fs: any = (this.$refs.fileInput as Vue & { files: () => any }).files 和 this.file = fs[0]

最后如下效果，代码就可以正常运行了

  private fileChange() {
    if ((this.$refs.fileInput as Vue & { files: () => any }).files.length > 0) {
      const fs: any = (this.$refs.fileInput as Vue & { files: () => any }).files
      this.file = fs[0]
      const reader = new FileReader()
      const self = this
      reader.readAsDataURL(this.file)
      reader.onload = (e: any) => {
        const dataBase64: any = reader.result
        self.userInfo.avatar = dataBase64
      }
    }
  }

参考来源

https://stackoverflow.com/questions/52109471/typescript-in-vue-property-validate-does-not-exist-on-type-vue-element