<form 
  method="post" 
  enctype="multipart/from-data"
  action="api/upload"
>
  <input type="file name="file">
  <button type="submit">Submit</button>
</form>

var formdata = new FormData(); // 创建FormData对象
formdata.append("name","laotie"); // 通过append()方法添加新的属性值
... // 更多方法请点下面链接

<input type="file" name="file" @change.prevent="handleFileChange">
// 创建一个file类型的input，用于触发文件上传，后面可以把input隐藏掉，自定义好看的样式
// 自定义样式的时候可以用slot区分不同上传状态的样式（loading，success，defult）

const handleFileChange = (e:Event)=>{
  const target = e.target as HTMLInputElement
  const files = Array.from（target.files）// 注意这里取得的是一个类数组
  if(files){
    // 取得文件
    const uploadedFile = files[0]
    
    if(!validateFormat) return
    // ...这里只是提供一种思路，具体校验不再讲述
    // 在这里做一些上传文件前的校验，比如文件格式，大小等，
    // 不符合要求的话就不在继续发送请求
    
    const formData = new FormData()
    formData.append(uploadedFile.name,uploadedFile)
    
    axios.post('/upload',formData,{
      headers:{
         // 注意设置编码类型
        'Content-Type': 'multipart/form-data'
      }
    }).then(res=>{
      console.log('上传成功')
    }).catch(error =>{
      // 文件上传失败
    }).finally(()=>{
      // 文件上传完成，无论成功还是失败
      // 这里可以清除一下input.value
    })
  }
}

// Upload.vue
<template>
  <div class="upload-container">
    <div class="upload-box" @click.prevent="triggerUpload" v-bind="$attrs">
      <slot name="loading" v-if="fileStatus==='loading'">
        <button class="btn btn-primary">上传中</button>
      </slot>
      <slot name="uploaded" v-else-if="fileStatus==='success'" :uploadedData="fileData">
        <button class="btn btn-primary">上传成功</button>
      </slot>
      <slot v-else name="default">
        <button class="btn btn-primary">点击上传</button>
      </slot>
    </div>
    <input type="file" class="file-input d-none" name="file" ref="uploadInput" @change="hanldeInput"/>
  </div>
</template>
<script lang="ts">
import { defineComponent, ref, PropType, watch } from 'vue'
import axios from 'axios'
type UploadStatus = 'ready' | 'loading' | 'success' | 'error'
type FunctionProps = (file:File) => boolean
export default defineComponent({
  name: 'Upload',
  inheritAttrs: false,
  props: {
    // 上传的url
    action: {
      type: String,
      required: true
    },
    // 上传之前的校验，是一个返回布尔值的函数
    beforeUpload: {
      type: Function as PropType<FunctionProps>
    },
    // 上传好的数据，用来判断状态或做初始化展示
    uploadedData: {
      type: Object
    }
  },
  emits: ['file-uploaded-success', 'file-uploaded-error'],
  setup(props, ctx) {
    const uploadInput = ref<null | HTMLInputElement>(null)
    const fileStatus = ref<UploadStatus>(props.uploadedData ? 'success' : 'ready')
    const fileData = ref(props.uploadedData)
    watch(() => props.uploadedData, (val) => {
      if (val) {
        fileStatus.value = 'success'
        fileData.value = val
      }
    })
    const triggerUpload = () => {
      if (uploadInput.value) {
        uploadInput.value.click()
      }
    }
    const hanldeInput = (e:Event) => {
      const target = e.target as HTMLInputElement
      const files = target.files
      console.log(target)
      if (files) {
        const uploadFile = Array.from(files)
        const validateFormat = props.beforeUpload ? props.beforeUpload(uploadFile[0]) : true
        if (!validateFormat) return
        fileStatus.value = 'loading'
        const formData = new FormData()
        formData.append('file', uploadFile[0])
        axios.post(props.action, formData, {
          headers: {
            'Content-Type': 'multipart/form-data'
          }
        }).then(res => {
          console.log('文件上传成功', res)
          fileStatus.value = 'success'
          fileData.value = res.data
          ctx.emit('file-uploaded-success', res.data)
        }).catch(error => {
          console.log('文件上传失败', error)
          fileStatus.value = 'error'
          ctx.emit('file-uploaded-error', error)
        }).finally(() => {
          console.log('文件上传完成')
          if (uploadInput.value) {
            uploadInput.value.value = ''
          }
        })
      }
    }

    return {
      uploadInput,
      triggerUpload,
      hanldeInput,
      fileStatus,
      fileData
    }
  }
})
</script>

<template>
  <div class="create-post-page">
    <upload
      action="/upload"
      :beforeUpload="beforeUpload"
      :uploadedData="uploadedData"
      @file-uploaded-success="hanldeUploadSuccess"
      class="d-flex align-items-center justify-content-center bg-light text-secondary w-100 my-4"
      >
      <template #uploaded="slotProps">
        <div class="uploaded-area">
          <img :src="slotProps.uploadedData.data.url"/>
          <h3>点击重新上传</h3>
        </div>
       </template>
       <template #default>
         <h2>点击上传头图</h2>
       </template>
       <template #loading>
         <div class="d-flex">
          <div class="spinner-border text-secondary" role="status">
            <span class="sr-only"></span>
          </div>
         </div>
       </template>
    </upload>
  </div>
</template>
<script lang="ts">
import { defineComponent, ref, onMounted } from 'vue'
import Upload from '../components/Upload.vue'
import createMessage from '../components/createMessage'

export default defineComponent({
  name: 'CreatePost',
  components: { Upload },
  setup() {
    const uploadedData = ref() //创建一个响应式数据
    let imageId = ''
    onMounted(() => {
      ....
      // 这里有逻辑省略了，取到初始化数据image
      if (image) {
        uploadedData.value = { data: image }
      }
    })
    // 上传前校验，返回布尔值
    const beforeUpload = (file:File) => {
      const res = beforeUploadCheck(file, {
        format: ['image/jpeg', 'image/png'],
        size: 1
      })
      const { error, passed } = res
      if (error === 'format') {
        createMessage('上传图片只能是JPG/PNG格式！', 'error')
      }
      if (error === 'size') {
        createMessage('上传图片大小不能超过1MB', 'error')
      }
      return passed
    }
    // 上传成功后拿到imageId就可以进行后续处理，创建表单啥的
    const hanldeUploadSuccess = (res:ResponeseProps<ImageProps>) => {
      createMessage(`上传图片ID ${res.data._id}`, 'success')
      if (res.data._id) {
        imageId = res.data._id
      }
    }
    return {
      beforeUpload,
      hanldeUploadSuccess,
      uploadedData
    }
  }
})
</script>
<style>
.create-post-page{
  padding:0 20px 20px;
}
.create-post-page .upload-box{
  height:200px;
  cursor: pointer;
  overflow: hidden;
}
.create-post-page .upload-box img{
  width: 100%;
  height: 100%;
  object-fit: cover;
}
.uploaded-area{
  position: relative;
}
.uploaded-area:hover h3{
  display: block;
}
.uploaded-area h3{
  display: none;
  position: absolute;
  color: #999;
  text-align: center;
  width: 100%;
  top:50%
}
</style>