Multer Overview | 點燈坊

We can use Multer with Express to upload files from the browser and receive files from the server.

Version

Node 16.16.0
Express 4.17.1
Alpine 3.10.3

Alpine

overview000

Upload single file by Alpine and Express.

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <script src="https://unpkg.com/alpinejs" defer></script>
    <title>Upload</title>
  </head>
  <body x-data="fileUpload">
    <input x-ref="upload" type="file" />
    <button @click="onClick">Upload</button>
    <div x-text="resMsg"></div>
  </body>
  <script>
    let fileUpload = {
      resMsg: '',
      async onClick() {
        let formData = new FormData()
        formData.append('file', this.$refs.upload.files[0])

        try {
          let res = await fetch(`http://localhost:8080/upload`, {
            method: 'POST',
            body: formData,
          })

          let data = await res.json()
          this.resMsg = data.message
        } catch (e) {
          console.error(e)
        }
      },
    }
  </script>
</html>

Line 8

<body x-data="fileUpload">
  <input x-ref="upload" type="file" />
  <button @click="onClick">Upload</button>
  <div x-text="resMsg"></div>
</body>

x-data：with fileUpload Object to control the component
x-ref：to set the name for the DOM element
@click：upload the file to the server
x-text：display the result message

Line 17

let formData = new FormData();
formData.append("file", this.$refs.upload.files[0]);

fetch() accepts FormData as argument for body
Append file key for uploaded file

Line 20

try {
  let res = await fetch(`http://localhost:8080/upload`, {
    method: 'POST',
    body: formData,
  })

  let data = await res.json()
  this.resMsg = data.message
} catch (e) {
  console.error(e)
}

Use fetch() API with method POST and body formData
response.json()：fetch() returns Promise with Response Object, use json() to convert Response Object to JSON Object

Express

import express, { json } from 'express'
import cors from 'cors'
import multer from 'multer'

let app = express()
app.use(cors())
app.use(json())

let opts = multer({ dest: 'uploads/' }).single('file')

app.post('/upload', opts, (req, res) => {
  res.json({ message: 'Successfully uploaded files' })
})

app.listen(8080, () => {
  console.log('Node listen on port: 8080')
})

Line 1

import express, { json } from 'express'
import cors from 'cors'
import multer from 'multer'

Import express、cors and multer

Line 5

let app = express()

Create app Object for Express

Line 6

app.use(cors())

Create cors middleware by cors()
Use cors middleware by Express

Line 7

app.use(json())

Create json middleware by json(). This is a built-in middleware by Express
Use json middleware by Express

Line 9

let opts = multer({ dest: "uploads/" }).single("file")

Create opts Object for Multer
dest：set folder to place files
single：upload single file by Multer, the argument for single() must depend on the name for the input specified in formData in Alpine

Line 12

app.post('/upload', opts, (req, res) => {
  res.json({ message: 'Successfully uploaded files' })
})

Create /upload POST with Multer options
Return JSON Object with successful message

Line 16

app.listen(8080, () => {
  console.log('Node listen on port: 8080')
})

Express listens on port 8080

Conclusion

Multer is a famous middleware for Express. We can use Alpine as the client and Express as the server to upload files

Reference

Express, Multer
LogRocket, Multer: Easily upload files with Node.js and Express