check magic number before upload

index.js

var express = require("express")
var multer = require('multer')
var app = express()
var path = require('path')
var fs = require('fs')

var ejs = require('ejs')
app.set('view engine', 'ejs')

var MAGIC_NUMBERS = {
    jpg: 'ffd8ffe0',
    jpg1: 'ffd8ffe1',
    png: '89504e47',
    gif: '47494638'
}

function checkMagicNumbers(magic) {
    if (magic == MAGIC_NUMBERS.jpg || magic == MAGIC_NUMBERS.jpg1 || magic == MAGIC_NUMBERS.png || magic == MAGIC_NUMBERS.gif) return true
}

app.get('/api/file', function (req, res) {
    res.render('index')
})

var upload = multer({
    storage: multer.memoryStorage()
}).single('userFile')

app.post('/api/file', function (req, res) {
    upload(req, res, function (err) {
        var buffer = req.file.buffer
        var magic = buffer.toString('hex', 0, 4)
        if (checkMagicNumbers(magic)) {
            fs.writeFile('./uploads/' + req.file.fieldname + '-' + Date.now() + path.extname(req.file.originalname), buffer, 'binary', function (err) {
                if (err) throw err
                res.end('File is uploaded')
            })
        } else {
            res.end('File is no valid')
        }
    })
})

var port = process.env.PORT || 8080
app.listen(port, function () {
    console.log('Node.js listening on port ' + port)
})