how can i add multiple images and preview them using react and express?

Server

Since you are using multer, change the upload.single() function to upload.array().

For example:

app.post("/addItem", 
  upload.array('product-image', 4), // 'product-image' is field name and 4 is the max number of files allowed
  (req, res) => {
    console.log(req.files);
    // ... rest of the logic
  }
)

Check out docs for upload.array()

Client

Change current <input> to allow multiple files:

<input type="file" name="product-image" onChange={this.fileChangeHandler} multiple>

Now save all the images user picked not only the event.target.files[0]:

fileChangeHandler(event) {
    let files = event.target.files
    this.setState({ selectedFiles: files })
}

Now add them in FormData and upload as usual:

let formData = new FormData()
formData.append("product-image", this.state.selectedFiles)

That's it! Hope it helps.

PS: I don't think files should be added to state. You can simply add them to a class variable. In this answer I explained why and how to do that.

Update:

You need to loop over the files now. Your /addItem endpoint's code will look something like this:

app.post("/addItem", upload.array('product-image', 4), (req, res) => {
  console.log(req.files);

  let paths = [];

  req.files.forEach(file => {
    console.log("new file location", file.path)

    let extension = file.originalname.split(".").pop()
    fs.rename(file.path, file.path + "." + extension, () => {})
    paths.push("/" + file.filename + "." + extension);
  });

  console.log("body", req.body)

  let itemToStore = {
    paths: paths,  // notice this `paths` now, it was `path`
    description: req.body.description
  }

  console.log("we are adding", itemToStore)
  itemData.push(itemToStore)
  console.log("updated itemData:", itemData)
  res.send(JSON.stringify(itemData))
})

I didn't modify your code, just added a loop. Your 'path' of undefined error should go away.