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How to upload a file

Files can be sent as is:

<!doctype html>
<html lang="en">
  <body>
    <input type="file" onchange="upload(this.files)" />

    <script src="/path/to/socket.io.js"></script>
    <script>
      const socket = io();

      function upload(files) {
        socket.emit("upload", files[0], (status) => {
          console.log(status);
        });
      }
    </script>
  </body>
</html>

The file will be received as a Buffer on the server side:

import { writeFile } from "fs";

io.on("connection", (socket) => {
  socket.on("upload", (file, callback) => {
    console.log(file); // <Buffer 25 50 44 ...>

    // save the content to the disk, for example
    writeFile("/tmp/upload", file, (err) => {
      callback({ message: err ? "failure" : "success" });
    });
  });
});

Additional notes​

maxHttpBufferSize limit​

While uploading a file, you might reach the maxHttpBufferSize value, which is the maximum allowed message size in bytes. It defaults to 1 MB.

You can increase this value, according to your use case:

import { Server } from "socket.io";

const io = new Server({
  maxHttpBufferSize: 1e8 // 100 MB
});

Reference: maxHttpBufferSize option

Over the wire​

Like other binary structures (ArrayBuffer, Blob), the arguments of the emit() method will be sent as two WebSocket frames over the wire:

First, a text frame:

451-["upload",{"_placeholder":true,"num":0}]
||||└─ JSON-encoded payload with placeholders for binary attachments
||||
|||└─ separator
||└─ number of binary attachments
|└─ socket.io BINARY EVENT packet type
└─ engine.io MESSAGE packet type

And then a binary frame (one per binary structure):

<0x25 0x50 0x44 ...>

Depending on your use case, you may change this behavior by using a custom parser.