Magic Bytes is a javascript library analyzing the first bytes of a file to tell you its type. The procedure is based on https://en.wikipedia.org/wiki/List_of_file_signatures.

Run npm install magic-bytes.js

On server:

import filetype from 'magic-bytes.js'

filetype(fs.readFileSync("myimage.png")) // ["png"]

Using HTML:

<input type="file" id="file" />

 <script src="node_modules/magic-bytes.js/dist/browser.js" type="application/javascript"></script>
<script>
    document.getElementById("file").addEventListener('change', (event, x) => {
      const fileReader = new FileReader();
      fileReader.onloadend = (f) => {
        const bytes = new Uint8Array(f.target.result);
        console.log("Possible filetypes: " + filetypeinfo(bytes))
      }
      fileReader.readAsArrayBuffer(event.target.files[0])
    })
</script>

The following functions are availble:

• filetypeinfo(bytes: number[]) Contains typeinformation like name, extension and mime type: [{typename: "zip"}, {typename: "jar"}]
• filetypenames(bytes: number[]) : Contains type names only: ["zip", "jar"]
• filetypemime(bytes: number[]) : Contains type mime types only: ["application/zip", "application/jar"]
• filetypeextensions(bytes: number[]) : Contains type extensions only: ["zip", "jar"]

Both function return an empty array [] otherwise, which means it could not detect the file signature. Keep in mind that txt files for example fall in this category.

You don't have to load the whole file in memory. For validating a file uploaded to S3 using Lambda for example, it may be
enough to load the files first 100 bytes and validate against them. This is especially useful for big files.

see examples for practical usage.

Run npm test

See examples/

The create-snapshot.js creates a new tree. The tree has a similar shape to the following

{
  "0x47": {
    "0x49": {
      "0x46": {
        "0x38": {
          "0x37": {
            "0x61": {
              "matches": [
                {
                  "typename": "gif",
                  "mime": "image/gif",
                  "extension": "gif"
                }
              ]
            }
          },
        }
      }
    }
  }
}

It acts as a giant lookup map for the given byte signatures. To check all available entries, have a look at pattern-tree.js and its generated pattern-tree.snapshot, which acts as a static resource.

Please refer to src/pattern-tree.js

• Specialize type detection(like zip) using offset-subtrees
• Add encoding detection