I am trying to get mimeType from the image buffer or ArrayBuffer. But I am facing a strange issue.

I am using the following code to convert the signature bytes to hex string.

const uint8Array = new Uint8Array([137, 80, 78, 71]);
const signatureLength = 4;
const signature1 = uint8Array.slice(0, signatureLength).map(byte => {
  const signature = byte.toString(16).padStart(2, "0");
  console.log("signature --- ", signature, byte, typeof signature);
  return signature;
});
const signature = signature1.join("").toUpperCase();
console.log("signature 1 --- ", signature1, signature);

I am trying to get mimeType from the image buffer or ArrayBuffer. But I am facing a strange issue.

I am using the following code to convert the signature bytes to hex string.

const uint8Array = new Uint8Array([137, 80, 78, 71]);
const signatureLength = 4;
const signature1 = uint8Array.slice(0, signatureLength).map(byte => {
  const signature = byte.toString(16).padStart(2, "0");
  console.log("signature --- ", signature, byte, typeof signature);
  return signature;
});
const signature = signature1.join("").toUpperCase();
console.log("signature 1 --- ", signature1, signature);

As you can see in the console output, 78 is actually converted to '4e', but in the map result it is saved as 0. This behaviour seems very strange. What is going on here?

• javascript
• node.js
• typescript
• mime-types
• uint8array

• It's not at all strange that mapping over Uint8Array gives you a Uint8Array array. Your title can use improvement because right now it's not at all informative to what you're trying to convey here. – VLAZ Commented Mar 9 at 13:31
• Thanks for the comment. Can you please suggest a better title. – Mayank Kumar Chaudhari Commented Mar 9 at 16:40

1 Answer 1

Uint8Array is not a regular array. TypedArray.prototype.map() does not return an array of transformed values, instead it returns another TypedArray of the same type.

This is causing the map to convert '4e' to 0.

The solution if to use Array.from(slicedArray);