How to convert a hex string to a byte and a byte to a hex string in Javascript?

Here's a node.js specific approach, taking advantage of the the Buffer class provided by the node standard lib.

https://nodejs.org/api/buffer.html#buffer_buffers_and_character_encodings

To get the byte (0-255) value:

Buffer.from('80', 'hex')[0];
// outputs 128

And to convert back:

Buffer.from([128]).toString('hex');
// outputs '80'

To convert to utf8:

Buffer.from('80', 'hex').toString('utf8');

You can make use of Number.prototype.toString and parseInt.

The key is to make use of the radix parameters to do the conversions for you.

var bytestring = Number('0x' + hexstring).toString(10);    // '128'
parseInt(bytestring, 2).toString(16);  // '80'

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