how to use node js buffer with a conversion map

I am following a tutorial about nodejs. In the tutorial it has this example:

const fs = require('fs');

const conversionMap = {
  '88': '65',
  '89': '66',
  '90': '67',
};

fs.readFile(__filename, (err, buffer) => {
  let tag = buffer.slice(-4, -1);

  for(let i=0;i < tag.length; i++) {
    tag[i] = conversionMap[tag[i]];
  }

  console.log(buffer.toString());
});

// TAG: XYZ

I am pretty sure I know what this example is doing. It is reading the file, then allocating it to a buffer and writing it out. I know that it finds the TAG via slice(-4, -1) (The tag it 1 up from the bottom and for over). I am just not sure it changes it.

Thanks in advance for the help!

via Zoe Carver