Javascript Caesars Cipher algorithm code break down needed?

Question

I am a freecodecamp camper. And I need a help to understand how does the below code works which is an advanced solution.

function rot13(str) { // LBH QVQ VG!
  str.replace(/[A-Z]/g, L => String.fromCharCode((L.charCodeAt(0) % 26) + 65));
}

// Change the inputs below to test
console.log(rot13("AAA !"));

I do have a little idea on why we are using modulus 26 and then adding 65. but I dont understand what L=> is actually doing. I also understand a bit of regular expression. Please break down this code.

Barmar · Accepted Answer · 2017-11-18 07:48:09Z

2

L => declares an arrow function, which is a shorthand syntax for:

function(L) {
    return String.fromCharCode((L.charCodeAt(0) % 26) + 65));
}

When you use a function as the replacement argument in String.prototype.replace(), it gets called on each match of the regular expression, and the return value is used as the replacement.

So the regular expression matches each uppercase letter in str. It calls that function, which calculates the new character code using modulus and addition, converts it back to a character, and that returned character becomes the replacement.

However, the function won't work as written -- it needs to return the result of str.replace().

function rot13(str) { // LBH QVQ VG!
  return str.replace(/[A-Z]/g, L => String.fromCharCode((L.charCodeAt(0) % 26) + 65));
}

// Change the inputs below to test
console.log(rot13("ABMNTZ !"));

edited Nov 18, 2017 at 7:48

answered Nov 18, 2017 at 7:15

Barmar

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1 Comment

Wodin Over a year ago

Looks like there's something missing here: which calculates the new character code by ,

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