Seeking an explanation of Caesar Cipher code

Question

After trying (and failing) to code a Caesar Cipher assignment for the Odin Project exercise, I finally caved in and looked up the answer. However, I do not quite understand it.

I am seeking an explanation of each line does and why it works. The code I copied has some brief descriptions of what each line does, but I still don't understand how it all works.

const caesar = function (str, amount) {
        // Wrap the amount
        if (amount < 0) {
          return caesar(str, amount + 26);
        }
      
        // Make an output variable
        var output = "";
      
        // Go through each character
        for (var i = 0; i < str.length; i++) {
          // Get the character we'll be appending
          var c = str[i];
      
          // If it's a letter...
          if (c.match(/[a-z]/i)) {
            // Get its code
            var code = str.charCodeAt(i);
      
            // Uppercase letters
            if (code >= 65 && code <= 90) {
              c = String.fromCharCode(((code - 65 + amount) % 26) + 65);
            }
      
            // Lowercase letters
            else if (code >= 97 && code <= 122) {
              c = String.fromCharCode(((code - 97 + amount) % 26) + 97);
            }
          }
      
          // Append
          output += c;
        }
      
        // All done!
        return output;
      };

Answer 1

The first if-statement:

if (amount < 0) {
  return caesar(str, amount + 26)
}

Makes sure that the shifting amount is 0 and above by calling itself until it is. Then the following line loops through all the characters in the entire string.

for (var i = 0; i < str.length; i++) {

For every character it checks if its a letter using something called a regex (Google for more information)

if (c.match(/[a-z]/i)) {

The line

var code = str.charCodeAt(i);

Gets the number representing the character at position "i" in the string. The number is the way the computer represents letters and other characters. Upper- and lowercase characters have two completely different numbers associated with them. That is what the two following if-statements is for. I will explain the one for lowercase letters, you should be able to see how the uppercase one works as well.

c = String.fromCharCode(((code - 65 + amount) % 26) + 65);

It starts by subtracting 65 from the number. This is because the first lowercase letter "a" has a value of 65. After that it shifts the result by "amount". The %-sign might seem weird. But all it does is divide the two sides and returning the "rest", the residual number. For example if we write:

5 % 2

It is equal to 1. This is done to "loop" the number and keep it between 0 and 26. After that it adds back the 65 and turns the number back to a character. The last line:

output += c;

Adds the character to the resulting string. Hope this helped you out!