How can i fix this message decoder/encoder?

Uhm so basically this is a script for encoding and decoding a message in a webpage using JS. When i decode a message it just returns "undefined(first letter of message)undefinedundefined.....". I know the code is messy, wrote this at like 3 am. Please help. Can't find the problem, pretty new to JS.

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<!-- language: lang-js -->

let dstring = "";
let estring = "";
let letter = "";
let letternum = 0;
let decodedtext = "";
let encodedtext = "";
let currentdt = "";
let currentcharat = 0;
const encoded = [
"lsdjf",
"flwek",
"sdlfj",
"pjnse",
"dsfnn",
"dsnfe",
"sjndv",
"sdfew",
"sdfjn",
"sfnkh",
"ådpfk",
"qpwkd",
"sorif",
"efjnf",
"pijgn",
"sijfb",
"feuwh",
"renfv",
"soduh",
"osdnv",
"ksdjn",
"tonrn",
"frjng",
"fjenw",
"fnkrj",
"vrejl",
"fgwei"
];
alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');
function find(input) {
for (let i = 0; i < encoded.length; i++) {
if (input === encoded[i]) {
//alert(alphabet[i]); (for debugging)
return alphabet[i];
}
}
}
function encode() {
estring = prompt("enter a string to encode, can only include non capital letters A-Z or SPACE");
for (i = 0; i < estring.length; i++) {
letter = estring.charAt(i);
if (letter.charCodeAt(0) === 32) {
letternum = 26;
} else {
letternum = letter.charCodeAt(0) - 97
}
encodedtext = encodedtext + encoded[letternum];
}
alert(encodedtext)
}
function decode() {
decodedtext = "";
dstring = prompt("enter a string to decode");
for (l = 0; l < dstring.length / 5; l++) {
currentdt = "";
for (i = 0; i < 5; i++) {
currentdt = currentdt + dstring.charAt(l * i);
}
decodedtext = decodedtext + find(currentdt)
}
alert(decodedtext);
}
encode();
decode();

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So as I said, when trying to decode something like "soduhksdjnsoduhundefinedsdlfjpijgnpjnsedsfnn" (which would translate to "code") it just returns undefinedCundefinedundefined...... I've tried a number of methods, my code worked in a very different way at first with the character "|" marking where a letter ends and the character "§" marking when the message ended.

Firstly, <code>soduhksdjnsoduh<strong>undefined</strong>sdlfjpijgnpjnsedsfnn</code> isn't a valid input for your decoder, it has undefined in its text. It's not clear how you produced this with an input of code as the encoder actually produces an output of sdlfjpijgnpjnsedsfnn for code.

Next, your indexing logic isn't correct:

dstring.charAt(l*i)

When l is 0, you'll be doing dstring.charAt(0) for all 5 iterations of your inner loop. When l is 1, you'll get the first 5 characters of your input, when it's 2 however, you'll then be accessing indexes 0, 2, 4, 6, 8. As you can see, you're repeating indexes. This leads to you constructing invalid currentdt values that when passed to find(currentdt), can't be found in the encoded array that find searches, leading to the undefined values.

What you really want to be doing is offseting i with + based on the number of chunks you've already computed (l*5):

It should be:

dstring.charAt((l*5)+i)

In addition to this, your alphabet array/string is missing a space character. When you run find() with the space encoding alphabet for the space fgwei encoding you'll end up searching for the character at index 26, which doesn't exist currently. You should make the 26th character a space for your decoder to work correctly with space characters. In addition to this, there is no need to turn your string into an array here. Indexing with bracket notation [i] works fine on strings:

'abcdefghijklmnopqrstuvwxyz ';
                           ^--- space added here at the 26th index.

Lastly, your loop variables created with for(l = 0; ...) and for(i = 0; ...) mean l and i are global as you're not defining them using var or let. While this doesn't change much in your code, it could easily break things down the line, so it's best to ensure you use var or let:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let dstring = "";
let estring = "";
let letter = "";
let letternum = 0;
let decodedtext = "";
let encodedtext = "";
let currentdt = "";
let currentcharat = 0;
const encoded = [ "lsdjf", "flwek", "sdlfj", "pjnse", "dsfnn", "dsnfe", "sjndv", "sdfew", "sdfjn", "sfnkh", "ådpfk", "qpwkd", "sorif", "efjnf", "pijgn", "sijfb", "feuwh", "renfv", "soduh", "osdnv", "ksdjn", "tonrn", "frjng", "fjenw", "fnkrj", "vrejl", "fgwei" ];
// updated to use `const`, avoids potentially overwriting `alphabet` unintentionally.
// Also remove `.split()` as it's not needed and added space
const alphabet = 'abcdefghijklmnopqrstuvwxyz '; 
function find(input) {
for (let i = 0; i < encoded.length; i++) {
if (input === encoded[i]) {
return alphabet[i];
}
}
}
function encode() {
estring = prompt("enter a string to encode, can only include non capital letters A-Z or SPACE");
for (let i = 0; i < estring.length; i++) {
letter = estring.charAt(i);
if (letter.charCodeAt(0) === 32) {
letternum = 26;
} else {
letternum = letter.charCodeAt(0) - 97
} 
encodedtext = encodedtext + encoded[letternum];
}
alert(encodedtext);
}
function decode() {
decodedtext = "";
dstring = prompt("enter a string to decode", "sdlfjpijgnpjnsedsfnn");
for (let l = 0; l < dstring.length/5; l++) { // added `let`
currentdt = "";
for (let i = 0; i < 5; i++) { // added `let` 
currentdt = currentdt + dstring.charAt((l*5)+i); // changed computation for offset
}
decodedtext = decodedtext + find(currentdt)
}
alert(decodedtext);
}
decode();

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