Calculating Vigenere Cipher's (Gravity Falls) via a TS script

vigenere.ts

function vigenere(text: string, key: string, DecryptMode: boolean, alphabet: string) {
	const textMask = getTextMask(text, alphabet);
	const normalizedText = getNormalizedText(text, textMask);

	if (!(normalizedText.length > 0 && key.length > 0))
		return;

	const transformedText = DecryptMode ? decrypt(normalizedText, alphabet, key) : encrypt(normalizedText, alphabet, key);
	return restoreText(text, transformedText, textMask);
}

function getTextMask(text:string, alphabet:string) {
	const alphabetUpper = alphabet.toUpperCase();
	const alphabetLower = alphabet.toLowerCase();
	
	const arr = text.split('').map((currentValue) => {
		const isUpperLetter = alphabetUpper.includes(currentValue);
		const isLowerLetter = alphabetLower.includes(currentValue);
		let l = '-';
		if (isUpperLetter)
			l = 'A';
		else if (isLowerLetter)
			l = 'a';
		else
			l = '+';
		return l;
	});

	return arr.join('');
}

function getNormalizedText(text:string, textMask:string) {
	const arr = [];
	const length = textMask.length;
	for (let i = 0; i < length; i++) {
		if (textMask[i] === 'a' || textMask[i] === 'A')
			arr.push(text[i]);
	}

	return arr.join('').toLowerCase();
}

function restoreText(originalText:string, normalizedText:string, textMask:string) {
	const arr = originalText.split('');
	let j = 0;
	const length = textMask.length;
	for (let i = 0; i < length; i++) {
		switch (textMask[i]) {
			case 'a':
				arr[i] = normalizedText[j++]; break;
			case 'A':
				arr[i] = normalizedText[j++].toUpperCase(); break;
			case '-':
				arr[i] = '';
		}
	}

	return arr.join('');
}

const encrypt = (src:string, alphabet:string, key:string) => transform(src, alphabet, getKeyShifts(key, alphabet), false);
const decrypt = (src:string, alphabet:string, key:string) => transform(src, alphabet, getKeyShifts(key, alphabet), true);

function transform(src:string, alphabet:string, shifts:number[], invert:boolean) {
	let result = '';
	const keyLength = shifts.length;

	if (shifts.length == 0)
		return src;

	for (let i = 0; i < src.length; i++) {
		let shift = 0;
		if (i < keyLength) {
			shift = invert ? -shifts[i % keyLength] : shifts[i % keyLength];
		} else {
			shift = invert ? -alphabet.indexOf(result[i - keyLength]) : alphabet.indexOf(src[i - keyLength]);
		}
		result += transformChar(src[i], alphabet, shift);
	}

	return result;
}

function getKeyShifts(key:string, keyAlphabet:string) {
	const arr = new Array(key.length);
	for (let i = 0; i < key.length; i++) {
		const p = keyAlphabet.indexOf(key[i]);
		arr[i] = p;
	}

	return arr;
}

function transformChar(ch:string, alphabet:string, shift:number) {
	const p = (alphabet.indexOf(ch) + shift + alphabet.length) % alphabet.length;
	return alphabet[p];
}

console.log(vigenere('hi', 'die', false, 'abcdefghijklmnopqrstuvwxyz'))
console.log(vigenere('kq', 'die', true, 'abcdefghijklmnopqrstuvwxyz'))