most common word in a string with banned word

mostcommonword.ts

function mostCommonWord(paragraph: string, bannedWords?: string[]): string {
    let invalid = new Set(["!","?","'",",",";","."," "]);
    let banned = new Set(bannedWords);
    // set can be a type of a array
    let words = new Map();
    // map should return values and use set to config new values
    let buffer = ""
    let result = "";
    
    for(let ptr = 0; ptr < paragraph.length; ptr++) {
        // character is valid, append it to the buffer
        if (!invalid.has(paragraph[ptr])) {
            buffer += paragraph[ptr].toLowerCase();
            // unless it is the last character, end this iteration
            if (ptr !== paragraph.length) {
                continue;
            }
        }
        
        // character is invalid or is the last one
        if (buffer) {
            // if the word is not banned, increment it's count
            if (!banned.has(buffer)) {
                let occurrences = (words.get(buffer) || 0) + 1;
                let mostOccurrences = (words.get(result) || 0);
                words.set(buffer, occurrences);
                if (occurrences > mostOccurrences) {
                    result = buffer;
                }
            }
            buffer = "";
        }
    }

    console.log(result)
    return result;
};

mostCommonWord('Bob hit a ball, the hit BALL flew far after it was hit', ['hit'])