一个检索串提取代码示例是JAVA版本的，能不能用JS来写

public static String extractIndex(String encryptedData) {
    if (encryptedData == null || encryptedData.length() < 4) {
        return null;
    }
    char sepInData = encryptedData.charAt(0);
    if (encryptedData.charAt(encryptedData.length() - 2) != sepInDat a) {
        return null;
    }
    String[] parts = StringUtils.split(encryptedData, sepInData);
    if (sepInData == ‘$‘ || sepInData == ‘#‘) {
        return parts[0];
    } else {
        return parts[1];
    }
}

function extractIndex(s) {
    if (typeof s !== "string") return null
    const l = s.length
    if (l < 4) return null
    const c = s[0]
    if (s[l - 2] !== c) return null
    const p = s.split(c)
    return "$#".includes(c) ? p[1] : p [2]
}