//Sorted it into tokens for simplicity
var script = [
    "20","/","(","6","+","4",")","*","5","+","4"
];

//Currently processed token
var currentToken;
//Index in the script
var index = 0;

//Get next token
function nextToken()
{
    index += 1;
    currentToken = script[index];
}

//Check if current token is the same as the one we expect
function acceptToken(type)
{
    if(type == "num")
    {
        if(!isNaN(currentToken))
        {
            return true;
        }
    }
    else if(type == currentToken)
    {
        return true;
    }
    return false;
}

//acceptToken but throw error if the currentToken is unexpected
function expectToken(expected)
{
    if(acceptToken(expected))
    {
        return true;
    }
    console.log("Error: invalid token");
    return false;
}

//Parse factor (in this case either a number or a bracket)
function parseFactor()
{

    if(acceptToken("num"))
    {
        var num = parseInt(currentToken);
        nextToken();
        return num;
    }
    else if(acceptToken("("))
    {
        nextToken()
        var num = parseSum();
        expectToken(")")
        nextToken()
        return num;
    }
    else
    {
        nextToken();
        console.log("Error: invalid token");
    }
}

//Parse either division or multiplication
function parseTerm()
{
    var f1 = parseFactor();
    while(currentToken == "*" || currentToken == "/")
    {
        if(currentToken == "*")
        {
            nextToken();
            f1 *= parseFactor();
        }
        else if(currentToken == "/")
        {
            nextToken();
            f1 /= parseFactor();
        }
    }
    return f1;
}

//Parse addition/subtraction
function parseSum()
{
    if(currentToken == "+" || currentToken == "-")
    {
        nextToken();
    }
    var p1 = parseTerm();
    while(currentToken == "+" || currentToken == "-")
    {
        if(currentToken == "+")
        {
            nextToken();
            p1 += parseTerm();
        }
        else if(currentToken == "-")
        {
            nextToken();
            p1 -= parseTerm();
        }
    }
    return p1;
}

function main()
{
    currentToken = script[index];
    var result = parseSum();
    console.log(result);
}

main();