only + - ( )

basic calculator.js

function basicCalc(str) {

    let result = 0, length = str.length, sign = 1, char;
    let stack = [];

    for (let i = 0; i < length; i++) {
        char = str.charAt(i);
        if (char >= '0') {
            //find all the chars
            let num = 0;
            while(i < length && str[i] >= '0') {
                num = num * 10 + parseInt(str[i++]);
            }
            i--;
            result = result + sign * num;

        } else if (char == '+') {
            sign = 1;
        } else if (char == '-') {
            sign = -1;
        } else if (char == '(') {

            stack.push(result);
            stack.push(sign);

            result = 0;
            sign = 1;
        } else if (char == ')') {
            let stackSign = stack.pop();
            let stackResult = stack.pop();
            result = stackResult + result * stackSign;
        }
    }
    console.log("Result = ", result);
    return result;
}

basicCalc("1 + 12");
basicCalc(" 2-1 + 2 ");
basicCalc("(1+(4+5+2)-3)-(62+8)");

Result =  13
Result =  3
Result =  -61
[Finished in 0.1s]