basic-calculator-ii.js

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {

    let stack = [];
    let num = '';
    let sign = null
    // we loop till the full length of the array to account for last sign
    for (let i = 0; i <= s.length; i++) {
        const curr = s[i];
        //handle space
        if (curr === ' ') continue;
        //if char is a number
        if (!isNaN(curr)) num += curr;
        //if we have a  sign + - / *
        if (isNaN(curr)) {
            num = Number(num)
            switch (sign) {
                case '+':
                case null:
                    //we push the initial number into the stack
                    stack.push(num)
                    break;
                case '-':
                    //we push any values after the subtraction sign as negative
                    stack.push(-num)
                    break;
                case '*':
                    //we pop the stack then multiply and push back
                    stack.push(stack.pop() * num)
                    break;
                case '/':
                    //we pop the stack then devide and push back
                    stack.push(parseInt(stack.pop() / num, 10))
                    break;
            }
            // sign becomes current sign
            sign = curr;
            // we reset num
            num = '';
        }
    }
    //we reduce the array adding positive and negative numbers
    return stack.reduce((a, b) => {
        return a + b
    }, 0)
};