https://leetcode.com/problems/basic-calculator-ii/submissions/

basicCalculator2.js

/**
 * @param {string} s
 * @return {number}
 */

// Straight-up stack. O(n) time and space since every character
// is inspected at worst a fixed number of times. 
var calculate = function(s) {
    // question constraint: ignore unary negatives
    const parsed = parse(s); // remove spaces, convert strings to ints
    const stack = [];
    
    // evaluate multiplication and division
    for(let i=0; i<parsed.length; i++){
        const token = parsed[i];
        if(token === '/' || token === '*'){
            const [left, right] = [stack.pop(), parsed[i+1];
            stack.push(token === '*' ? (left * right) : Math.floor(left/right));
            i++;
        }else stack.push(token);
    }
    
    // evaluate addition and subtraction
    let result = stack[0];
    let op = null;
    for(let i=1; i<stack.length; i++){
        const token = stack[i];
        if(token === '+' || token === '-') op = token;
        else result = op === '+' ? result + token : result - token;
    }
    
    return result;
    
    function parse(str){
        let cleanStr = str.replace(/\s/g, ''); // remove all spaces
        let currentNum = [];
        const result = [];

        for(let i=0; i<cleanStr.length; i++){
            const token = cleanStr[i];
            if(token.match(/[0-9]/)) currentNum.push(token);
            else{
                if(currentNum.length){
                    const numStr = currentNum.join('');
                    result.push(parseInt(numStr, 10));
                    currentNum = [];
                }
                result.push(token);
            }
        }
        
        if(!currentNum.length) return result;
      
        const numStr = currentNum.join('');
        result.push(parseInt(numStr, 10));
        
        return result;
    }
};