Jan's Notation

function solve(input) {
    let result = [];

    let parser = {
        '+': function sum(num1, num2) {
            result.push(num1 + num2);
        },
        '-': function subtract(num1, num2) {
            result.push(num1 - num2);
        },
        '*': function multiply(num1, num2) {
            result.push(num1 * num2);
        },
        '/': function divide(num1, num2) {
            result.push(num1 / num2);
        }
    }

    for (const el of input) {
        if (typeof (el) === 'number') {
            result.push(el);
        } else {
            let lastNumber = result.pop();
            let secondLast = result.pop();
            if (lastNumber && secondLast) {
                parser[el](secondLast, lastNumber);
            } else {
                return 'Error: not enough operands!';
            }
        }
    }

    if (result.length > 1) {
        return 'Error: too many operands!';
    } else {
        return result[0];
    }
}