Stack

function main(allInstructions) {
    const AvailableOperations = {
        ['+']: (a, b) => a + b,
        ['-']: (a, b) => a - b,
        ['*']: (a, b) => a * b,
        ['/']: (a, b) => a / b,
    }

    const stack = [];

    for (const instruction of allInstructions) {
        const number = Number(instruction);

        if (isNumber(number)) {
            stack.push(number);
        } else {
            if (notEnoughNumbersInStack(stack)) {
                console.log('Error: not enough operands!');
                return; // potential bug
            }

            const rightOperand = stack.pop();
            const leftOperand = stack.pop();

            const currentOperation = AvailableOperations[instruction];

            const result = currentOperation(leftOperand, rightOperand);

            stack.push(result);
        }
    }

    if (stack.length === 1) {
        console.log(stack.pop());
    } else {
        console.log('Error: too many operands!');
    }

    function isNumber(potentialNumber) {
        return !isNaN(potentialNumber);
    }

    function notEnoughNumbersInStack(stack) {
        return stack.length < 2;
    }
}