JSFiddle - React, Tailwind, and code Playground

HTML

Input : <input type="text" id="input" value="a" />
<input id="calc" type="button" value="Submit" />
<br/>
Answer : <span id="result"></span>

JavaScript

document.getElementById("calc").onclick = evaluate;

function evaluate() {
    var postStr = document.getElementById("input").value; //initial postfix expression     
    var postArr = new Array();
    postArr = postStr.split(""); //string.split() turns string into array!

    var stack = [];
    var result;
    var firstNum;
    var secNum;
    //var k;
    for (var i = 0; i < postArr.length; i++) {
        if ((postArr[i] != "^") && (postArr[i] != "+") && (postArr[i] != "-") && (postArr[i] != "*") && (postArr[i] != "/")) {
            stack.push(+postArr[i]);
        }

        else if ((postArr[i] == "^") || (postArr[i] == "+") || (postArr[i] == "-") || (postArr[i] == "*") || (postArr[i] == "/")) {
            if (postArr[i] == '+') {
                firstNum = stack.pop();
                secNum = stack.pop();
                result = secNum + firstNum;
                stack.push(result);
            }
            else if (postArr[i] == '*') {
                firstNum = stack.pop();
                secNum = stack.pop();
                result = secNum * firstNum;
                stack.push(result);
            }
            else if (postArr[i] == '/') {
                firstNum = stack.pop();
                secNum = stack.pop();
                result = secNum / firstNum;
                stack.push(result);
            }
            else if (postArr[i] == '-') {
                firstNum = stack.pop();
                secNum = stack.pop();
                result = secNum - firstNum;
                stack.push(result);
            }
        }
    }
    
    var finalRes = stack.pop();
    document.getElementById("result").innerHTML = finalRes;
}