JSFiddle - React, Tailwind, and code Playground

JavaScript

function evaluatePostfix(str) {
  const operationMap = {
    '+': (a, b) => a + b,
    '-': (a, b) => a - b,
    '*': (a, b) => a * b,
    '/': (a, b) => a / b,
  };
  const operations = Object.keys(operationMap);
  const stack = [];
  for (let i = 0; i < str.length; i++) {
    const currentChar = str[i];
    if (operations.indexOf(currentChar) > -1) {
      const operand2 = stack.pop();
      const operand1 = stack.pop();
      const result = operationMap[currentChar](+operand1, +operand2);
      stack.push(result);
    } else {
      stack.push(currentChar);
    }
  }
  return stack[0];
}
/* Test run  */
console.log(evaluatePostfix('12+'));
console.log(evaluatePostfix('56+7*'))