Reverse Polish Notation expressions in JavaScript | Typescript | ES6

/**
 * 
 * @param str 
 * @returns 
 */
function reversePolish(str) {
    str = str.split(" "), result = [];
    if (str === "" || str === null) return;

    let haystack = str.map((i) => {
      return Number(i) ? Number(i) : i;
    });
    
    const operators = {
      "+": (a, b) => a + b,
      "-": (a, b) => a - b,
      "*": (a, b) => a * b,
      "/": (a, b) => a / b,
    };

    haystack.forEach(op => {
      result.push ( 
        operators[op] 
          ? operators[op](...result.splice(-2))
          : op
        )
    });
 
    return result[0];
}

console.log(reversePolish("3 4 + 2 * 1 +"));

Live Demo / Test / Refactoring