在不使用eval的情况下计算javascript中的字符串

var str = "1 + 2 * 3";
var result = new Function("return " + str)();
console.log(result); // 输出 7

var str = "1 + 2 * 3";
var result = eval(str);
console.log(result); // 输出 7

var str = "1 + 2 * 3";
var tokens = str.split(" ");
var stack = [];
var operators = {
  "+": function(a, b) { return a + b; },
  "-": function(a, b) { return a - b; },
  "*": function(a, b) { return a * b; },
  "/": function(a, b) { return a / b; }
};

tokens.forEach(function(token) {
  if (operators[token]) {
    var b = stack.pop();
    var a = stack.pop();
    var result = operators[token](a, b);
    stack.push(result);
  } else {
    stack.push(parseFloat(token));
  }
});

var result = stack.pop();
console.log(result); // 输出 7