var patrn = \\D; // this is the regular expression that removes the letters

这不是JavaScript中的有效正则表达式.

This is not a valid regular expression in JavaScript.

您还缺少代码结尾的右括号.

You are also missing a closing bracket in the end of your code.

一种更简单的解决方案是找到字符串中的所有整数，将它们转换为数字(例如，使用+运算符)并将它们求和(例如，使用reduce运算符).

A simpler solution would be to find all integers in the string, to convert them into numbers (e.g. using the + operator) and summing them up (e.g. using a reduce operation).

var str = "12sf0as9d";
var pattern = /\d+/g;
var total = str.match(pattern).reduce(function(prev, num) {
  return prev + +num;
}, 0);

console.log(str.match(pattern)); // ["12", "0", "9"]
console.log(total);              // 21