toFixed 方法不适用于用户输入，但适用于 javascript 中的其他变量，包括片段

Question

我想要反馈为什么当附加到用户输入 var usergb 时 toFixed 在我的 JS 警报中不起作用。是一件很奇怪的事情。有人鼓励我发帖，但我在这里没有看到任何其他关于用户输入变量的 toFixed 问题的问题。

var downloadspeed = function () {
        var usergb = prompt("How many gigabytes do you wish to download at 56 kilobits per second?");
        console.log(usergb);
        var hours = (usergb*8388608)/201600;
        console.log(hours);
        var days = (usergb*8388608)/4838400;
        console.log(days);

    //below in the alert is where you will see it, after the first concatenated string, I'd like to type usergb.toFixed(2) but it won't take.


        alert("The number of hours it will take to download a " + usergb + " GB file at 56 kilobits per second is " + hours.toFixed(2) + " hours. This might be a big number. So, in terms of days to download, it'll take "  + days.toFixed(2) + " days to download. This is why we have faster broadband speeds nowadays and why we have video streaming.");

    }
    downloadspeed();

我得到的错误是它不是一个函数。

格雷西，卢

Answer 1

toFixed 是一个数字上的方法...

prompt 返回一个字符串

试试parseFloat(usergb).toFixed(2)

Answer 2

你必须在号码上使用toFixed()：

parseFloat(usergb).toFixed(2)

提示函数返回一个字符串，不能对字符串使用toFixed()，只能对数字使用。

Answer 3

您需要将输入转换为数字：

var usergb = +prompt("How many gigabytes do you wish to download at 56 kilobits per second?");

Answer 4

您可以解析 Number 类型的输入。 Example plunkr

var downloadspeed = function() {
  var input = prompt("How many gigabytes do you wish to download at 56 kilobits per second?");

  var usergb = new Number(input);
  if(isNaN(usergb))
  {
    alert("You must input a number");
    return;
  }

  console.log(usergb);
  var hours = (usergb * 8388608) / 201600;
  console.log(hours);
  var days = (usergb * 8388608) / 4838400;
  console.log(days);

  //below in the alert is where you will see it, after the first concatenated string, I'd like to type usergb.toFixed(2) but it won't take.


  alert("The number of hours it will take to download a " + usergb + " GB file at 56 kilobits per second is " + hours.toFixed(2) + " hours. This might be a big number. So, in terms of days to download, it'll take " + days.toFixed(2) + " days to download. This is why we have faster broadband speeds nowadays and why we have video streaming.");

}
downloadspeed();