为什么此parseFloat无效？

Question

I'm very new to JavaScript so excuse my newbiness. But I'm currently running this code, to test some stuff out

var firstNumber = prompt("enter the first number ");
var secondNumber = prompt("enter the second number ");
var theTotal = firstNumber + secondNumber;
firstNumber = parseFloat(firstNumber);
secondNumber = parseFloat(secondNumber);
document.write(firstNumber + " added to " + secondNumber + " equals " +    theTotal);

Why doesn't that add the numbers up correctly? I know the proper way of doing it would be to wrap the parseFloat around the prompt, but I just want to know why this doesn't work. Thanks in advance.

Answer 1

Parse should be done before doing the add operation:

var firstNumber = prompt("enter the first number ");
var secondNumber = prompt("enter the second number ");
firstNumber = parseFloat(firstNumber);
secondNumber = parseFloat(secondNumber);
var theTotal = firstNumber + secondNumber;
document.write(firstNumber + " added to " + secondNumber + " equals " +    theTotal)

Answer 2

firstNumber + secondNumber正在添加strings ...尝试将firstNumber*1 + secondNumber*1快速转换为数字并查看结果。

Answer 3

When you add firstNumber to secondNumber, they're still strings; you're not converting them to numbers until after the addition. Do this instead:

firstNumber = parseFloat(firstNumber);
secondNumber = parseFloat(secondNumber);
var theTotal = firstNumber + secondNumber;
document.write(firstNumber + " added to " + secondNumber + " equals " +    theTotal);