Javascript wrong output in browser

Question

I'm working on a Celsius to Fahrenheit function in JavaScript and I wrote this code:

<!DOCTYPE html>
<html>
<head>
<title>Celius to Fahrenheit Converter</title>
</head>
<body>

<script type="text/javascript">

alert("Welcome");

function tempConverter() {
var degCent = prompt("Enter the degrees in Farenheit", 50)
var degFahren;

degFahren = 9/5 * (degCent + 32);

alert(degFahren);

}

tempConverter();

</script>

</body>
</html>

I tried using 50 as the default value and when i converted it shows 9057.6 instead of 122. Any Ideas?

prompt() returns a string and + also performs concatenation ("50" + 32 === "5032"). What's the fastest way to convert String to Number in JavaScript? — Jonathan Lonowski
– Jonathan Lonowski, Commented Jun 2, 2015 at 0:34

RaphaelDDL · Accepted Answer · 2015-06-02 00:36:57Z

2

You need to convert the input to an integer -

var degCent = parseInt(prompt("enter", 50) , 10);

edited Jun 2, 2015 at 0:36

RaphaelDDL

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answered Jun 2, 2015 at 0:36

rosscj2533

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Comments

Greg Hewgill · Accepted Answer · 2015-06-02 00:36:48Z

1

The degCent value is being treated as a string. Javascript helpfully does string concatenation when evaluating the expression:

(degCent + 32) = "5032"

Only when starting to do multiplication and division does it then convert the above result to a number to do arithmetic. This is an unexpected (but documented) behaviour of Javascript.

To fix this, force the degCent value to be a number by doing something like:

(+degCent + 32)

The leading + converts the string "50" to a number before adding 32.

answered Jun 2, 2015 at 0:36

Greg Hewgill

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3 Comments

defc0de Over a year ago

thanks for pointing that out, I'm just confused, why is it treating it as a string when the code for my Fahrenheit to Celsius function is working correctly? I have the exact code for it, only the equation is different..

Greg Hewgill Over a year ago

Because <opinion>Javascript has dumb type handling</opinion>. You just have to deal with it, Javascript can't really be fixed at this point.

defc0de Over a year ago

lol, ok I understand. as a beginner it's just making me so confused right away. Anyway thanks for all the help.

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