Addition, subtraction and multiplication of values in JavaScript

Question

var apples = prompt('Please enter no. of apples');
var oranges = prompt('Please enter no. of oranges');
var fruits = apples + oranges;


document.write(fruits);

Why does it work with - and * and not +?

Thanks!

See also http://stackoverflow.com/q/17498883 http://stackoverflow.com/q/10414675 http://stackoverflow.com/q/22704963 http://stackoverflow.com/q/12044595 http://stackoverflow.com/q/21215122 and many, many other similar questions — p.s.w.g, Aug 12 '14 at 01:20

tadman · Answer 1 · 2014-08-12T01:19:53.937

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You're adding two strings together to get another string. That's how JavaScript does it.

Maybe what you want is numbers:

var fruits = parseInt(apples, 10) + parseInt(oranges, 10);

As a note, using prompt to collect information is utterly barbaric. What you need to do is have two input boxes and a submit trigger that does the math, or since it's so trivial, hook it up to trigger on any change to either value. jQuery basics here.

edited Aug 12 '14 at 01:19

answered Aug 12 '14 at 01:12

tadman

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`var fruits = parseInt(apples, 10) + parseInt(oranges, 10);` – Oliboy50 Aug 12 '14 at 01:14
@Oliboy50 A good point! – tadman Aug 12 '14 at 01:19
Or `parseFloat` to parse floats. – Derek 朕會功夫 Aug 12 '14 at 03:42
In this parseInt what does the 10 represent? – zenarthra Aug 12 '14 at 13:26
It forces base-10, as `parseInt` will sometimes get ideas and use other bases. For example: `parseInt("0x100")` is 256, but `parseInt("0x100", 10)` is 0. – tadman Aug 12 '14 at 15:32
Thanks for the reply, what does parseInt do? What is radix and base? – zenarthra Aug 12 '14 at 22:17
It's just the JavaScript standard [`parseInt()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt) function. Radix/base is the number system in question. Normal numbers are base-10. Octal is base-8, hexadecimal is base-16. – tadman Aug 13 '14 at 16:21

score 1 · Answer 2 · answered Aug 12 '14 at 01:13

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You're doing string concatenation with the + instead of an addition. Parse to float or int.

var fruits = parseFloat(apples) + parseFloat(oranages);

answered Aug 12 '14 at 01:13

EasyCo

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score 0 · Answer 3 · answered Aug 12 '14 at 01:12

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prompt returns a string, thus it's appending the strings together. Javascript will convert to an int when you try other operators on the variables.

Use parseInt to make sure they're ints.

answered Aug 12 '14 at 01:12

ChrisGheen

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Addition, subtraction and multiplication of values in JavaScript

3 Answers3