cannot use + when try to add number

Question

After i selected option 1 and after i add the number like 100 it results like this 100100

This is the code:

var userAmount;
    var userMoney = 100;
    var totalMoney;
    alert("1. Add");
    alert("2. Subtraction");
    var userOption = prompt("Enter your selection: ");
        if (userOption == 1) {
            userAmount = prompt("Amount: ");
            var totalMoney = userMoney + userAmount;
            alert("Total is " + totalMoney);
        }
        else {
            userAmount = prompt("Amount: ");
            var totalMoney = userMoney - userAmount;
            alert("Total is" + totalMoney);
        }

`prompt` returns string, not a number – Grundy Dec 21 '13 at 13:08 — Grundy, Dec 21 '13 at 13:08

Rahul Tripathi · Accepted Answer · 2013-12-21T13:22:40.963

Because there is an implicit conversion when you are using +. In your case + is actually concatenating the values.

The function prompt returns a string so when you are adding it to an int it is concatenating the values. It takes both the values as string and concatenates the value instead of adding it.

MDN says that:

String operators

In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings. For example, "my " + "string" returns the string "my string".

You need to cast it like Number(userAmount) Something like:

var totalMoney = userMoney + Number(userAmount);

or

userAmount = parseFloat(prompt("Amount: "));

so you need to cast both the values to an int so that + operator behaves as addition instead of string concatenation.

@Mr.AZ: welcome to [so]. To mark a solution as the best one for you (the questioner), you can accept an answer by clicking the big tick just underneath the vote arrows on the left of an answer. I'd highly recommend taking a quick look at what [so] is [about] -- it's just a quick intro into how our community runs. Good luck -- it's great that we were able to answer your question! — Qantas 94 Heavy, Dec 24 '13 at 02:17

score 2 · Answer 2 · answered Dec 21 '13 at 13:08

A common mistake for people new to JavaScript is they forget almost every value taken from user input is a String. You'll need to convert these into numbers before you can use the addition + operator. This conversion can be done with the unary + or parseFloat, for example

userAmount = parseFloat(prompt("Amount: "));

When using Strings in JavaScript, the + operator is actually string concatenation

score 2 · Answer 3 · answered Dec 21 '13 at 13:09

2

You need to cast your values to integers first. Try using parseInt()

change:

var totalMoney = userMoney + userAmount;

to this:

var totalMoney = parseInt(userMoney, 10) + parseInt(userAmount, 10);

answered Dec 21 '13 at 13:09

John Lay

301
4
12

You shouldn't need to convert userMoney though - its already a number. – hugomg Dec 21 '13 at 13:15

score 1 · Answer 4 · answered Dec 21 '13 at 13:09

1

That's because the user given number is actually a string.

A quick way to fix it is to type:

var totalMoney = userMoney + userAmount * 1;

answered Dec 21 '13 at 13:09

pstenstrm

6,339
5
41
62

cannot use + when try to add number

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