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Hi! My problem is that when you prompt a number and add it to another prompt is uses the "+" operator to join the lines. I want it to add the numbers together. How can I do this?

alert(prompt('add a number') + prompt('add another number'));

Thanks in advance, TheCoder

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    The prompt() function always returns a string, so you have to explicitly make the return values be numbers. You can do that by writing +prompt("...") instead of just prompt("...") Commented Jul 26, 2019 at 15:50

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You can convert a string to a number with the + operator, like so:

+"50" + +"40" = 90

The result will be NaN if the strings can't be converted to a number (ie, +"fifty" => NaN), so you'll want to account for that in your code.

alert(+prompt('add a number') + +prompt('add another number'));
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4 Comments

I would note that using the + operator like this is technically considered a "hack" (even though it's defined in the ECMAScript specification and therefore isn't subject to breaking like many hacks), and it can make your code less readable. I would instead suggest using parseInt or parseFloat
That's a nice shorthand
@stevendesu In my experience, the + operator is far more common than the other methods, so I would say that although it may be hacky, it's an accepted standard practice and is therefore probably more recognizable (although admittedly less readable).
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You need to use parseInt or parseFloat

alert(parseInt(prompt('add a number')) + prompt(parseInt('add another number')));
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parseFloat()?
It's parseFloat() not parseDouble(), also those functions are not always the best things to use because they allow trailing non-numeric text; "123hello" will not cause an error.
I would maybe extend this answer by adding a check that the parseInt actually succeeded -- otherwise you risk adding NaN, or as @Pointy mentioned, falsely detecting 123hello as a number. Something like var input_one = prompt(...); var input_two = prompt(...); if (!isNaN(parseInt(input_one)) && parseInt(input_one).toString() === input_one && !isNaN(parseInt(input_two)) && parseInt(input_two).toString() === input_two) { alert(parseInt(input_one) + parseInt(input_two)); };
@stevendesu In something more complex, I agree checking for NaN does make sense... maybe throw an exception. But for a simple case, I think if you ask the user for 2 numbers to sum together and they enter bad input, NaN to me is fine