Javascript perform arithmetic on captured regexp

Question

I want to replace my title from its default to "*number* new messages | default"

Here is the code I have, it changes from default to 1 new messages fine, but it never goes above 1.

title = $('title').text();
regexp = /^(\d+)/
if (title.match(regexp))
 $('title').text().replace(regexp, (parseInt("1ドル")+1).toString())
else
 $('title').text('1 New Messages | Default')

• Brad Christie: Just curious, Why are you using match over test in this implementation? Also, can you provide an example (or examples) of what $('title').text() may have? (Finally, are you sure it's not #title or .title as the selector?)
• aziz punjani: Why are you doing this parseInt("1ドル") ? That will give you NaN
• aziz punjani: @Brad, title - is the title of the page, that selector should work.
• Brad Christie: @Interstellar_Coder: Touche, I had tunnel-vision and was thinking of only the body's contents. Good call.

Answer 1

You should be using a function as the second argument to replace, you also need to set the new value:

var title = $('title').text();
$('title').text(title.replace(regexp, function(m) { return parseInt(m, 10) + 1) }));

And, as usual, don't forget the radix argument when you call parseInt or you will be unpleasantly surprised sooner or later.

• Gazler: +1 I was not aware of the function as a 2nd argument. This is a neat solution.
• LanguagesNamedAfterCofee: Thanks, this worked! Also thanks for mentioning that I shouldn't assume that parseInt will assume base 10 by default in all browsers.
• mu is too short: @LanguagesNamedAfterCofee: I tend to think that leaving the second argument to parseInt optional is a bug in the JavaScript spec. You almost never want parseInt to guess the radix so parseInt's interface is (IMHO) broken by design. Can't change it now though :)

Answer 2

I came across this recently and the issue is to do with calling a function within the second argument of the replace function. The 1ドル is only valid if called directly in a string literal, not as a string argument to a function.

Another issue is that $('title').text().replace(regexp, (parseInt("1ドル")+1).toString()) will return a string. You need to pass the value you want to assign the text to as a function argument of the text() function like you have done in your else block.

Try this:

title = $('title').text();
regexp = /^(\d+)/
if (numToReplace = title.match(regexp))
 $(title).text(title.replace(regexp, (parseInt(numToReplace[1])+1).toString()))
else
 $('title').text('1 New Messages | Default')

And a JSFiddle: http://jsfiddle.net/KFW4G/

Answer 3

replace returns a new string, so you have to use text again to set the text to the result.