javascript problem with implicitly typed variables

Question

Forgive this novice question (novice in Javascript!).

In a html page I have a set of images with a name and a number: img1 img2 img3... img12... And there is a JS function where I need to iterate from (say) img5 to last img.

If I write this:

function iterateImg(objImgID) {var imgNum = objImgID.replace("img","");
    for (i=imgNum+1;i<20;i++)
    .../...

The variable "i" is treated as a string, and i+1 receive the value "51" if the passed object's ID is img5.

Of course I can still iterate from 1 to n and only perform the desired task when the value of i reaches 6, but that would be an ugly fix.

How can I fix this please? How can I create a variable that will be treated as an integer before I use it as seed? Besides, what would you recommend as best practice?

Don't forget to declare 'i' with var. If you don't, 'i' may get attached to the global scope (window). — Evert
– Evert, Commented Aug 29, 2011 at 19:14

Joseph Marikle · Accepted Answer · 2011-08-29 18:58:55Z

1

var imgNum = Number(objImgID.replace("img",""));

That'll do it.

Or better yet (because I love regex)

var imgNum = Number(objImgID.match(/[0-9]+/));

answered Aug 29, 2011 at 18:58

Joseph Marikle

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3 Comments

Didier Levy Over a year ago

Guys, I have to tank you all! 4 answers in less than 10 minutes, that's great!

JaredPar Over a year ago

Is there a functional difference between using Number and parseInt or is it just a matter of style?

JaredPar Over a year ago

For those interested. It looks like there is at least a minor functional difference. For example Number(true) === 1 and parseInt(true) results in NaN

JaredPar · Accepted Answer · 2011-08-29 18:59:41Z

1

Use parseInt to convert the string to a number

var imgNum = parseInt(objImgID.replace("img", ""));

answered Aug 29, 2011 at 18:59

JaredPar

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Comments

pimvdb · Accepted Answer · 2011-08-29 18:59:18Z

0

There are several ways to force JavaScript to convert it into a number

parseInt(x, 10) - the 10 is the base of the number
x - 0 - subtracting only works for numbers
x * 1 - multiplying also

answered Aug 29, 2011 at 18:59

pimvdb

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Darin Dimitrov · Accepted Answer · 2011-08-29 18:59:32Z

0

var imgNum = parseInt(objImgID.replace("img",""), 10);

Now when you do var i=imgNum+1, it will perform addition of 2 integers.

answered Aug 29, 2011 at 18:59

Darin Dimitrov

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2 Comments

Joseph Marikle Over a year ago

ok so why are you guys using parseInt() instead of Number()? I'm going to assume there's some kind of speed aspect or something?

Darin Dimitrov Over a year ago

@Joseph, performance would be pretty equivalent but if you are interested in concrete numbers take a look here: jsperf.com/number-vs-parseint-vs-plus/3

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