Use a variable for the index of an array in javascript

Question

Here is my code:

var span = getSpanWithClass("galleria-current");
var slideNumber = span.innerHTML
var imageIDDivs = document.getElementsByClassName("slideshowImage");
var singleimageidDiv = imageIDDivs[slideNumber]

If I use this, sigleImageidDiv doesn't have anything in it. If I just put 0 or 1 in like this:

var singleimageidDiv = imageIDDivs[1]

It works fine. slideNumber is 1, in my test cases.

I have tried these as well:

var singleimageidDiv = imageIDDivs[Number(slideNumber)]
var singleimageidDiv = imageIDDivs[parseInt(slideNumber)]

What is the proper way to use a variable as the index of an array?

• Wex: If you're copying the code directly from the source, you're missing semi-colons at the end of most of your lines. You should fix the syntax of your code before you start looking at other reasons why it might not be working.
• Josh Mein: Semicolons are not required in javascript; however, it is definitely good practice to use them.
• gdoron: @JoshMein. Here comes am not I am comments saying: "I uses semicolons only at Stack Overflow..."
• Bergi: What is the innerHTML of that span? That would be the information of interest to solve your problem.
• gdoron: @user1308743. Then it should work. DEMO

Answer 1

Parse to int with a radix.

This is the proper way of doing it:

imageIDDivs[parseInt(slideNumber, 10)];

Live DEMO If it doesn't work, then your problem is somewhere else.

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BTW indexes work with strings as well. DEMO

• Kyle: I tried parseInt without the ,10 and it did not work. with the ,10, it works fine. Thank you!
• gdoron: @user1308743. With which browser did you try it? 10 is the default radix. can you create a not working DEMO at jsfiddle.net ?

Answer 2

innerHTML returns the string inside the element, so if even if you have a number inside your element, it'll be returned as a string, that's why when you use that variable as an index for your array, you don't get a result. Using parseInt is an acceptable solution, you could also just do something like span.innerHTML * 1, that would force JS to treat the string as an integer.

• Bergi: Array indizes are strings as every other property names. No need for a number.
• apsillers: @Berji Didn't know that! Shoot.
• Deleteman: Almost, but not exactly. Arrays are objects, and thus, if you use a string as an index, you actually add a property that you could retrieve using that same string index. That being said, "1" and 1 are two different indizes for an array.
• apsillers: Are they? This fiddle seems to disagree, but maybe I am misunderstanding you.
• Deleteman: @apsillers Your example works for me too, but everything I've read about JS says otherwise, don't know what to tell you.

Answer 3

I think, you have an error in your getSpanWithClass function.