• 0

Passing a variable to a jQuery selector


Question

I have some JavaScript code that loops through a record set using a for loop. The ID of each record is assigned dynamically using this script. For example:

id="header1", id="header2", id="header3"

The numeral at the end of the id is assigned via script.

I also need to select these IDs via jQuery but I do not understand how to insert a variable into my jQuery selector.

I have tried the following (simplified for clarity):

$("#header[i]")...;
$("#header"+[i])...;
$("#header+[i]")...;
$("#header"+i)...;

None of these things seem to work as jQuery doesn't select the objects I'm trying to get.

Any ideas on how to dynamically create jQuery objects using a counter variable?

10 answers to this question

Recommended Posts

  • 0

I can't get this technique to work.

My code:

var currentIMHeader = "IM-header"+[i];
			var currentIMData = "IMData"+[i];
			alert("binding a function to "+currentIMData);
			$(currentIMHeader).toggle( function() {
				alert("showing "+currentIMData);
				$(currentIMData).show();
				}, function() {
				alert("hiding "+currentIMData);
				$(currentIMData).hide();
				});

The alerts are there for testing purposes. I have also tried passing a "#" to the selector as I am trying to select based on ID.

  • 0

This is a basic thing on JavaScript syntax, not on jQuery really.

var i = 5;

$("#header"+i).fadeOut();

I see no reason why this would not work - fading out the element with an ID of header5.

Edited by Rob
  • 0
  Rob said:
This is a basic thing on JavaScript syntax, not on jQuery really.

var i = 5;

$("#header"+i).fadeOut();

I see no reason why this would not work - fading out the element with an ID of header5.

It might work outside of a for loop but it doesn't seem to work in one.

The following code with hardcoded jQuery selectors should bind a function to the "defName0" object but it doesn't bind when used within a for loop.

$("#defName0").bind("click", function() {
			 $("#defValue0").fadeIn("normal");
			 });

I'm completely stuck.

Is there some basic incompatibility betwen standard JS For loops and jQuery? jQuery has an "Each()" function that I have not used yet - is this the path to salvation?

  • 0

Quick usage note:

$(".headers").fadeOut();

This would fade out all elements with a class of 'headers'.

$(".headers").each(function() { 
  alert("Fading out element: "+$(this).attr("id"));
  $(this).fadeOut(); 
} );

That would find all elements with a class of 'headers', alert to the screen their ID, and fade them out.

  • 0

To append on what Rob posted, you can put a variable in function() to have an iterated value for the loop

$(".headers").each(function(i) {
  alert("Fading out element "+i+": "+$(this).attr("id"));
  $(this).fadeOut();
} );

Do note that it works like an indexed array, so i will start at 0, not at 1.

  • 0
  raskren said:
When passing the counter variable "i" to my jQuery function is there a need to reference the variable using brackets ""?

$(".headers").each(function([i]) {

No, the only time (that I know of) that it is necessary is for arrays or lists:

var locations = Array();
locations[0] = 'Neowin.net';

var sites = ["Neowin.net", "Microsoft.com"];

  • 0

Long explanation incoming.

jQuery is JavaScript. All JavaScript rules apply to everything jQuery. jQuery doesn't change how the language (and interpreters) function and behave. Case in point:

var i = 5
$("#myElement" + i).bind(...);

In the above code, "#myElement" + i always returns the string #myElement5. Therefore, the jQuery function $() receives the string #myElement5 as a parameter.

The code your provided has some issues with it. I won't say that what you did is wrong, because you wrote it the way you thought it would work. It's wrong in the sense that the interpreter interprets the code differently than what you thought it would. Your code follows; I added the for loop because I assumed that's what you're doing:

for (var i = 0; i < 5; i++) {
	var currentIMHeader = "IM-header"+[i];
	var currentIMData = "IMData"+[i];
	alert("binding a function to "+currentIMData);

	$(currentIMHeader).toggle(
		function() {
			alert("showing "+currentIMData);
			$(currentIMData).show();
		}, 
		function() {
			alert("hiding "+currentIMData);
			$(currentIMData).hide();
		}
	);
}

First, is used incorrectly. The [] operators are for arrays (and objects, but we won't go there) to pick an item in the array at a specific index. For instance, say I have an array with ten items. You use the [] operators to pick which item you want from the array:

alert(myArray[5]); // alerts the sixth item in the array

You don't receive an error from the usages above, but it is incorrect.

The issues in your code are scope and closures. Variables defined in a function are function scoped... meaning any variable you define in the function can be used anywhere in that function as long as the variable is defined. Take the following code samples as an example:

alert(i); // alerts undefined

for (var i = 0; i < 5; i++) {
	alert(i); // alerts 0, 1, 2, 3, or 4
}

alert(i); // alerts 5 (because the loop iterated i to 5, then exited because 5 isn't less than 5)

...

alert(foo); // undefined

if (true) {
	var foo = "bar";
}

alert(foo); // alerts bar

Even though the i and foo variables are defined within a block (designated by {}) of code, they can be used outside that block and still retain their value. That in itself doesn't cause the problem you see, but throw in a closure and things can start to behave differently than you'd think.

Looking back at your code, the function declarations for the toggle() method are closures. The problem comes from using the currentIMData variable within the enclosed functions. Each function passed to the toggle() method contains a reference to the same currentIMData variable, and currentIMData's value changes with each iteration of the loop. So in actuality, after the loop exists, this is what the the interpreter sees for every heading when toggle() is called:

function() {
	alert("showing IMData5");
	$("IMData5").show();
}, 
function() {
	alert("hiding IMData5");
	$("IMData5").hide();
}

The way to get around this issue is, ironically, another closure; a function to return a function. The code below works:

for (var i = 0; i < 5; i++) {
	var currentIMHeader = "IM-header"+ i;
	var currentIMData = "IMData"+ i;
	alert("binding a function to "+currentIMData);

	$(currentIMHeader).toggle(
		function(elementId) { // function that accepts a parameter
			return function() { // the function to return
				alert("showing "+ elementId);
				$(elementId).show(); // use the variable from the parameter
			};
		}(currentIMData), // call the function and pass currentIMData as the parameter
		function(elementId) { // function that accepts a parameter
			return function() { // the function to return
				alert("hiding "+ elementId);
				$(elementId).hide(); // use the variable from the parameter
			};
		}(currentIMData) // call the function and pass currentIMData as the parameter
	);
}

This topic is now closed to further replies.
  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Posts

    • It's weird how some people don't know how to read the article... "If you remember the tests SpaceX was performing around 2012 with Grasshopper, well Honda is at about the same stage with its reusable rocket."
    • Microsoft Project 2021 Professional for Windows now just $9.97 by Steven Parker Today's highlighted deal comes from our Apps & Software section of the Neowin Deals store, where you can save $240 off Microsoft Project 2021 Professional for Windows. Be the boss of any project with Microsoft Project Professional 2021. This powerful yet easy-to-use app lets you stay on track, from small tasks to larger projects. Manage your projects more efficiently with the right timelines, budgets, and resources. You'll get automated scheduling tools as well as built-in reports to help you reduce inefficiencies and make better, informed decisions. Project management has never been this easy with Microsoft Project Pro. Lifetime license for Microsoft Project Professional 2021 One-time purchase installed on 1 Windows PC for use at home or work Instant Delivery & Download – access your software license keys and download links instantly Free customer service – only the best support! Why Microsoft Project Professional 2021? Pre-built templates help you get your project started on the right track Sync with Project Online and Project Server Submit timesheets to capture time spent on project and non-project work Run what-if scenarios to make the most of your task assignments Auto-populate start and end dates based on dependencies Visually represent complex schedules with built-in multiple timelines Supports Long-Term Servicing Channel (LTSC) and is compatible with Office LTSC and Office 2021 Good to know Length of access: lifetime Redemption deadline: redeem your code within 30 days of purchase Access options: desktop Max number of device(s): 1 Version: 2021 Pro Updates included Microsoft Project 2021 Professional for Windows normally costs $249.99, but it can be yours for just $14.97 for a limited time, that's a saving of $235. For terms, specifications, and license info please click the link below. Use MSO5 when checking out for additional $5 off. Coupon Expires June 29 Get Microsoft Project 2021 Professional for Windows for just $9.97, or learn more Although priced in U.S. dollars, this deal is available for digital purchase worldwide. We post these because we earn commission on each sale so as not to rely solely on advertising, which many of our readers block. It all helps toward paying staff reporters, servers and hosting costs. Other ways to support Neowin Whitelist Neowin by not blocking our ads Create a free member account to see fewer ads Make a donation to support our day to day running costs Subscribe to Neowin - for $14 a year, or $28 a year for an ad-free experience Disclosure: Neowin benefits from revenue of each sale made through our branded deals site powered by StackCommerce.
    • Weird how some people don't need to crash 7 rockets before they get it right.
    • Or it's a bug that can easily be fixed with an update. My guess is the limitation is there to prevent game makers from doing dumb things like having a racing game constantly rumble while the car is in motion. Sounds like the threshold or detection needs to be tweaked if it is triggering for no reason.
    • Nice I guess. Azure's DaaS feature has supported that for years. It's so odd to me how MS is deploying Azure and 365 DaaS as two totally different services with different features. I would think the 365 cloud desktops would just be a slightly dumbed down frontend that manages the same service.
  • Recent Achievements

    • Week One Done
      slackerzz earned a badge
      Week One Done
    • Week One Done
      vivetool earned a badge
      Week One Done
    • Reacting Well
      pnajbar earned a badge
      Reacting Well
    • Week One Done
      TBithoney earned a badge
      Week One Done
    • First Post
      xuxlix earned a badge
      First Post
  • Popular Contributors

    1. 1
      +primortal
      676
    2. 2
      ATLien_0
      286
    3. 3
      Michael Scrip
      224
    4. 4
      +FloatingFatMan
      197
    5. 5
      Steven P.
      137
  • Tell a friend

    Love Neowin? Tell a friend!