How to generate dynamic strings in javascript closure call

Question

I'm creating a long list of select items and wanted to create a closure call to add each option, then queue the call (via a setTimer) so the browser would not hang.

My implementation works great, but here is what has me puzzled, the following code:

var mainList = $('#mainList');
for (var i=0;i < 100000; i++) {
  var self = mainList, addOption = function() {
    $(self).append('<option value=' + i + '>' + i + '</option>');
   };

  $.queue.add(addOption, this);
}

generates:

<option value='100000'>100000</option>
<option value='100000'>100000</option>
<option value='100000'>100000</option>  etc...

Where I would like to have it generate the options:

<option value='1'>1</option>
<option value='2'>2</option>
<option value='3'>3</option>  etc...

I'm struggling with how the closure is executed, it makes sense that the addOption() method is called when i == 100000, but I would like the call to reflect the value of i at the time it is queued up.

Is there an easy trick I am missing here?

score 2 · Accepted Answer · edited May 23 '17 at 12:06

2

Set addOption by IIFE which returns a function

var mainList = $('#mainList');
for (var i = 0; i < 100000; i++) {
    var self = mainList,
        addOption = (function(i) {
            return function() {
                $(self).append('<option value=' + i + '>' + i + '</option>');
            }
        })(i);
    $.queue.add(addOption, this);
}

edited May 23 '17 at 12:06

Community

1
1

answered Aug 27 '15 at 20:57

AmmarCSE

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Pure awesomeness @AmmarCSE, thanks a lot. I'm going to read up on the IIFE and how the js executes right now, that's exactly the functionality I was looking for. – Mark Kadlec Aug 27 '15 at 21:05
@MarkKadlec, great, glad it helped :) – AmmarCSE Aug 27 '15 at 21:07

score 2 · Answer 2 · answered Aug 27 '15 at 21:24

Prior to the introduction of Function.prototype.bind, it was certainly necessary to employ a closure specifically to trap one or more vars in situations like this.

Now, that approach is considered to be a bit clumsy.

You can define a basic addOption function, then spawn versions of it in the loop, using addOption.bind() to bind in both mainList as "thisArg", and i as the first formal variable :

var mainList = $('#mainList');
var addOption = function(i) {
    this.append('<option value=' + i + '>' + i + '</option>');
};
for (var i=0; i<100000; i++) {
    $.queue.add(addOption.bind(mainList, i), this);
}

That's very nice and looks cleaner (and easier to read) than my older way. Thanks! — Mark Kadlec, Aug 27 '15 at 21:31

How to generate dynamic strings in javascript closure call

2 Answers2