javascript - $getJSON and for loop issue

Question

This is to populate a table with the amount of results that are returned from the MediaWiki API query /api.php?action=query&list=querypage&qppage=BrokenRedirects. The number of results is then added to the id, for example:

// BrokenRedirects
$.getJSON('/api.php?action=query&list=querypage&qppage=BrokenRedirects&format=json', function (data) {
    $('#BrokenRedirects').text(data.query.querypage.results.length);
});

But as it's being repeated another 7 times I made the arguments for qppage into an array and used a for loop to shorten overall code.

var array = ['BrokenRedirects',
             'DoubleRedirects',
             'Unusedcategories',
             'Unusedimages',
             'Wantedcategories',
             'Wantedfiles',
             'Wantedpages',
             'Wantedtemplates'];

for (var i = 0; i < array.length; i++) {
    $.getJSON('/api.php?action=query&list=querypage&qppage=' + array[i] + '&format=json', function (data) {
        $('#' + array[i]).text(data.query.querypage.results.length);
    });
}

The first, unlooped, version works. But when I added a loop it didn't. The $getJSON part executes, but it then fails to add the resultant data to the id. I ran it through JSLint which apart from complaining about functions in a loop and declaring var i with var array returned little help. I'm relatively inexperienced with javascript so thought perhaps a variable can't be used twice within a loop? Other than that, maybe something to do with using an id within a loop?

Answer 1

That's a classical problem : i has the value of end of loop when the callback is called.

You can fix it like this :

for (var i = 0; i < array.length; i++) {
    (function(i) { // protects i in an immediately called function
      $.getJSON('/api.php?action=query&list=querypage&qppage=' + array[i] + '&format=json', function (data) {
        $('#' + array[i]).text(data.query.querypage.results.length);
      });
    })(i);
}

2018 addendum:

There's now another cleaner solution in today's browsers: use let instead of var:

for (let i = 0; i < array.length; i++) {
    $.getJSON('/api.php?action=query&list=querypage&qppage=' + array[i] + '&format=json', function (data) {
        $('#' + array[i]).text(data.query.querypage.results.length);
    });
}