The is main query() function is a conditional function that determines whether the current query is the "main" query, such as when it is being evaluated inside of a loop (as opposed to a secondary query).
string|string[] $post types = ""; is singular(string|string[]): bool. determines whether the request is for a single post of any post type that already exists (post, attachment, page, custom post types).
$("#checkAll").change(function(){
if (! $('input:checkbox').is('checked')) {
$('input:checkbox').prop('checked',true);
} else {
$('input:checkbox').prop('checked', false);
}
});
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