WordPress at a glance
filter is not described

pre_get_document_title filter-hook . WP 4.4.0

Filters the document title before it is generated.

Passing a non-empty value will short-circuit wp_get_document_title(), returning that value instead.

Usage

add_filter( 'pre_get_document_title', 'filter_function_name_8042' );
function filter_function_name_8042( $title ){
	// filter...

	return $title;
}
$title(string)
The document title.
Default: empty string

Where the hook is called

wp_get_document_title()
pre_get_document_title
wp-includes/general-template.php 959
$title = apply_filters( 'pre_get_document_title', '' );

Where the hook is used (in WP core)

Does not used.