WordPress at a glance
function is not described

WP_Customize_Setting::_preview_filter() public WP 3.4.0

Callback function to filter non-multidimensional theme mods and options.

If switch_to_blog() was called after the preview() method, and the current site is now not the same site, then this method does a no-op and returns the original value.

{} It's a method of the class: WP_Customize_Setting{}

No Hooks.

Return

Mixed. New or old value.

Usage

$WP_Customize_Setting = new WP_Customize_Setting();
$WP_Customize_Setting->_preview_filter( $original );
$original(mixed) (required)
Old value.

Changelog

Since 3.4.0 Introduced.

Code of WP_Customize_Setting::_preview_filter() WP 5.7.2

<?php
public function _preview_filter( $original ) {
	if ( ! $this->is_current_blog_previewed() ) {
		return $original;
	}

	$undefined  = new stdClass(); // Symbol hack.
	$post_value = $this->post_value( $undefined );
	if ( $undefined !== $post_value ) {
		$value = $post_value;
	} else {
		/*
		 * Note that we don't use $original here because preview() will
		 * not add the filter in the first place if it has an initial value
		 * and there is no post value.
		 */
		$value = $this->default;
	}
	return $value;
}