Source

File: wp-includes/class-wp-customize-setting.php

public function value() {
		$id_base      = $this->id_data['base'];
		$is_core_type = ( 'option' === $this->type || 'theme_mod' === $this->type );

		if ( ! $is_core_type && ! $this->is_multidimensional_aggregated ) {

			// Use post value if previewed and a post value is present.
			if ( $this->is_previewed ) {
				$value = $this->post_value( null );
				if ( null !== $value ) {
					return $value;
				}
			}

			$value = $this->get_root_value( $this->default );

			/**
			 * Filters a Customize setting value not handled as a theme_mod or option.
			 *
			 * The dynamic portion of the hook name, `$id_base`, refers to
			 * the base slug of the setting name, initialized from `$this->id_data['base']`.
			 *
			 * For settings handled as theme_mods or options, see those corresponding
			 * functions for available hooks.
			 *
			 * @since 3.4.0
			 * @since 4.6.0 Added the `$this` setting instance as the second parameter.
			 *
			 * @param mixed                $default The setting default value. Default empty.
			 * @param WP_Customize_Setting $this    The setting instance.
			 */
			$value = apply_filters( "customize_value_{$id_base}", $value, $this );
		} elseif ( $this->is_multidimensional_aggregated ) {
			$root_value = self::$aggregated_multidimensionals[ $this->type ][ $id_base ]['root_value'];
			$value      = $this->multidimensional_get( $root_value, $this->id_data['keys'], $this->default );

			// Ensure that the post value is used if the setting is previewed, since preview filters aren't applying on cached $root_value.
			if ( $this->is_previewed ) {
				$value = $this->post_value( $value );
			}
		} else {
			$value = $this->get_root_value( $this->default );
		}
		return $value;
	}