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function is not described

wp_oembed_ensure_format() WP 4.4.0

Ensures that the specified format is either 'json' or 'xml'.

No Hooks.

Return

String. The format, either 'xml' or 'json'.

Usage

wp_oembed_ensure_format( $format );
$format(string) (required)
The oEmbed response format. Accepts 'json' or 'xml'.

Code of wp oembed ensure format: wp-includes/embed.php WP 5.2.2

<?php
function wp_oembed_ensure_format( $format ) {
	if ( ! in_array( $format, array( 'json', 'xml' ), true ) ) {
		return 'json';
	}

	return $format;
}