Source

File: wp-includes/functions.php

function wp_send_json( $response, $status_code = null ) {
	if ( defined( 'REST_REQUEST' ) && REST_REQUEST ) {
		_doing_it_wrong(
			__FUNCTION__,
			sprintf(
				/* translators: 1: WP_REST_Response, 2: WP_Error */
				__( 'Return a %1$s or %2$s object from your callback when using the REST API.' ),
				'WP_REST_Response',
				'WP_Error'
			),
			'5.5.0'
		);
	}

	if ( ! headers_sent() ) {
		header( 'Content-Type: application/json; charset=' . get_option( 'blog_charset' ) );
		if ( null !== $status_code ) {
			status_header( $status_code );
		}
	}

	echo wp_json_encode( $response );

	if ( wp_doing_ajax() ) {
		wp_die(
			'',
			'',
			array(
				'response' => null,
			)
		);
	} else {
		die;
	}
}