Source

File: wp-includes/general-template.php

function wp_default_editor() {
	$r = user_can_richedit() ? 'tinymce' : 'html'; // Defaults.
	if ( wp_get_current_user() ) { // Look for cookie.
		$ed = get_user_setting( 'editor', 'tinymce' );
		$r  = ( in_array( $ed, array( 'tinymce', 'html', 'test' ), true ) ) ? $ed : $r;
	}

	/**
	 * Filters which editor should be displayed by default.
	 *
	 * @since 2.5.0
	 *
	 * @param string $r Which editor should be displayed by default. Either 'tinymce', 'html', or 'test'.
	 */
	return apply_filters( 'wp_default_editor', $r );
}