How to show top-level product categories in wordpress with get_term_children()?

I know that i can use get_term_children() function to get childs for some parent for some taxonomy. For example, the taxonomy can be a product category.

For example, to display children product categories for a parent product category with ID = 70 i can use:

$taxonomyName = "product_cat";
    $termID = "70";
    $termchildren = get_term_children( $termID, $taxonomyName );
    var_dump($termchildren);
    echo '<ul>';
    foreach ($termchildren as $child) {
        $term = get_term_by( 'id', $child, $taxonomyName );
        echo '<li><a href="' . get_term_link( $term->term_id, $term->taxonomy ) . '">' . $term->name . '</a></li>';
    }
    echo '</ul>';

I tried, the code is working. But what is the id of "THE VERY-VERY TOP / BASE / 0-level" product category in wp? I want to show only first-level it's parents. I tried "0" and "-1" as $termID, but no luck - it shows nothing. What is the right value?

Try this:

<?php

  $args = array(
         'taxonomy'     => 'product_cat',
         'orderby'      => 'name',
         'show_count'   => false, // true or false
         'pad_counts'   => false, // true or false
         'hierarchical' => false, // true or false
         'title_li'     => '',
         'hide_empty'   => true // true or false
  );
 $product_categories = get_categories( $args );
 echo '<ul>';
 foreach ($product_categories as $category) {
    if($category->category_parent == 0) { //this checks for 1st level that you wanted
      echo '<a href="'. get_term_link($category->term_id, 'product_cat') .'">'. $category->name .'</a>';
    }       
 }
 echo '</ul>';
 ?>

References:

1. get_categories
2. get_terms