Auto set default variants for WooCommerce products. Copy to woocommerce/single-product/add-to-cart/variable.php

variable.php

<?php
/**
Put this file to folder of your theme to overide default option
woocommerce/single-product/add-to-cart/variable.php
*
* Check "IMPORTANT HERE" part to set default variant with attribute. Else code will set first variant to default.
*
*
* By Anthony Pham from SalesGen.io
*
 */

defined( 'ABSPATH' ) || exit;

global $product;

$attribute_keys  = array_keys( $attributes );
$variations_json = wp_json_encode( $available_variations );
$variations_attr = function_exists( 'wc_esc_json' ) ? wc_esc_json( $variations_json ) : _wp_specialchars( $variations_json, ENT_QUOTES, 'UTF-8', true );

do_action( 'woocommerce_before_add_to_cart_form' ); ?>

<form class="variations_form cart" action="<?php echo esc_url( apply_filters( 'woocommerce_add_to_cart_form_action', $product->get_permalink() ) ); ?>" method="post" enctype='multipart/form-data' data-product_id="<?php echo absint( $product->get_id() ); ?>" data-product_variations="<?php echo $variations_attr; // WPCS: XSS ok. ?>">
	<?php do_action( 'woocommerce_before_variations_form' ); ?>

	<?php if ( empty( $available_variations ) && false !== $available_variations ) : ?>
		<p class="stock out-of-stock"><?php echo esc_html( apply_filters( 'woocommerce_out_of_stock_message', __( 'This product is currently out of stock and unavailable.', 'woocommerce' ) ) ); ?></p>
	<?php else : ?>
		<table class="variations" cellspacing="0">
			<tbody>
				<?php 
				// IMPORTANT HERE!!!!
				$default_option = array('hoodie', 's'); //Forcus variant will be default ELSE set first variant to default.
				
				foreach ( $attributes as $attribute_name => $options ) : ?>
					<tr>
						<td class="label"><label for="<?php echo esc_attr( sanitize_title( $attribute_name ) ); ?>"><?php echo wc_attribute_label( $attribute_name ); // WPCS: XSS ok. ?></label></td>
						<td class="value">
							<?php
								$selected = $options[0];

								foreach ($options as $option) {
									if ( in_array(strtolower($option), $default_option) ) {
										$selected = $option;
										break;										
									}
								}

								wc_dropdown_variation_attribute_options(
									array(
										'options'   => $options,
										'attribute' => $attribute_name,
										'product'   => $product,
										'selected'   => $selected,
									)
								);
								echo end( $attribute_keys ) === $attribute_name ? wp_kses_post( apply_filters( 'woocommerce_reset_variations_link', '<a class="reset_variations" href="#">' . esc_html__( 'Clear', 'woocommerce' ) . '</a>' ) ) : '';
							?>
						</td>
					</tr>
				<?php endforeach; ?>
			</tbody>
		</table>

		<div class="single_variation_wrap">
			<?php
				/**
				 * Hook: woocommerce_before_single_variation.
				 */
				do_action( 'woocommerce_before_single_variation' );

				/**
				 * Hook: woocommerce_single_variation. Used to output the cart button and placeholder for variation data.
				 *
				 * @since 2.4.0
				 * @hooked woocommerce_single_variation - 10 Empty div for variation data.
				 * @hooked woocommerce_single_variation_add_to_cart_button - 20 Qty and cart button.
				 */
				do_action( 'woocommerce_single_variation' );

				/**
				 * Hook: woocommerce_after_single_variation.
				 */
				do_action( 'woocommerce_after_single_variation' );
			?>
		</div>
	<?php endif; ?>

	<?php do_action( 'woocommerce_after_variations_form' ); ?>
</form>

<?php
do_action( 'woocommerce_after_add_to_cart_form' );