example_add_to_cart_button.php

<?php

$product_id = 147; // This is just for demonstration, you would use whatever product ID the page is referencing.  if the page is the product page itself, you can use get_the_ID();
// we output a different version of the button, depending on whether the item is in the cart already
// note the class on the button... we are using that class to target our javascript handler
// note the data-attribute, that's how the product ID gets to our AJAX handler
if( !my_custom_cart_contains( $product_id ) ) {
	?>
	<button class="my-custom-add-to-cart-button" data-product-id="<?php echo $product_id; ?>">add to cart</button>
	<?php
} else {
	?>
	<button class="my-custom-add-to-cart-button" data-product-id="<?php echo $product_id; ?>" disabled="disabled">added to cart</button>
	<?php
}
?>

wc_ajax_add_to_cart.php

<?php
// this handler adds our custom javascript to fire AJAX requests when we click one of these buttons
add_action('wp_footer', 'my_custom_wc_button_script');
function my_custom_wc_button_script() {
	?>
	<script>
		jQuery(document).ready(function($) {
			var ajaxurl = "<?php echo esc_attr( admin_url( 'admin-ajax.php' ) ); ?>"; // get the url we use to submit AJAX
			$( document.body).on('click', '.my-custom-add-to-cart-button', function(e) { // i made this delegated, rather than a traditional click handler, so you could add additional buttons via AJAx, if you ever wanted to
				e.preventDefault(); // stop the click from doing normal button things
				var $this = $(this);
				if( $this.is(':disabled') ) { // don't do anything if the button is disabled (item is in cart)... this could be changed to toggle whether you are in cart or not
					return;
				}
				var id = $(this).data("product-id"); // get the product ID from the button
				var data = { // prep our AJAX request
					action     : 'my_custom_add_to_cart', // This is the AJAX function we define in PHP below
					product_id : id
				};
				$.post(ajaxurl, data, function(response) {
					if( response.success ) {
						// we added to cart so change the message, and make sure no one can add again
						$this.text("added to cart");
						$this.attr('disabled', 'disabled');
						// make woocommerce update cart counts in the menu widget
						$( document.body ).trigger( 'wc_fragment_refresh' );
					}
				}, 'json');
			})
		});
	</script>
	<?php
}
// here's where we define the ajax processing functions.. the part after wp_ajax, and wp_ajax_nopriv has to match the action we used in our javascript
add_action('wp_ajax_my_custom_add_to_cart', "my_custom_add_to_cart");
add_action('wp_ajax_nopriv_my_custom_add_to_cart', "my_custom_add_to_cart");
function my_custom_add_to_cart() {
	// just setting up data to return... we override this as we go
	$retval = array(
		'success' => false,
		'message' => ""
	);
	
	if( !function_exists( "WC" ) ) {
		// check if woocommerce is installed
		$retval['message'] = "woocommerce not installed";
	} elseif( empty( $_POST['product_id'] ) ) {
		// check product id was sent
		$retval['message'] = "no product id provided";
	} else {
		$product_id = $_POST['product_id'];
		// my_custom_cart_contains is defined below.. checks if the cart contains a product
		if( my_custom_cart_contains( $product_id ) ) {
			// make sure we can't add a product twice
			$retval['message'] = "product already in cart";
		} else {
			// we are good to add to cart
			$cart = WC()->cart;
			//add_to_cart returns an id, but we only need to make sure it doesn't return false... hence casting to a boolean
			$retval['success'] = (bool) $cart->add_to_cart( $product_id );
			if( !$retval['success'] ) {
				// the add to cart failed
				$retval['message'] = "product could not be added to cart";
			} else {
				// the add succeeded
				$retval['message'] = "product added to cart";
			}
		}
	}
	// we send the data back to javascript by outputting it as JSON, and exiting.
	echo json_encode( $retval );
	wp_die();
}
// check if cart contains product
function my_custom_cart_contains( $product_id ) {
	$cart = WC()->cart;
	$cart_items = $cart->get_cart();
	if( $cart_items ) {
		foreach( $cart_items as $item ) {
			$product = $item['data'];
			if( $product_id == $product->id ) {
				return true;
			}
		}
	}
	return false;
}