Magento – Magento add-to-cart buttons on homepage don’t update cart

The solution (for no clarified reason) was to enable all Magento caching options. This solved the issue for me.

Tested this again in a new install: Fresh Luma templated with new product widget on the front page, with caching disabled. The 'Add to Cart' button wouldn't function as expected.

Enabled all Magento2 caching options, and the problem was immediatly fixed

First, change Add to Cart Button to Buy Now.

Then add a hidden input field to the #product_addtocart_form form. Name should be return_url and its value should be <?php echo Mage::getUrl('checkout/onepage')?>":

<input type="hidden" value="<?php echo Mage::getUrl('checkout/onepage')?>" name="return_url" />

If you want both Add to Cart and Buy now, add the buy now button to addtoCart.phtml

<button type="button" 
        title="<?php echo $buttonTitle ?>"
        class="button btn-cart" 
        onclick="productBuyNowForm.submit(this)">
    <span><span>Buynow</span></span>
</button>

On that buttons onlick event, a custom VarienForm is used, productBuyNowForm.submit(this), which will submit this form and redirect to checkout onepage.

To define this custom form, add the following JavaScript code at view.phtml

var productBuyNowForm = new VarienForm('product_addtocart_form');
productBuyNowForm.submit = function (button, url) {
    if (this.validator.validate()) {
        var form = this.form;
        var oldUrl = form.action;

        if (url) {
            form.action = url;
        }

        /* add return Url */
        var inputreturn= document.createElement("input");
        inputreturn.type = "hidden";
        inputreturn.name = "return_url";
        inputreturn.value = "<?php echo Mage::getUrl('checkout/onepage')?>";
        document.getElementById('product_addtocart_form').appendChild(inputreturn);
        /* add return Url */
        // Append a line break 
        var e = null;
        try {
            this.form.submit();
        } catch (e) {
        }
        this.form.action = oldUrl;
        if (e) {
            throw e;
        }

        if (button && button != 'undefined') {
            button.disabled = true;
        }
    }
}.bind(productBuyNowForm);