How to Minify and Optimize JavaScript in Magento 2

Add the following in your local.xml layout file (app/design/frontend/{interface}/{theme}/layout/local.xml) right under the <layout> tag this:
For product pages

<catalog_product_view>
    <reference name="head">
        <block type="core/text" name="some_name" as="some_name">
            <action method="setText">
                <text><![CDATA[YOUR TEXT GOES HERE]]></text>
            </action>
        </block>
    </reference>
</catalog_product_view>

For the checkout page is the same thing, except you need to replace the top tag with <checkout_onepage_index>. So the section above becomes:

<checkout_onepage_index>
    <reference name="head">
        <block type="core/text" name="some_name" as="some_name">
            <action method="setText">
                <text><![CDATA[YOUR TEXT GOES HERE]]></text>
            </action>
        </block>
    </reference>
</checkout_onepage_index>

it works the same for any other page. You just need to know the correct page handle (top tag). For index is cms_index_index, for cart is checkout_cart_index.

If you use a cms site, you can create a cmsyourname.phtml. Now you can insert your jquery in the top of the file.

In your cms page in the backend, you have to add this:

{{block class="Magento\Framework\View\Element\Template" name="cmsyourname" template="Magento_Cms::html/cmsyourname.phtml"}}