<?xml version='1.0'?>
<layout version='0.1.0'>
<catalog_product_view>
<reference name='root'>
<reference name='head'>
<action method='addItem'>
<type>skin_js</type>
<name>js/customjs.js</name>
</action>
</reference>
</reference>
</catalog_product_view>
</layout>
File path: skin/design/frontend/MageCheck/MCTheme/layout/local.xml
<?xml version='1.0'?>
<page xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' layout='1column' xsi:noNamespaceSchemaLocation='../../../../../../../lib/internal/Magento/Framework/View/Layout/etc/page_configuration.xsd'>
<head>
<link src='MageCheck_Seasons::customjs.js'/>
</head>
<referenceBlock name='content'>
<block template='content.phtml' class='MageCheck\Seasons\Block\Main' name='magecheck_seasons_block_main' />
</referenceBlock>
</page>
File path: app/code/MageCheck/Seasons/view/frontend/magecheck_seasons_index_index.xml
alert('Hello MageCheck!');
File path: skin/design/frontend/MageCheck/MCTheme/js/customjs.js
alert('Hello MageCheck!');
File path: app/code/MageCheck/Seasons/view/frontend/web/customjs.js
The last thing you have to do is to clean and flush your Magento cache:
php bin/magento cache:clean
php bin/magento cache:flush
This is the simplest method to add Javascript file from layout in Magento!
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