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By default, the JavaScript tracking code is embedded within a common JavaScript file on all of your SharePoint WFEs (Web Front Ends), which typically is one of the following locations relevant to your software versions:
For SharePoint 2016 only:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\init.js
For SharePoint 2013 only:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\init.js
For SharePoint 2010 only:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\[language code]\init.js
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