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Use the Run JavaScript on Webpage action in Shortcuts on iPhone or iPad
Use the Run JavaScript on Webpage action to retrieve specific data from a website, or to modify the contents of a webpage.

The Run JavaScript on Webpage action contains a text field where you write your own script. You run the shortcut from the Safari app. Before you can run JavaScript from a shortcut you must turn on Allow Running Scripts. See Advanced Privacy and security settings in Shortcuts on iPhone or iPad.
Note: You can also use the Run JavaScript on Webpage action in SFSafariViewController. For more information, see https://developer.apple.com/documentation/safariservices/sfsafariviewcontroller.