In this episode, I cover the 'browser.execute' command, which can be used to run custom JavaScript on the page you're testing.
WebdriverIO Wednesdays is an "every-other-weekly" live stream going through various WebdriverIO concepts, focused around real-world test scripts. http://learn.webdriver.io
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https://www.youtube.com/watch?v=GpXKmVL17Tc