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var buttons = document.getElementsByClassName("_jvpff");
console.log(buttons); //added after error from command below
document.getElementsByClassName returns a live
HTMLNodeList which updates as elements become available, and the console has its own quirks for when the contents of an object are evaluated and updated. If
undefined at the time of trying to access it, then it's
undefined at that time. Which means you need to wait with your script execution until the DOM is ready and the element actually exists. Either put your scripts at the bottom of the page, or wait for the DOM to be ready by listening for the