questions are as follows
document.getElementsByTagName("iframe")[0]
document.getElementsByTagName("iframe")Code and screenshot
var ifr = document.getElementsByTagName("iframe");
console.log(ifr);
console.log(ifr[0]); 
document.getElementsByTagName("iframe")[0]
document.getElementsByTagName("iframe")var ifr = document.getElementsByTagName("iframe");
console.log(ifr);
console.log(ifr[0]);
are you sure which undefined is the result of the execution of ifr [0]? Not your console.log (xxxx)?
document.getElementsByTagName ('iframe'); returns a dynamic collection whose contents change each time the value is taken.
if between console.log (ifr); and console.log (ifr [0]), the iframe in the page is removed from the document, then console.log (ifr [0]) becomes undefined.. You can output ifr again after console.log (ifr [0]) to determine whether this situation has been sent.
console.log(ifr);
console.log(ifr[0]);
console.log(ifr); // ifr has not been reproduced. I have tried it on this page and output it successfully. Please check the code
lz
frames || window.frames iframeframes[index] || window.frames[index] || window.frames[iframe_id] iframeframes[index].frameElement.contentWindow iframewindowframes[index].frameElement.contentWindow.frames[sub_index].frameElemnet.contentWindow...I hope it will be useful to you
Today, I encountered the same problem, and it is an occasional problem. Ios appears frequently. There was no such problem before. Since yesterday, the frequency of problems in testing has become higher. At the same time, the browser test on mac will show that getElementsByTagName ('iframe') [0] cannot get the corresponding iframe tag, and I don't know whether the landlord has solved the problem
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