Gmail getElementById('canvas_frame') keeps returning null in Google Chrome

问题

i'm trying to write JavaScript code for a Gmail extension, and when I'm trying to get the canvas frame I, "getElementById" keeps returning the null value.

Here is the code I'm using:

var doc2 = document.getElementById('canvas_frame').contentDocument;

I'm getting the following error:

"Uncaught TypeError: Cannot read property 'contentDocument' of null"

How can I solve this?

回答1:

Because the iframe with ID canvas_frame does not need to exist in Gmail.

To get a reference to the relevant document, you can first try to get a reference to iframe#canvas_frame, and if that fails, check if the current context is correct:

var doc2 = document.getElementById('canvas_frame');
if (doc2) {
    doc2 = doc2.contentDocument;
} else if (document.getElementById('js_frame')) {
    // If #canvas_frame does not exist, but #js_frame does, then Gmail renders
    // everything in the main (=top) frame
    doc2 = document;
} // else not Gmail's document, and doc2 === null

来源：https://stackoverflow.com/questions/14032301/gmail-getelementbyidcanvas-frame-keeps-returning-null-in-google-chrome