Why is chrome.runtime undefined?

Question

I am creating a Google Chrome extension (my first one) and I want to send messages from the extension to the current tab.

I am following the documentation:

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Answer 1

or it could just be a Heisenbug which only appears under certain circumstances. in my case, closing the chrome://extensions tab and refreshing my target caused chrome.runtime to be available again. Why is chrome.runtime undefined in the content script?

Answer 2

If you're inserting JavaScript into a page with a <script> tag, it executes in the page's context.

Sometimes it is desirable: that's the only way to access page-level JavaScript objects.

But in your case it means that the code does not have access to Chrome APIs, as it is "the same" as the page's code.

You need to look into communicating between page-level and context scripts, or between page-level and background (spoiler, in most cases needs a context script proxy anyway).