Google's John Mueller warned one site owner not to load "not available" in the JavaScript before the actual content that loads is available. It can lead Google to believe the page does not exist, preventing indexing and ranking in Google Search.
To be clear, that site is changing "not available' via JavaScript and not publishing that via JavaScript.
A Reddit thread named "Google Might Think Your Website Is Down" is where this discussion came up.
John Mueller replied to the concern saying, "Is that your site? I'd recommend not using JS to change text on your page from "not available" to "available" and instead to just load that whole chunk from JS."
He explained "That way, if a client doesn't run your JS, it won't get misleading information."
He said this was similar to Google's advice on not using noindex tags on JS pages. John added, "This is similar to how Google doesn't recommend using JS to change a robots meta tag from "noindex" to "please consider my fine work of html markup for inclusion" (there is no "index" robots meta tag, so you can be creative)."
In short, by the time Google crawls the page, it only sees it is not available and then it goes away. It doesn't want to wait longer for a different message to come up, because that is the message Google received.
Forum discussion at Reddit.
