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A hatred such as he had never known before was coursing through Harry like poison.

As I know, the phrase "such as" can mean "for example" or 'like' and is followed by a list or a noun, pronoun and etc. But in this sentence, it's followed by a clause. It looks to me that it's like saying: A hatred that he had never known before was coursing through Harry like poison. It seems to function as "that" in this case.

How should we understand it in this sentence?

It means "like" here as well, more or less. A wooden paraphrase would be of a kind that he had never known.

as is a clause subordinator.

You hear contemporary speakers asking "Such as?" when they want to elicit examples from the person they're speaking with, and so it's easy to think that those words belong together, but if you were to parse that sentence in slo-mo it would be:

A .. hatred .. such ..... as he had never known.

Here's the pattern: A [noun] + such as [subject] had never [known, experienced, seen, etc.] is common in English writing.

• Wealth such as the country had never seen etc.


• Destruction such as the state had never experienced etc.


• Love such as he had never known etc.


• Good fortune such as they had never had etc.

This pattern is often seen in formal and literary writing.

Consider this:

A decision //such as this// was unusual.
such as this= like the one.

A decision //such as the one he was making// was unusual.
such as the one he was making=like the one he was making

In writing, "such as" would be preferred to "like", as strictly speaking, like is for similes (He looks like a rat.) and such as is for providing an example that stands out. (A man such as he had never run for office.)

such as can be followed by a noun or a clause.

"A hatred that he had never known before" is quite specific.  There is only one hatred in that phrasing.  "A hatred such as he had never known before" has a broader meaning.  Not only has he never known this hatred, but he has never known any other hatred like it.  That meaning which "such as" shares with "for example" and "like" hasn't been lost. 

The word "such" is an adjective.  On its own it usually occurs as a premodifier, as in "such hatred" or "such a hatred".  Here it takes a natural (but optional) post-positive position because it is directly modified by the following prepositional phrase. 

The word "as" is a preposition.  However, the clause that serves as its argument is not an object.  Rather, it is a complement.  Here it serves as the postcedent of the anaphoric adjective "such". 

We should take the subject of this sentence to mean a hatred of a kind that he had never known before, probably a hatred of a greater strength than he had ever known before.