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To collect data from all line items, you can use api.find – but iterating over all items is a very bad idea. It will be very slow, possibly even unusable. A better option here is aggregation queries; here you can use api.find with the fields parameter which supports aggregation functions too. To overcome the limitation of api.find (results are by default limited to 200), you can use pagination.
For details see Tips on Data Querying with api.find().