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The part of a URL after the question mark that sends extra information to the server — like writing a note on the outside of an envelope.
Key-value pairs appended to a URL after '?' (e.g., ?page=2&sort=name). In Express, they are parsed into req.query. Query strings are used for filtering, pagination, and non-sensitive parameters since they are visible in the URL.
Express parses query strings via the qs library (by default), which supports nested objects (a[b]=c) and arrays (a[]=1&a[]=2). The queryParser setting can be swapped to the simpler querystring module. Query strings are limited by URL length (typically 2,048-8,192 bytes depending on the server). Because they appear in server logs and browser history, they must never contain secrets or PII.