Summary qs.stringify with allowEmptyArrays: true writes the key of an empty array without encoding it, even though keys are encoded by default. A key that contains & (or the configured delimiter), =, or # therefore becomes query string syntax instead of data, so a caller-supplied key name can add parameters to the serialized query string, or cut it short. Details In lib/stringify.js, the allowEmptyArrays branch returns adjustedPrefix + '[]' early, before the code that passes every other key through encoder and formatter. So { "x&k=v": ["1"] } serializes as x%26k%3Dv%5B0%5D=1, but { "x&k=v": [] } serializes as x&k=v[]. The early return applies at any depth ({ "a": { "b&k=v": [] } } gives a[b&k=v][]), under every arrayFormat, with allowDots, and with a custom encoder, which is never called…