The Jiangmen Underground Neutrino Observatory (JUNO) collaboration has completed the construction of a 20-kt liquid scintillator detector in a laboratory 700 m (1800~m.w.e.) underground to address many important topics in neutrino and astro-particle physics. Initial testing and performance studies show that key specifications have been mostly met. The detector is clean enough to start the reactor antineutrino oscillation analysis. The energy non-linearity is known to be smaller than 1%. For events in radius smaller than 16 m, the residual energy non-uniformity scanned along the z-axis is smaller than 0.5%. The light yield is larger than 1600 p.e./MeV for $^{68}$Ge calibration source located at the centre of the detector, while it is larger than 1800 p.e./MeV for neutrons from the AmBe or AmC calibration sources, which is better than expectations. The energy resolution for $\alpha$ from $^{214}$Po is about 3% at 0.92 MeV, while it is about 3.4% at 2$\times$0.511 MeV for $^{68}$Ge. We are excited to have started the physics data taking, and the analysis of the first data and the understanding of the detector performance is in progress.