In the frame of an European Space Agency (ESA) contract, a consortium of three European research institutes (Laboratoire d'Astrophysiqu de Marseille, Centre de Recherche Astronomique de Lyon and the University of Durham) and the Cybernetix company have designed, manufactured and tested a prototype of an Integral Field Unit (IFU) for the NIRSpec instrument of the future James Webb Space Telescope (JWST). After a brief presentation of the optical design of this prototype, which is based on the advanced slicer concept, we will focus on the optical tests of this prototype. We will first present the tests peformed at LAM on the individual optical elements prior to their integration in the mechanical structure, as well as the alignment tests conducted as part of the integration procedure. We will then describe the tests and their results in the visible of the complete IFU system both at room temperature (tests performed at CRAL) and at operating temperature (30 K, tests peformed at the University of Durham). Briefly, these tests included: measurements of the characteristics (position, shape, size ...) of the pseudo-slit of the IFU prototype; measurement of the point-spread-function at different locations within its field of view; and measurement of the position, shape and size of the exit pupils. Last, we will conclude on the TRL6 readiness of the advanced image slicer technique and we will provide a glimpse of how wide-spread this technique is becoming both for ground- and space-based applications.