Exploring the concept further: a reflection essentially captures an image of the surroundings from a specific point of view. If you wish to use a planar mesh as a mirror, it requires positioning a camera at that exact spot, rendering the scene into a texture during the animation loop, and then integrating that texture into the material for the planar mesh. Additionally, I suggest delving into along with the examples highlighted by WestLangley.
Experimenting with different configurations can yield varying results; to achieve a less reflective outcome, consider attempting:
var mirrorMaterial = new THREE.MeshBasicMaterial( { color: 0x111111, envMap: mirrorCamera.renderTarget } );
or
var mirrorMaterial = new THREE.MeshPhongMaterial( { emissive: 0x111111, envMap: mirrorCamera.renderTarget } );