Reflective material in THREE.js

https://threejs.org/examples/#webgl_materials_cubemap
I did it with the above example:

new THREE.MeshLambertMaterial ({
    map: texture,    
    envMap: scene.background,      
    combine: THREE.MixOperation,     
    reflectivity: .5     
})

The key variable, as i understand, is THREE.MixOperation

To go into a bit of the theory: a reflection is basically an image of the scene taken from a certain position. So if you want a planar mesh to serve as a mirror, you'll have to add a camera at that position, have it render the scene to a texture in the animation loop, and then use that texture in the material for the planar mesh. I would also recommend looking at http://stemkoski.github.io/Three.js/Reflection.html in addition to the examples WestLangley mentioned.

Also, play around with settings; for a less reflective effect, for example, try:

var mirrorMaterial = new THREE.MeshBasicMaterial( { color: 0x111111, envMap: mirrorCamera.renderTarget } );

or

var mirrorMaterial = new THREE.MeshPhongMaterial( { emissive: 0x111111, envMap: mirrorCamera.renderTarget } );