Q:

Why does not the ambient light work in this example?

In the following example, the ambient light is not working (everything is black). Why is this happening? And how do I fix it? 
If I put spot light, it works, so it must be something wrong with the ambient light, but I followed the docs ... =:O
<html>
    <head>
        <title>My first three.js app</title>
        <style>
            body { margin: 0; }
            canvas { width: 100%; height: 100% }
        </style>
    </head>
    <body>
        <script src="js/three.js"></script>
        <script>
            var scene = new THREE.Scene();
            var camera = new THREE.PerspectiveCamera( 75, window.innerWidth/window.innerHeight, 0.1, 1000 );

            var renderer = new THREE.WebGLRenderer();
            renderer.setSize( window.innerWidth, window.innerHeight );
            document.body.appendChild( renderer.domElement );

            var geometry = new THREE.BoxGeometry( 1, 1, 1 );

            material = new THREE.MeshStandardMaterial( {
                color: 0x0c79bf,
                    roughness: 0.71,
                    metalness: 1,
                    normalScale: new THREE.Vector2( 1, - 1 ), // why does the normal map require negation in this case?
                    side: THREE.DoubleSide
                } );

            var cube = new THREE.Mesh( geometry, material );
            scene.add( cube );

            var alight = new THREE.AmbientLight( 0x404040);
            scene.add( alight );

            camera.position.z = 5;

            var animate = function () {
                requestAnimationFrame( animate );

                cube.rotation.x += 0.01;                
                cube.rotation.y += 0.01;

                renderer.render(scene, camera);
            };

            animate();
        </script>
    </body>
</html>

A:

Ambient light in three.js is a simple model of indirect light which is reflected by the material diffusely.
In your example, you have set the material metalness property to 1; that is, you are modeling a pure metal. Pure metals do not reflect light diffusely -- they only reflect light specularly.
When using MeshStandardMaterial, you should always specify an environment map (material.envMap) so your metallic materials have something to reflect.
three.js r.89

