While making a little game, I ran into an issue when using polygons where some moving objects would collide with a static body and slowly move through it as if it were a dense medium. This issue did not occur when the static body was created with the SetAsBox method. When creating a shape as a box, the moving objects bounced off correctly.

var slantFixtureDef = new b2FixtureDef;
slantFixtureDef.density =     1.0;
slantFixtureDef.friction =    0.5;
slantFixtureDef.restitution = 0.2;
slantFixtureDef.shape = new b2PolygonShape;
slantFixtureDef.shape.SetAsBox(10, 1);

What I really wanted was to use vertices for this shape so that I could rotate it how I wanted.

var points = [
    {x: 0, y: 0},
    {x: 0, y: 0.5},
    {x: 5, y: 0.5},
    {x: 5, y: 0}

var vecs = [];
points.forEach(function(e, i, arr) {
    var vec = new b2Vec2(e.x, e.y);
    vecs[i] = vec;

slantFixtureDef.shape.SetAsArray(vecs, vecs.length);

But creating a shape like this caused the problem of dynamic bodies not bouncing off this static body. The issue turned out to be that the vertices were specified in the wrong order. Box2dWeb requires you to specify the vertices in clockwise order even though the C++ version requires counter-clockwise order.

var points = [
    {x: 0, y: 0},
    {x: 5, y: 0},
    {x: 5, y: 0.5},
    {x: 0, y: 0.5}

Simple as that!