Get the inner angle at point "b" in degrees

inner-angle.js

function inner_angle(a,b,c){ // inner angle at point b in degrees
    var v1=[a[0] - b[0], a[1] - b[1]],
        v2=[c[0] - b[0], c[1] - b[1]];
        
    return Math.acos(math.inner(v1, v2) / (math.norm(v1) * math.norm(v2))) * 180 / Math.PI;
}

var math={
    norm: function(v){
        var s=0, n=v.length;

        for(var i=0; i < n; ++i){
            s += Math.pow(v[i], 2);
        }

        return Math.sqrt(s);
    },
    inner: function(a,b){
        var s=0;

        for(var i=0; i < a.length; ++i){
            s += (a[i]*b[i]);
        }

        return s;
    }
}