I'm super new to WebGL and trying to run the below portion of my code, but it's not working. I'm following an exercise in my textbook for approximating a sphere object using subdivision. Trying to follow the same basic concept that I used for the Sierpinski Gasket.

    //Specifiy four vertices for a tetrahedron
    var va = vec4(0.0, 0.0, -1.0, 1);
    var vb = vec4(0.0, 0.942809, 0.333333, 1);
    var vc = vec4(-0.816497, -0.471405, 0.333333, 1);
    var vd = vec4(0.816497, -0.471405, 0.333333, 1);
    
    var pointsArray = [];
    var numTimesToSubdivide = 8;
    
    //Adding vertex positions to a vertex array
    function triangle(a, b, c)
    {
        pointsArray.push(a);
        pointsArray.push(a);
        pointsArray.push(a);
        index += 3;
    }
    
    //Tetrahedron midpoint subdivision
    function divideTriangle(a, b, c, count)
    {
        if(count > 0) {
            var ab = normalize(mix(a, b, 0.5), true);
            var ac = normalize(mix(a, c, 0.5), true);
            var bc = normalize(mix(b, c, 0.5), true);
            divideTriangle(a, ab, ac, count - 1);
            divideTriangle(ab, b, bc, count - 1);
            divideTriangle(bc, c, ac, count - 1);
            divideTriangle(ab, bc, ac, count - 1);
        }
        else {
            triangle(a, b, c);
        }
    }
    
    //Dividing four sides of the tetrahedron
    function tetrahedron(a, b, c, d, n)
    {
        divideTriangle(a, b, c, n);
        divideTriangle(d, c, b, n);
        divideTriangle(a, d, b, n);
        divideTriangle(a, c, d, n);
    }

    //Initiate recursion
    tetrahedron(va, vb, vc, vd, numTimesToSubdivide);
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(pointsArray), gl.STATIC_DRAW);
    gl.drawArrays(gl.TRIANGLES, 0, pointsArray.length / 2);
}

Getting this error

Uncaught ReferenceError: index is not defined

Appreciate any help.