Coming from the Flash background, I'm used to creating a fragment shader in the following manner:
filters = [];
filters.push(new BasicFilter(SomeTexture));
filters.push(new NormalMapFilter(SomeOtherTexture));
myShader = new Shader(filters);
As a result, I could combine a number of effects freely and easily without the need of writing a large separate shader each time.
In case of threejs, I noticed that for complex visual effects, a single shader is written, like here: http://threejs.org/examples/#webgl_materials_bumpmap_skin
Is it possible to write e.g. a bump map shader and environment map shader separately and then combine them dynamically when needed? What would be the most proper way of doing this?
It is not possible directly in three.js. Three creates shaders by passing the shader code as a string directly to a WebGL shader object to compile. There is no mechanism to automatically build complex shaders, you have to write your own. Three.js conveniently adds a couple of uniforms/attributes to the shader, but you have to write what's done with them.