The GSN Composer is an online node-based visual programming environment that allows generating custom nodes using GLSL/WebGL. For every uniform variable that is created within the GLSL shader code editor, an input slot is automatically added to the corresponding shader node, which can be connected to other nodes of the dataflow graph. This makes online shader prototyping very fast and intuitive and frees the developer of writing many lines of support code to fill the GLSL uniform variables with values. The GSN Composer requires no login and is free-of-charge. All that is needed to get started is a web-browser and your creativity. Several simple GLSL examples are provided such that this tool is also suited as starting point for GLSL/WebGL beginners.