Three.js provides a number of built-in attributes you can use within your vertex and fragment shaders.

If you're using a BufferGeometry object for your mesh, you'll have access to the following within your vertex shader:

attribute vec3 position;
attribute vec3 normal;
attribute vec2 uv;

The position attribute represents a vertex's position within a BufferGeometry. For example, if we were to log the contents of position, we'd get something like vec3(0.3, 0.2, 0.0)—the first argument represents the vertex's X position in 3D space, the second represents the Y position, and the third the Z position.

One of the nice things about GLSL is that you can insert position, a type of vec3, directly into a vec4, and it will fill up the first three values automatically.

Replace the first three values of the vec4 below with the position attribute passed through to us automatically by Three.js.