Inverse kinematics aren’t supported by three.js at this time. There are some libraries that can perform calculations in two dimensions, but because these calculations can be expensive, I recommend another way.

Use Blender since it supports inverse kinematics in three dimensions, and create animations before exporting to three.js. By doing IK in Blender, you are effectively baking the inverse kinematics into the animations to be used later. You won’t have the flexibility of dynamic IK, but your animations will be easier to make and look more natural.

Until someone builds an IK library for the 3 dimensions in the browser, I’m afraid we’ll just have to wait. One good thing about using baked animations though is being able to use three.js’s animation mixer class which allows you to fade between animations like a DJ fades between tracks.

Here’s an example of a model with IK animations and the mixer class.

Update: Here’s a promising library from github user glumb. https://github.com/glumb/kinematics