The EXT_frag_depth extension is part of the WebGL API and enables to set a depth value of a fragment from within the fragment shader.

API Fragment Shader Reference WebGL WebGL extension

The WebGLRenderingContext.createShader() method of the WebGL API creates a WebGLShader that can then be configured further using WebGLRenderingContext.shaderSource() and WebGLRenderingContext.compileShader().

API Graphics Method Reference Shader WebGL