The WebGLRenderingContext.activeTexture() method of the WebGL API specifies which texture unit to make active.

texture

The texture unit to make active. The value is a gl.TEXTUREI where I is within the range from 0 to gl.MAX_COMBINED_TEXTURE_IMAGE_UNITS - 1.

None (undefined).

If texture is not one of gl.TEXTUREI, where I is within the range from 0 to gl.MAX_COMBINED_TEXTURE_IMAGE_UNITS - 1, a gl.INVALID_ENUM error is thrown.

The following call selects gl.TEXTURE1 as the current texture. Subsequent calls that modify the texture state will affect this texture.

gl.activeTexture(gl.TEXTURE1);

The number of texture units is implementation dependent, you can get this number with the help of the MAX_COMBINED_TEXTURE_IMAGE_UNITS constant. It is, per specification, at least 8.

gl.getParameter(gl.MAX_COMBINED_TEXTURE_IMAGE_UNITS);

To get the active texture, query the ACTIVE_TEXTURE constant.

gl.activeTexture(gl.TEXTURE0);
gl.getParameter(gl.ACTIVE_TEXTURE);
// returns "33984" (0x84C0, gl.TEXTURE0 enum value)

• WebGLRenderingContext.getParameter()