projection.clipExtent([extent])
If extent is specified, sets the projection’s viewport clip extent to the specified bounds in pixels and returns the projection. The extent bounds are specified as an array [[x₀, y₀], [x₁, y₁]], where x₀ is the left-side of the viewport, y₀ is the top, x₁ is the right and y₁ is the bottom. If extent is null, no viewport clipping is performed. If extent is not specified, returns the current viewport clip extent which defaults to null. Viewport clipping is indep