zoom.extent()

zoom.extent([extent])

If extent is specified, sets the viewport extent to the specified array of points [[x0, y0], [x1, y1]], where [x0, y0] is the top-left corner of the viewport and [x1, y1] is the bottom-right corner of the viewport, and returns this zoom behavior. The extent may also be specified as a function which returns such an array; if a function, it is invoked for each selected element, being passed the current datum d and index i, with the this context as the current DOM element.

If extent is not specified, returns the current extent accessor, which defaults to [[0, 0], [width, height]] where width is the client width of the element and height is its client height; for SVG elements, the nearest ancestor SVG element’s width and height is used. In this case, the owner SVG element must have defined width and height attributes rather than (for example) relying on CSS properties or the viewBox attribute; SVG provides no programmatic method for retrieving the initial viewport size. Alternatively, consider using element.getBoundingClientRect. (In Firefox, element.clientWidth and element.clientHeight is zero for SVG elements!)

The viewport extent affects several functions: the center of the viewport remains fixed during changes by zoom.scaleBy and zoom.scaleTo; the viewport center and dimensions affect the path chosen by d3.interpolateZoom; and the viewport extent is needed to enforce the optional translate extent