brush.extent()

brush.extent([extent])

If extent is specified, sets the brushable extent to the specified array of points [[x0, y0], [x1, y1]], where [x0, y0] is the top-left corner and [x1, y1] is the bottom-right corner, and returns this brush. 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:

function extent() {
  var svg = this.ownerSVGElement || this;
  return [[0, 0], [svg.width.baseVal.value, svg.height.baseVal.value]];
}

This default implementation requires that the owner SVG element 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 brush extent determines the size of the invisible overlay and also constrains the brush selection; the brush selection cannot go outside the brush extent.