``` const svg = d3.create("svg") .attr("viewBox", [0, 0, width, height]) .property("value", []); ``` this part set the svg element, but I don't know why have to set the property of `value` to an empty array?

The svg element's value property informs the contents of the selection variable, which will then be updated when we brush (with the call to "dispatch"). Since we want to begin with an empty selection, we have to set it explicitly to [] (rather than undefined).