source: node_modules/d3-scale/src/threshold.js

import {bisect} from "d3-array";
import {initRange} from "./init.js";

export default function threshold() {
  var domain = [0.5],
      range = [0, 1],
      unknown,
      n = 1;

  function scale(x) {
    return x != null && x <= x ? range[bisect(domain, x, 0, n)] : unknown;
  }

  scale.domain = function(_) {
    return arguments.length ? (domain = Array.from(_), n = Math.min(domain.length, range.length - 1), scale) : domain.slice();
  };

  scale.range = function(_) {
    return arguments.length ? (range = Array.from(_), n = Math.min(domain.length, range.length - 1), scale) : range.slice();
  };

  scale.invertExtent = function(y) {
    var i = range.indexOf(y);
    return [domain[i - 1], domain[i]];
  };

  scale.unknown = function(_) {
    return arguments.length ? (unknown = _, scale) : unknown;
  };

  scale.copy = function() {
    return threshold()
        .domain(domain)
        .range(range)
        .unknown(unknown);
  };

  return initRange.apply(scale, arguments);
}