index.js

const d3 = require('d3-geo')
Object.assign(d3, require('d3-geo-projection'))

module.exports = reproject

/**
 * Reprojects the given geometry coordinate array _in place_, with
 * unprojectable points or degenerate geometries removed. If both
 * `options.forward` and `options.reverse` are supplied, then `forward` is
 * performed first.
 *
 * @param {Object} options
 * @param {Function|string} [options.forward] The forward projection to use.
 * @param {Function|string} [options.reverse] The reverse projection to use.
 * @param {Object} [options.projections] A map of named projections to use.  If provided, then string values of `options.forward` or `options.reverse` will be used as keys to look up the projection function in `options.projections`.  For an extensive list provided by d3-geo-projection, use `require('dirty-reprojectors/projections')`.
 * @param {Object} geometry A GeoJSON geometry object
 */
function reproject (options, geometry) {
  let streams = []

  if (options.forward) {
    let proj = options.forward
    if (typeof proj === 'string') {
      proj = options.projections[proj]
    }
    streams.push(proj.stream, flipY())
  }

  if (options.reverse) {
    let proj = options.reverse
    if (typeof proj === 'string') {
      proj = options.projections[proj]
    }
    streams.push(reverse(proj), flipY())
  }

  streams.reverse()

  const projection = {
    stream: function (output) {
      return streams.reduce((combined, s) => s(combined), output)
    }
  }

  return d3.geoProject(geometry, projection)
}

function reverse (projection) {
  let prev = []
  return d3.geoTransform({
    point: function (x, y) {
      // prevent wrapping due to precision issues
      x = clamp(x, -20037508.34278924, 20037508.34278924)
      var reversed = projection.invert([x, y])

      // drop points that fail to project or are identical to the previous
      // point
      if (!reversed || (reversed[0] === prev[0] && reversed[1] === prev[1])) {
        return
      }

      prev = reversed
      this.stream.point(reversed[0], reversed[1])
    }
  }).stream
}

function flipY () {
  return d3.geoTransform({
    point: function (x, y) {
      this.stream.point(x, -y)
    }
  }).stream
}

function clamp (value, min, max) {
  if (value > max) {
    return max
  } else if (value < min) {
    return min
  } else {
    return value
  }
}