winnow/lib/calculate-class-breaks/filter-and-validate-classification-features.js

Apply sql-like filters to GeoJSON

const _ = require('lodash')

module.exports = function filterAndValidateClassificationFeatures (features, classificationField) {
  return features.filter(feature => {
    return !shouldSkipFeature({ feature, classificationField })
  }).map(feature => {
    validateClassificationValue(feature, classificationField)
    return Number(feature.properties[classificationField])
  })
}

function validateClassificationValue ({ properties }, classificationField) {
  const value = properties[classificationField]
  if (_.isNaN(Number(value))) {
    throw new TypeError(`Cannot use non-numeric classificationField, ${classificationField}: "${value}"`)
  }
}

function shouldSkipFeature ({ feature: { properties }, classificationField }) {
  const value = properties[classificationField]
  return value === undefined || value === null
}