How to use the @mapbox/mapbox-gl-style-spec.function.isFunction function in @mapbox/mapbox-gl-style-spec

import Stroke from 'ol/style/Stroke';
import Icon from 'ol/style/Icon';
import Text from 'ol/style/Text';
import Circle from 'ol/style/Circle';
import Point from 'ol/geom/Point';
import derefLayers from '@mapbox/mapbox-gl-style-spec/deref';
import spec from '@mapbox/mapbox-gl-style-spec/reference/latest';
import {
  expression, Color,
  function as fn,
  featureFilter as createFilter
} from '@mapbox/mapbox-gl-style-spec';
import mb2css from 'mapbox-to-css-font';
import {deg2rad, defaultResolutions, getZoomForResolution, wrapText, applyLetterSpacing} from './util';

const isFunction = fn.isFunction;
const convertFunction = fn.convertFunction;
const isExpression = expression.isExpression;
const createPropertyExpression = expression.createPropertyExpression;

const types = {
  'Point': 1,
  'MultiPoint': 1,
  'LineString': 2,
  'MultiLineString': 2,
  'Polygon': 3,
  'MultiPolygon': 3
};

const expressionData = function(rawExpression, propertySpec) {
  const compiledExpression = createPropertyExpression(rawExpression, propertySpec);
  if (compiledExpression.result === 'error') {