How to use the vega-scale.isDiscrete function in vega-scale

function parseScaleRange(spec, scope, params) {
  var range = spec.range,
      config = scope.config.range;

  if (range.signal) {
    return scope.signalRef(range.signal);
  } else if (isString(range)) {
    if (config && config.hasOwnProperty(range)) {
      spec = extend({}, spec, {range: config[range]});
      return parseScaleRange(spec, scope, params);
    } else if (range === 'width') {
      range = [0, {signal: 'width'}]
    } else if (range === 'height') {
      range = isDiscrete(spec.type)
        ? [0, {signal: 'height'}]
        : [{signal: 'height'}, 0]
    } else {
      error('Unrecognized scale range value: ' + stringValue(range));
    }
  } else if (range.scheme) {
    params.scheme = isArray(range.scheme)
      ? parseArray(range.scheme, scope)
      : parseLiteral(range.scheme, scope);
    if (range.extent) params.schemeExtent = parseArray(range.extent, scope);
    if (range.count) params.schemeCount = parseLiteral(range.count, scope);
    return;
  } else if (range.step) {
    params.rangeStep = parseLiteral(range.step, scope);
    return;
  } else if (isDiscrete(spec.type) && !isArray(range)) {

? [0, {signal: 'height'}]
        : [{signal: 'height'}, 0]
    } else {
      error('Unrecognized scale range value: ' + stringValue(range));
    }
  } else if (range.scheme) {
    params.scheme = isArray(range.scheme)
      ? parseArray(range.scheme, scope)
      : parseLiteral(range.scheme, scope);
    if (range.extent) params.schemeExtent = parseArray(range.extent, scope);
    if (range.count) params.schemeCount = parseLiteral(range.count, scope);
    return;
  } else if (range.step) {
    params.rangeStep = parseLiteral(range.step, scope);
    return;
  } else if (isDiscrete(spec.type) && !isArray(range)) {
    return parseScaleDomain(range, spec, scope);
  } else if (!isArray(range)) {
    error('Unsupported range type: ' + stringValue(range));
  }

  return range.map(v => (isArray(v) ? parseArray : parseLiteral)(v, scope));
}

function singularDomain(domain, spec, scope) {
  var data = scope.getData(domain.data);
  if (!data) dataLookupError(domain.data);

  return isDiscrete(spec.type)
      ? data.valuesRef(scope, domain.field, parseSort(domain.sort, false))
      : isQuantile(spec.type) ? data.domainRef(scope, domain.field)
      : data.extentRef(scope, domain.field);
}

function multipleDomain(domain, spec, scope) {
  var data = domain.data,
      fields = domain.fields.reduce(function(dom, d) {
        d = isString(d) ? {data: data, field: d}
          : (isArray(d) || d.signal) ? fieldRef(d, scope)
          : d;
        dom.push(d);
        return dom;
      }, []);

  return (isDiscrete(spec.type) ? ordinalMultipleDomain
    : isQuantile(spec.type) ? quantileMultipleDomain
    : numericMultipleDomain)(domain, scope, fields);
}