How to use the postman-collection.ItemGroup function in postman-collection

pList.upsert(certificate); // $ExpectType boolean | null
pmCollection.PropertyList.isPropertyList(pList); // $ExpectType boolean

// CertificateList Tests
const certList = new pmCollection.CertificateList(null, [pList]);
certList.resolveOne("string"); // $ExpectType CertificateDefinition
pmCollection.CertificateList.isCertificateList(certList); // $ExpectType boolean

// ItemGroupDefinition Tests
const igDef: pmCollection.ItemGroupDefinition = {};
igDef.item; // $ExpectType (ItemGroupDefinition | ItemDefinition)[] | undefined
igDef.auth; // $ExpectType RequestAuthDefinition | undefined
igDef.event; // $ExpectType EventDefinition[] | undefined

// ItemGroup Tests
const ig = new pmCollection.ItemGroup();
ig.auth; // $ExpectType RequestAuth | undefined
ig.items; // $ExpectType PropertyList
ig.events; // $ExpectType EventList

ig.authorizeRequestsUsing("string"); // $ExpectType void
ig.authorizeRequestsUsing({}); // $ExpectType void
ig.authorizeRequestsUsing("string", new pmCollection.VariableList(ig, [])); // $ExpectType void

ig.forEachItem((el) => {}); // $ExpectType void
ig.forEachItemGroup((el) => {}); // $ExpectType void

ig.oneDeep("string"); // $ExpectType Certificate

pmCollection.ItemGroup.isItemGroup(ig); // $ExpectType boolean

// CollectionDefinition Tests

itemGroup,
      subChild,
      item,
      i,
      requestCount;

    // it's a folder
    if (resource.type === 'item-group') {
      // 3 options:

      // 1. folder with more than one request in its subtree
      // (immediate children or otherwise)
      if (resource.requestCount > 1) {
        // only return a Postman folder if this folder has>1 requests in its subtree
        // otherwise we can end up with 10 levels of folders with 1 request in the end
        itemGroup = new sdk.ItemGroup({
          name: this.insertSpacesInName(resource.name)
          // TODO: have to add auth here (but first, auth to be put into the openapi tree)
        });

        // recurse over child leaf nodes
        // and add as children to this folder
        for (i = 0, requestCount = resource.requests.length; i < requestCount; i++) {
          itemGroup.items.add(
            this.convertChildToItemGroup(openapi, resource.requests[i])
          );
        }

        // recurse over child folders
        // and add as child folders to this folder
        for (subChild in resource.children) {
          if (resource.children.hasOwnProperty(subChild) && resource.children[subChild].requestCount > 0) {

convertChildToItemGroup: function(child) {
        var thisItemGroup, thisItem, rCount;
        // child will be a folder or request
        // depending on the type

        if (child.type === "group") {
            // folder
            thisItemGroup = new ItemGroup({
                name: child.name
            });
            for(var subChild in child.children) {
                if(child.children.hasOwnProperty(subChild)) {
                    thisItemGroup.items.add(
                    	this.convertChildToItemGroup(child.children[subChild])
                	);
                }
            }
            for(var i = 0, rCount = child.requests.length; i < rCount; i++) {
                thisItemGroup.items.add(this.convertChildToItemGroup(child.requests[i]));
            }
            return thisItemGroup;
        }
        else {
            // request