Angular 2 performance issue (rendering)

Question

I'm doing a project in Angular 2 and I'm getting an issue that makes my pages lose a lot of performance. So, I have a table and everytime I scroll (up or down) it keeps calling one of my functions. This is the one:

.component.ts

entriesToShow(): Array<any>{
    let entries = [];
    let i = 0;
    if(Number(this.startingEntry)+Number(this.numOfEntries) <this.totalEntries)
        this.lastShowingEntry = Number(this.startingEntry)+Number(this.numOfEntries);
    else
        this.lastShowingEntry = this.totalEntries;

    if(this.dataTable != null && this.dataTable.aaData != null){
        for(i = this.startingEntry; i< this.lastShowingEntry; i++){
            entries.push(this.dataTable.aaData[i]);
        }
        this.lastShowingEntry = i;
        return entries;
    }else
        return null;
}

And in my html I got something like this:

<div class="col-md-12" *ngIf="isDataTableAvailable">
     <table id="table-EDIImports" class="table table-bordered display" cellspacing="0">
         <thead>
              <tr>
                   <th><strong>{{ 'POINT_OF_SALE' | translate }}</strong></th>

                    (more fields)

                    <th *ngIf="mode=='EDIT'"></th>
             </tr>
             <tr *ngFor="let obj of entriesToShow()" [ngSwitch]="obj.Status">
                    <th>{{ obj.PointOfSell }}</th>
                    <th *ngIf="obj.LineID == editRowId && selectedField == 'NIF' && mode=='EDIT'">
                        <input type="text" [(ngModel)]="valueToSave" value="{{ obj.NIF }}">
                     </th>
                        (more fields)
              </tr>
           </thead>
           <tbody></tbody>
       </table>
 </div>

Any advice to make my page stop calling entriesToShow() everytime I scroll up/down?

Thank for the help.

EDIT: Removed some extra code

Answer 1

Don't call the entriesToShow() function from your template! It cause's the function to be called every change detection! Instead you should store the data in a variable in your component and the ngFor should iterate over him. .component.ts

        entries:Array<any>;

        ngOnInit(){
           this.entries=this.entriesToShow();
        }
        entriesToShow(): Array<any>{
            let entries = [];
            let i = 0;
            if(Number(this.startingEntry)+Number(this.numOfEntries) <this.totalEntries)
                this.lastShowingEntry = Number(this.startingEntry)+Number(this.numOfEntries);
            else
                this.lastShowingEntry = this.totalEntries;

            if(this.dataTable != null && this.dataTable.aaData != null){
                for(i = this.startingEntry; i< this.lastShowingEntry; i++){
                    entries.push(this.dataTable.aaData[i]);
                }
                this.lastShowingEntry = i;
                return entries;
            }else
                return null;
        }

.html

<div class="col-md-12" *ngIf="isDataTableAvailable">
     <table id="table-EDIImports" class="table table-bordered display" cellspacing="0">
         <thead>
              <tr>
                   <th><strong>{{ 'POINT_OF_SALE' | translate }}</strong></th>

                    (more fields)

                    <th *ngIf="mode=='EDIT'"></th>
             </tr>
             <tr *ngFor="let obj of entries" [ngSwitch]="obj.Status">
                    <th>{{ obj.PointOfSell }}</th>
                    <th *ngIf="obj.LineID == editRowId && selectedField == 'NIF' && mode=='EDIT'">
                        <input type="text" [(ngModel)]="valueToSave" value="{{ obj.NIF }}">
                     </th>
                        (more fields)
              </tr>
           </thead>
           <tbody></tbody>
       </table>
 </div>