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Nigro Simone
Nigro Simone

Posted on

Angular global ngFor track by property directive with strict type checking.

When displaying a list of data (at least somewhat large lists) you should be using Angular's trackBy feature which looks something like:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackBy: trackById">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppListComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];

  public trackById(index: number, item: Item) {
    return item.id;
  }
}
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Unfortunately, Angular forces us to write a tracking function in each component in which we want to make use of trackBy. With ng-for-track-by-property you could just handle this entirely in the template by passing a property like this:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackByProperty: 'id'">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppListComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}
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ng-for-track-by-property has strict type checking and only available property are allowed
Image description

Get Started

Step 1: install ng-for-track-by-property

npm i ng-for-track-by-property
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Step 2: Import NgForTrackByPropertyModule into your app module, eg.:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
import { CommonModule } from '@angular/common';
import { NgForTrackByPropertyModule } from 'ng-for-track-by-property';

@NgModule({
  declarations: [AppComponent],
  imports: [
    BrowserModule,
    CommonModule,
    NgForTrackByPropertyModule,
  ],
  providers: [],
  bootstrap: [AppComponent],
  ],
})
export class AppModule { }
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Step 3: add trackByProperty to your ngFor, eg.:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackByProperty: 'id'">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}
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you can also track by index with trackByIndex, eg.:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackByIndex">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}
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since track by property id is a very common case, there is also trackById:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackById">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}
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See:

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