track-by-id.directive.ts

import {Directive, Host, Input} from '@angular/core';
import {NgForOf} from '@angular/common';

// Usage:
// *ngFor="let message of messages; trackById: 'created'"

@Directive({
  selector: '[ngForTrackById]'
})
export class TrackByIdDirective<T> {

    @Input('ngForTrackById')
    property;

    constructor(
        @Host()
        private ngFor: NgForOf<T>
    ) {
        this.ngFor.ngForTrackBy = (index: number, item: T) => item[this.property];
    }

}