interface TrackByFunction {
}
An optional function passed into NgForOf that defines how to track items in an iterable (e.g. fby index or id)
exported from core/index, defined in core/src/change_detection/differs/iterable_differs.ts
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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/docs/ts/latest/api/core/index/TrackByFunction-interface.html