What is the difference between [ngFor] and [ngForOf] in angular2?

Question

As per my understanding, Both are doing the same functions. But,

• ngFor would be works like as collections?.

• ngForOf would be works like as generics?.

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Is my understanding is correct? or Could you please share more difference's(details) about ngFor and ngForOf?

Calen

Good question! i also wanted to ask! btw, your links for both collections and generics are same – Tundra

@Tundra Typo issue. Sorry for that. Now I have updated my question. thanks – Calen

github.com/angular/angular/commit/… – Burgoyne

@yurzui, good catch. So it seems as though the class was previously NgFor and it got changed in the move to angular v4, and the key thing is that we as a community were none the wiser. Awesome backwards compatibility – Cartie

Answer 1

ngFor and ngForOf are not two distinct things - they are actually the selectors of the NgForOf directive.

If you examine the source, you'll see that the NgForOf directive has as its selector: [ngFor][ngForOf] , meaning that both attributes need to be present on an element for the directive to 'activate' so to speak.

When you use *ngFor, the Angular compiler de-sugars that syntax into its cannonical form which has both attributes on the element.

So,

  <div *ngFor="let item of items"></div>

desugars to:

 <template [ngFor]="let item of items">
     <div></div>
 </template>

This first de-sugaring is due to the '*'. The next de-sugaring is because of the micro syntax: "let item of items". The Angular compiler de-sugars that to:

 <template ngFor let-item="$implicit" [ngForOf]="items">
   <div>...</div>
 </template>

(where you can think of $implicit as an internal variable that the directive uses to refer to the current item in the iteration).

In its canonical form, the ngFor attribute is just a marker, while the ngForOf attribute is actually an input to the directive that points to the the list of things you want to iterate over.

You can check out the Angular microsyntax guide to learn more.

Cartie

Isn't ngFor the actual directive whereas ngForOf is "just" a property of the "de-sugared" version? – Vandalism

well, technically, ngForOf is the name of the class that implements the directive (again, see the source). It's just that no one actually refers to it by that name - ngFor is basically how its referred to in all documentation and conversations. – Cartie

Your desugaring examples are both quite wrong. I don't think there are 2 steps, and the end result would be <template ngFor let-item="$implicit" [ngForOf]="items"> </template> – Marieann

@GünterZöchbauer, you are right - i put the directives in the wrong place - i updated the code examples. And you're probably also right that the de-sugaring happens in just one step, but i think in this case, it's useful to separate the desugaring of the structural directive from the desugaring of the let micro-syntax to show that they are (relatively) independent things – Cartie

Answer 2

ngFor is a structural directive of Angular which replaces the ng-repeat attribute of AngularJS

You can use ngFor as a shorthand

<li *ngFor="let item of items">{{item.name}}</li>

or as the longhand version

<template ngFor let-item="$implicit" [ngForOf]="items">
  {{item.name}}
</template>

Vandalism

shorthand and longhand means no of data size? or what? – Calen

It means: Difference of usage for the same purpose and effect – Vandalism

Okay now I understand it. But whenever am seeing this line in the document ngForOf: NgIterable<T>: The value of the iterable expression. Useful when the expression is more complex then a property access, for example when using the async pipe (userStreams | async). really not understandable. – Calen

Let's say your items collection is not a simple collection but rather a collection which is not yet available and will only be available after some asynchronous call. So you won't have items yet but only an Observable or Promise (let's call it futureItems). In that case you'd like to use the async pipe let item of (futureItems|async) which is not possible. But you can use the longhand version with [ngForOf]="futureItems | async" – Vandalism

Answer 3

In my opinion what I got from the angular document,

• [ngFor] is not type safe
• [NgForOf] is type safe

Because both class details are little different

• ngFor Class type is any type

• But ngForOf class type is generic ngForOf : NgIterable<T>

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ngForOf looks like generics which I have already mentioned in my question.

Calen

Interesting. However, since they are used from within templates, this matters less in practice than it otherwise would. That said, the number of Angular APIs, including ones not intended for template use such as FormBuilder, which are not type checked is troubling. – Dvina

Answer 4

NgFor can iterate an array but in ngContainer, ngContent we cannot iterate ngFor directly so For iterating we can use [ngForOf] . Here i is variable, ratings is an array.

Ex.

 <ng-template ngFor let-i [ngForOf]="ratings">
                  <option *ngIf="i<=22" [ngValue]="i">{{i}}:00 Hrs.</option>
   </ng-template>

At this example, I want to apply a ngFor and ngIf simultaneously, so i used it.

The other view is that at a normal if we apply ngFor then at rendering it converts it into

<template ngFor let-i [ngForOf]="ratings">
   <HTML Element>...</HTML Element>
 </template>

Compress

Answer 5

Explicit version

• (<template ngFor ...>) allows to apply the directive to multiple elements at once

Implicit version

• only wraps the element where it is applied

Verb