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Angular 17 for loop example | @for block - Repeaters | track expression | @empty block

B Built-in control flow and the NgIf, NgSwitch and NgFor structural directives:

1.     The @if block replaces *ngIf for expressing conditional parts of the UI.

2.     The @switch block replaces ngSwitch with major benefits.

3.     The @for block replaces *ngFor for iteration, and has several differences compared to its structural directive NgFor predecessor.

4.     The track setting replaces NgFor's concept of a trackBy function.

Before Angular 17, the for loop looks like:

<li *ngFor="let user of users; index as i; first as isFirst">

  {{i}}/{{users.length}}. {{user}} <span *ngIf="isFirst">default</span>

</li>

 

From Angular 17, @for block - Repeaters

The @for repeatedly renders the content of a block for each item in a collection. The collection can be represented as any JavaScript alterable but there are performance advantages of using a regular Array.

A basic @for loop looks like:

@for (item of items; track item.id) {

  {{ item.name }}

}

What Is ‘track’?

The track setting replaces NgFor's concept of a trackBy function.

The value of the ‘track’ expression determines a key used to associate array items with the views in the DOM. 

Inside @for contents, several implicit variables are always available:

1.      $count:         Number of items in a collection iterated over

2.      $index:         Index of the current row

3.      $first:            Whether the current row is the first row

4.      $last:             Whether the current row is the last row

5.      $even:          Whether the current row index is even

6.      $odd:            Whether the current row index is odd


These variables are always available with these names, but can be aliased via a let segment:

@for (item of items; track item.id; let idx = $index, e = $even) {

  Item #{{ idx }}: {{ item.name }}

}


@empty block

You can optionally include an @empty section immediately after the @for block content. The content of the @empty block displays when there are no items:

A basic @empty block looks like:

@for (item of items; track item.name) {

  <li> {{ item.name }} </li>

} @empty {

  <li> There are no items. </li>

}


Let see the other Example,

@for (user of users; track user.id) {

  {{ user.name }}

} @empty {

  Empty list of users

}


Examples: Loop over an array of users with Built-in for loop using @for:

const users = [

  { id: 1, name: ‘Anil’ },

  { id: 2, name: 'Reena' },

  { id: 3, name: 'Aradhya' },

  { id: 4, name: 'Shiva' },

];

<ul>

  @for (user of users; track user.id) {

    <li>{{ user.name }}</li>

  } @empty {

    <span>Empty list of users</span>

  }

</ul>


The output looks like this:

1)     Anil

2)     Reena

3)     Aradhya

4)     Shiva

 

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