Angular4: How do I *ngFor, for every number in an int?

Question

I have an int property named "count" in my component.

I would like to display a p tag X amount of times, where X is the int my count property equals. Is there really no simple way to do this, besides messing with fake arrays?

Answer 1

You could easily do it with an pipe filter which transforms an empty array to an number of childs depending on a filter param = number.

Pipe filter

import { Pipe, PipeTransform } from '@angular/core';  

@Pipe({  
    name: 'range',  
    pure: false  
})  

export class RangePipe implements PipeTransform {  
    transform(items: any[], quantity: number): any {  
      items.length = 0;
      for (let i = 0; i < quantity; i++) {
        items.push(i);
      }
      return items;
    }  
}  

View

<div *ngFor="let n of [] | range:100"></div>

Answer 2

Plnkr: https://plnkr.co/edit/Yn775KSbBeUPeyaI9sep?p=preview

You can create another variable called countObservable

  countObservable = Observable.range(0, this.count).toArray();

Use async for in HTML

  <p *ngFor="let num of countObservable | async" >Hello {{num}}</h2>

Update

If we need to update the number we can use flatMap.

Instead of above code for countObservable, use this

count$= new BehaviorSubject(10);
countObservable$ = 
  this.count$.flatMap(count => Observable.range(0, count).toArray()) ;

To change the number value, just update count$

this.count$.next(newNum);

Answer 3

I kind of disliked the approach of creating an empty array of size n every time that I wanted to render an element n times, so I created a custom structural directive:

import { Directive, Input, TemplateRef, ViewContainerRef, isDevMode, EmbeddedViewRef } from '@angular/core';

export class ForNumberContext {
  constructor(public count: number, public index: number) { }
  get first(): boolean { return this.index === 0; }

  get last(): boolean { return this.index === this.count - 1; }

  get even(): boolean { return this.index % 2 === 0; }

  get odd(): boolean { return !this.even; }
}

@Directive({
  selector: '[ForNumber]'
})
export class ForNumberDirective {


 @Input() set forNumberOf(n: number) {
    this._forNumberOf = n;
    this.generate();
  }

  private _forNumberOf: number;

  constructor(private _template: TemplateRef<ForNumberContext>,
    private _viewContainer: ViewContainerRef) { }

  @Input()
  set ngForTemplate(value: TemplateRef<ForNumberContext>) {
    if (value) {
      this._template = value;
    }
  }

  private generate() {
    for (let i = 0; i < this._forNumberOf; i++) {
      this._viewContainer.createEmbeddedView(this._template, new ForNumberContext(this._forNumberOf, i));
    }
  }

}

And then u can use it as follows:

<ng-template ForNumber [forNumberOf]="count" let-index="index">
<span>Iteration: {{index}}!</span></ng-template>

Please note, I havent tested it extensively so I cant promise that its bulletproof :)

Answer 4

I solved it using :

In TS:

months = [...Array(12).keys()];

In Template:

<p *ngFor="let month of months">{{month+1}}</p>

Answer 5

According Angular documentation:

createEmbeddedView() Instantiates an embedded view and inserts it into this container. it accepts a context object as second parameter: abstract createEmbeddedView(templateRef: TemplateRef, context?: C, index?: number): EmbeddedViewRef.

When angular creates template by calling createEmbeddedView it can also pass context that will be used inside ng-template.

Using context optional parameter, you may use it in the component extracting it within the template just as you would with the *ngFor.

app.component.html:

 <p *for="randomNumber; let i = index; let first = first; let last = last; let even = even, let odd = odd; length = length">
   index :{{i}},
   length:{{length}},
   is first : {{first}},
   is last : {{last}},
   is even : {{even}},
   is odd : {{odd}}
 </p>

for.directive.ts:

import { Directive, Input, TemplateRef, ViewContainerRef, EventEmitter } from '@angular/core';

@Directive({
  selector: '[for]'
})
export class ForDirective {

  constructor(
    private templateRef: TemplateRef<any>,
    private viewContainer: ViewContainerRef) { }

  @Input('for') set loop(num: number) {
    for (var i = 0; i < num; i++)
      this.viewContainer.createEmbeddedView(
        this.templateRef,
        {
          index: i,
          odd: i % 2 == 1,
          even: i % 2 == 0,
          first: i == 0,
          last: i == num,
          length: num - 1,
        }
      );
  }
}