Angular 2: default radio button selection

Question

I have two radio buttons (they're not dynamically generated):

<input type="radio" name="orderbydescending" [(ngModel)]="orderbydescending" [value]="['+recordStartDate']">Ascending<br>
<input type="radio" name="orderbydescending" [(ngModel)]="orderbydescending" [value]="['-recordStartDate']">Descending

How do I make one of the radio buttons checked by default?

Thank you!

Edit

The button's values are being passed through this pipe (i.e. not a component, per se...not sure this is worth mentioning?). The app is pretty simple, and the radio buttons are just hardcoded into app.component. Is the pipe the correct place to initialize which radio button is checked by default?

import { Pipe, PipeTransform } from '@angular/core';
@Pipe({name: 'orderBy', pure: false})
export class OrderByPipe implements PipeTransform {

static _OrderByPipeComparator(a:any, b:any):number{

  if((isNaN(parseFloat(a)) || !isFinite(a)) || (isNaN(parseFloat(b)) || !isFinite(b))){
    //Isn't a number so lowercase the string to properly compare
    if(a.toLowerCase() < b.toLowerCase()) return -1;
    if(a.toLowerCase() > b.toLowerCase()) return 1;
  }
  else{
    //Parse strings as numbers to compare properly
    if(parseFloat(a) < parseFloat(b)) return -1;
    if(parseFloat(a) > parseFloat(b)) return 1;
  }

  return 0; //equal each other
}

transform(input:any, [config = '+']): any{

    if(!Array.isArray(input)) return input;

    if(!Array.isArray(config) || (Array.isArray(config) && config.length == 1)){
        var propertyToCheck:string = !Array.isArray(config) ? config : config[0];
        var desc = propertyToCheck.substr(0, 1) == '-';

        //Basic array
        if(!propertyToCheck || propertyToCheck == '-' || propertyToCheck == '+'){
            return !desc ? input.sort() : input.sort().reverse();
        }
        else {
            var property:string = propertyToCheck.substr(0, 1) == '+' || propertyToCheck.substr(0, 1) == '-'
                ? propertyToCheck.substr(1)
                : propertyToCheck;

            return input.sort(function(a:any,b:any){
                return !desc
                    ? OrderByPipe._OrderByPipeComparator(a[property], b[property])
                    : -OrderByPipe._OrderByPipeComparator(a[property], b[property]);
            });
        }
    }
    else {
        //Loop over property of the array in order and sort
        return input.sort(function(a:any,b:any){
            for(var i:number = 0; i < config.length; i++){
                var desc = config[i].substr(0, 1) == '-';
                var property = config[i].substr(0, 1) == '+' || config[i].substr(0, 1) == '-'
                    ? config[i].substr(1)
                    : config[i];

                var comparison = !desc
                    ? OrderByPipe._OrderByPipeComparator(a[property], b[property])
                    : -OrderByPipe._OrderByPipeComparator(a[property], b[property]);

                //Don't return 0 yet in case of needing to sort by next property
                if(comparison != 0) return comparison;
            }

            return 0; //equal each other
        });
    }
 }

Edit 2

So in component.app.ts I've edited my export class AppComponent{ to the following:

export class AppComponent {
  artists = ARTISTS;
  currentArtist: Artist;
  orderbydescending = ['-recordStartDate'];

showArtist(item) {
  this.currentArtist = item;

} }

This works in terms of preventing the previous errors, but it doesn't actually make the radio button selected. It still appears as though it's unselected - even though it functions as though it is. Does this make sense?

Answer 1

Try this in the component

radioValue = {valueAsc: 'Asc', valueDesc: 'Desc'} ;
orderbydescending = 'Asc';

and in the template put this

<input type="radio" name="radioGroup" [(ngModel)]="orderbydescending" [value]="radioValue.valueAsc">Ascending

<input type="radio" name="radioGroup" [(ngModel)]="orderbydescending" [value]="radioValue.valueDesc">Descending

With this the first radio button is checked, if you don't want any radio button selected assign to the variable orderbydescending null.

orderbydescending = 'null;

Answer 2

If orderbydescending and recordStartDate are members of your component class:

@Component({
    ...
})
export class MyComponent {
    public recordStartDate: any = ... 
    public orderbydescending: any = +recordStartDate;
    ...
}

the appropriate radio button will be checked if you assign the radio button values with [value]:

<input type="radio" name="order" [(ngModel)]="orderbydescending" [value]="+recordStartDate">Ascending<br>
<input type="radio" name="order" [(ngModel)]="orderbydescending" [value]="-recordStartDate">Descending

In the case shown above, the ascending order radio button will be checked by default because the orderbydescending variable is initially set to +recordStartDate.

Note: The variables in my sample code are of type any because I don't know exactly what kind of data you are using. Your data will probably have a more specific data type.

Answer 3

If you're doing this in Angular 2+ with 2 way binding, in the component where this HTML is being used, you could just try initializing the value associated with the input.

// in your component ts file
orderbydescending: boolean = true;

and you could leave the HTML the same. Although, you seem to have 2 radio buttons associated with the same data value, orderbydescending. I don't know if that's what you intend, but it looks like it could cause problems.

Here's some code from my personal side project to give you a better idea.

@Component({
  selector: 'gmu-home-page',
  templateUrl: './home-page.component.html',
  styleUrls: ['./home-page.component.css']
})
export class HomePageComponent implements OnInit {
  // here you would put your variables
  myFlag: boolean = true;
  constructor() { }
  ngOnInit() {
  }
}