如何在Angular2中为@Input设置默认参数？

Question

Say I have a component that will display a name property, so it roughly goes like this:

 import {Component, Input} from 'angular2/core'; @Component({ selector: 'demo', template: `<div>{{name}}</div>`, styles: [``], }) export class Demo { @Input() name: string; }

The problem is, how could I display [noname] when someone using this component but not passing any name property?

The only solution comes to mind is using logic operator inside template like {{ name || '[noname]' }} {{ name || '[noname]' }} .

Answer 1

尝试

@Input() name: string = 'noname';

Answer 2

You can intercept @Input() with a setter and have it backed by a private field. In setter, you do a nullcheck so field gets set only to a non-null value. As last, you bind your template to a private fied in which you have set initial value.

Answer 3

with angular 8 and the default tslint settings your IDE will notice:

so it´s okay to just write:

@Input() addclass = '';

without any "type annotation".

Answer 4

In the component you should initialize like:

@Input () name:String='';

In the HTML you can use:

{{ name ===''?  'empty string': name }}

Answer 5

I think you can use your idea of using the template. So it would be:

In Component:

@Input () name:String;

In Template:

<div>{{ name != '' ? name : '[no name]' }}</div>

That would check to see if the name is empty, and use '[no name]' or insert the name if the name is passed.

Answer 6

Here is the proper solution to this. (ANGULAR 2 to 9)

Addressing solution: To set a default value for @Input variable. If no value passed to that input variable then It will take the default value.

Example:

I have an object interface named bike:

export interface bike {
  isBike?: boolean;
  wheels?: number;
  engine?: string;
  engineType?: string;
}

You made a component named app-bike where you need to pass the properties of bike with @input decorator of angular. But you want that isBike and Wheels properties must have a default value (ie.. isBike: true, wheels: 2)

export class BikeComponent implements OnInit {
  private _defaultBike: bike = {
    // default isBike is true
    isBike: true,
    // default wheels  will be 2
    wheels: 2
  };

  @Input() newBike: bike = {};

  constructor() {}

  ngOnInit(): void {

   // this will concate both the objects and the object declared later (ie.. ...this.newBike )
   // will overwrite the default value. ONLY AND ONLY IF DEFAULT VALUE IS PRESENT

    this.newBike = { ...this._defaultBike, ...this.newBike };
   //  console.log(this.newBike);
  }
}

For more detailed article refer to this .

Refer Destructuring assignment from here