如何为 Angular 2 组件属性设置默认值？

Question

When writing Angular 2.0 components, how does one set default values for properties?

For example - I want to set foo to 'bar' by default, but the binding might immediately resolve to 'baz' . How does this play out in the lifecycle hooks?

@Component({  
    selector: 'foo-component'
})
export class FooComponent {
    @Input()
    foo: string = 'bar';

    @Input()
    zalgo: string;

    ngOnChanges(changes){
          console.log(this.foo);
          console.log(changes.foo ? changes.foo.previousValue : undefined);
          console.log(changes.foo ? changes.foo.currentValue : undefined);
    }
}

Given the following templates, this is what I expect the values would be. Am I wrong?

<foo-component [foo] = 'baz'></foo-component>

Logged to console:

'baz'
'bar'
'baz'

<foo-component [zalgo] = 'released'></foo-component>

Logged to console:

'bar'
undefined
undefined

Answer 1

That is interesting subject. You can play around with two lifecycle hooks to figure out how it works: ngOnChanges and ngOnInit .

Basically when you set default value to Input that's mean it will be used only in case there will be no value coming on that component. And the interesting part it will be changed before component will be initialized.

Let's say we have such components with two lifecycle hooks and one property coming from input .

@Component({
  selector: 'cmp',
})
export class Login implements OnChanges, OnInit {
  @Input() property: string = 'default';

  ngOnChanges(changes) {
    console.log('Changed', changes.property.currentValue, changes.property.previousValue);
  }

  ngOnInit() {
    console.log('Init', this.property);
  }

}

Situation 1

Component included in html without defined property value

As result we will see in console: Init default

That's mean onChange was not triggered. Init was triggered and property value is default as expected.

Situation 2

Component included in html with setted property <cmp [property]="'new value'"></cmp>

As result we will see in console:

Changed new value Object {}

Init new value

And this one is interesting. Firstly was triggered onChange hook, which setted property to new value , and previous value was empty object ! And only after that onInit hook was triggered with new value of property .

Answer 2

Here is the best solution for this. (ANGULAR All Version)

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 .

I have provided solution for this kind of similar question. You can find the full solution from here

export class CarComponent implements OnInit {
  private _defaultCar: car = {
    // default isCar is true
    isCar: true,
    // default wheels  will be 4
    wheels: 4
  };

  @Input() newCar: car = {};

  constructor() {}

  ngOnInit(): void {

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

    this.newCar = { ...this._defaultCar, ...this.newCar };
   //  console.log(this.newCar);
  }
}