Tips for passing an object by value to an Angular2+ component

Question

Tips for passing an object by value to an Angular2+ component

Imagine having a scenario where I create a component that takes a Foo instance and generates a form for editing.

export class ChildComponent implements OnInit {
  @Input() foo : Foo;
  @Output() onChange : EventEmitter<Foo> = new EvenEmitter<Foo>();

  constructor() {
  }

  ngOnInit() {
  }
}

I then include this ChildComponent within a ParentComponent.

<div id="parent">
  <app-child-component [foo]="parentFoo"></app-child-component>
</div>

Although I used one-way binding, the fact that foo is an object passed by reference means any changes made to it in ChildComponent will also affect ParentComponent.

Is there a way to avoid this or pass by value instead? Are there any recommended practices?

javascript angular typescript parameter-passing pass-by-value

Answer 1

Answer №1

Here is the solution I have come up with:

class ChildComponent {
  private _bar: Bar;
  
  @Input()
  set bar(value: Bar) { this._bar = Object.create(value || null); }
  get bar(): Bar { return this._bar; }

  @Output() onUpdate: EventEmitter<Bar> = new EventEmitter<Bar>();

  constructor() {}

  ngOnInit() {}
}

While this code works, it feels like a lot for just one attribute. It makes me wonder why Angular does not have built-in functionality like an @InputByValue decorator.

I initially thought using two-way binding [(bar)]="bar" would automatically reflect changes from child to parent. Am I missing something or is there a reason not to approach it this way?

Answer 2

Here is the solution I have come up with:

class ChildComponent {
  private _bar: Bar;
  
  @Input()
  set bar(value: Bar) { this._bar = Object.create(value || null); }
  get bar(): Bar { return this._bar; }

  @Output() onUpdate: EventEmitter<Bar> = new EventEmitter<Bar>();

  constructor() {}

  ngOnInit() {}
}

While this code works, it feels like a lot for just one attribute. It makes me wonder why Angular does not have built-in functionality like an @InputByValue decorator.

I initially thought using two-way binding [(bar)]="bar" would automatically reflect changes from child to parent. Am I missing something or is there a reason not to approach it this way?

Answer 3

Answer №2

If you want to avoid directly modifying the input reference, consider creating a new object instead. One way to do this is by utilizing the Spread syntax to clone the object and then work with the new copy in your template.

@Input('foo') fooRef : Foo;
foo: Foo;
.
.
.
ngOnInit() {
    // Using spread syntax to create a new object
    this.foo = {...this.fooRef};
}

Answer 4

If you want to avoid directly modifying the input reference, consider creating a new object instead. One way to do this is by utilizing the Spread syntax to clone the object and then work with the new copy in your template.

@Input('foo') fooRef : Foo;
foo: Foo;
.
.
.
ngOnInit() {
    // Using spread syntax to create a new object
    this.foo = {...this.fooRef};
}

Answer 5

Answer №3

According to Gunter's explanation found in this response: Angular2: Pass by reference to interact between components

When dealing with primitive values (such as strings, numbers, booleans, and object references), they are passed by value (copied). On the other hand, objects and arrays are passed by reference, meaning both components receive a reference to the same object instance.

This concept is not specific to Angular; rather, it reflects how JavaScript, including TypeScript, functions at its core.

In practice, when passing an object into a child component and making changes there, you are essentially modifying the original object, resulting in synchronized values across components.

However, if your intention is to create a separate copy of the object, you will need to generate a local copy using methods like deep copying within the child component. To see this process in action, refer to the following Stackblitz example demonstrating the technique: https://stackblitz.com/edit/angular-axr5zf.

Answer 6

According to Gunter's explanation found in this response: Angular2: Pass by reference to interact between components

When dealing with primitive values (such as strings, numbers, booleans, and object references), they are passed by value (copied). On the other hand, objects and arrays are passed by reference, meaning both components receive a reference to the same object instance.

This concept is not specific to Angular; rather, it reflects how JavaScript, including TypeScript, functions at its core.

In practice, when passing an object into a child component and making changes there, you are essentially modifying the original object, resulting in synchronized values across components.

However, if your intention is to create a separate copy of the object, you will need to generate a local copy using methods like deep copying within the child component. To see this process in action, refer to the following Stackblitz example demonstrating the technique: https://stackblitz.com/edit/angular-axr5zf.

Answer 7

Answer №4

One way to duplicate your object is by using the spread syntax in JavaScript - simply write `const copy = { ...original }`. This method will create an exact copy of your object.

Answer 8

One way to duplicate your object is by using the spread syntax in JavaScript - simply write `const copy = { ...original }`. This method will create an exact copy of your object.

Tips for passing an object by value to an Angular2+ component

Answer №1

Answer №2

Answer №3

Answer №4

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