property-binding-best-practices.md
# Property binding best practices
By following a few guidelines, you can use property binding in a way that helps you reduce bugs and keep your code readable.
<div class="alert is-helpful">
See the <live-example name="property-binding"></live-example> for a working example containing the code snippets in this guide.
</div>
## Avoid side effects
Evaluation of a template expression should have no visible side effects.
Use the syntax for template expressions to help avoid side effects.
In general, the correct syntax prevents you from assigning a value to anything in a property binding expression.
The syntax also prevents you from using increment and decrement operators.
### An example of producing side effects
If you had an expression that changed the value of something else that you were binding to, that change of value would be a side effect.
Angular might or might not display the changed value.
If Angular does detect the change, it throws an error.
As a best practice, use only properties and methods that return values.
## Return the proper type
A template expression should result in the type of value that the target property expects.
For example, return:
* a `string`, if the target property expects a string
* a `number`, if it expects a number
* an `object`, if it expects an object.
### Passing in a string
In the following example, the `childItem` property of the `ItemDetailComponent` expects a string.
<code-example header="src/app/app.component.html" path="property-binding/src/app/app.component.html" region="model-property-binding"></code-example>
Confirm this expectation by looking in the `ItemDetailComponent` where the `@Input()` type is `string`:
<code-example header="src/app/item-detail/item-detail.component.ts (setting the @Input() type)" path="property-binding/src/app/item-detail/item-detail.component.ts" region="input-type"></code-example>
The `parentItem` in `AppComponent` is a string, which means that the expression, `parentItem` within `[childItem]="parentItem"`, evaluates to a string.
<code-example header="src/app/app.component.ts" path="property-binding/src/app/app.component.ts" region="parent-data-type"></code-example>
If `parentItem` were some other type, you would need to specify `childItem` `@Input()` as that type as well.
### Passing in an object
In this example, `ItemListComponent` is a child component of `AppComponent` and the `items` property expects an array of objects.
<code-example header="src/app/app.component.html" path="property-binding/src/app/app.component.html" region="pass-object"></code-example>
In the `ItemListComponent` the `@Input()`, `items`, has a type of `Item[]`.
<code-example header="src/app/item-list.component.ts" path="property-binding/src/app/item-list/item-list.component.ts" region="item-input"></code-example>
Notice that `Item` is an object and it has two properties, an `id` and a `name`.
<code-example header="src/app/item.ts" path="property-binding/src/app/item.ts" region="item-class"></code-example>
In `app.component.ts`, `currentItems` is an array of objects in the same shape as the `Item` object in `items.ts`, with an `id` and a `name`.
<code-example header="src/app.component.ts" path="property-binding/src/app/app.component.ts" region="pass-object"></code-example>
By supplying an object in the same shape, you meet the expectations of `items` when Angular evaluates the expression `currentItems`.
<!-- links -->
<!-- external links -->
<!-- end links -->
@reviewed 2022-02-28