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