Just as TypeScript catches type errors in your code, Angular checks the expressions and bindings within the templates of your application and can report any type errors it finds. Angular currently has three modes of doing this, depending on the value of the fullTemplateTypeCheck
and strictTemplates
flags in the TypeScript configuration file.
In the most basic type-checking mode, with the fullTemplateTypeCheck
flag set to false
, Angular validates only top-level expressions in a template.
If you write <map [city]="user.address.city">
, the compiler verifies the following:
user
is a property on the component class.user
is an object with an address property.user.address
is an object with a city property.The compiler does not verify that the value of user.address.city
is assignable to the city input of the <map>
component.
The compiler also has some major limitations in this mode:
*ngIf
, *ngFor
, other <ng-template>
embedded view.#refs
, the results of pipes, the type of $event
in event bindings, and so on.In many cases, these things end up as type any
, which can cause subsequent parts of the expression to go unchecked.
If the fullTemplateTypeCheck
flag is set to true
, Angular is more aggressive in its type-checking within templates. In particular:
*ngIf
or *ngFor
) are checked.any
).The following still have type any
.
$event
object.Angular version 9 maintains the behavior of the fullTemplateTypeCheck
flag, and introduces a third "strict mode". Strict mode is a superset of full mode, and is accessed by setting the strictTemplates
flag to true. This flag supersedes the fullTemplateTypeCheck
flag. In strict mode, Angular version 9 adds checks that go beyond the version 8 type-checker. Note that strict mode is only available if using Ivy.
In addition to the full mode behavior, Angular version 9:
@Input()
s.strictNullChecks
flag when validating the above.NgFor
).$event
in component/directive, DOM, and animation event bindings.document.createElement
would return for that tag).*ngFor
The three modes of type-checking treat embedded views differently. Consider the following example.
interface User { name: string; address: { city: string; state: string; } }
<div *ngFor="let user of users"> <h2>{{config.title}}</h2> <span>City: {{user.address.city}}</span> </div>
The <h2>
and the <span>
are in the *ngFor
embedded view. In basic mode, Angular doesn't check either of them. However, in full mode, Angular checks that config
and user
exist and assumes a type of any
. In strict mode, Angular knows that the user
in the <span>
has a type of User
, and that address
is an object with a city
property of type string
.
When enabling the new strict mode in version 9, you might encounter template errors that didn't arise in either of the previous modes. These errors often represent genuine type mismatches in the templates that were not caught by the previous tooling. If this is the case, the error message should make it clear where in the template the problem occurs.
There can also be false positives when the typings of an Angular library are either incomplete or incorrect, or when the typings don't quite line up with expectations as in the following cases.
null | undefined
if the library was not written with strictNullChecks
in mind).<input disabled>
.$event.target
for DOM events (because of the possibility of event bubbling, $event.target
in the DOM typings doesn't have the type you might expect).In case of a false positive like these, there are a few options:
$any()
type-cast function in certain contexts to opt out of type-checking for a part of the expression.strictTemplates: false
in the application's TypeScript configuration file, tsconfig.json
.false
.strictTemplates
and strictNullChecks
together, you can opt out of strict null type checking specifically for input bindings via strictNullInputTypes
.Strictness flag | Effect |
---|---|
strictInputTypes | Whether the assignability of a binding expression to the @Input() field is checked. Also affects the inference of directive generic types. |
strictInputAccessModifiers | Whether access modifiers such as private /protected /readonly are honored when assigning a binding expression to an @Input() . If disabled, the access modifiers of the @Input are ignored; only the type is checked. |
strictNullInputTypes | Whether strictNullChecks is honored when checking @Input() bindings (per strictInputTypes ). Turning this off can be useful when using a library that was not built with strictNullChecks in mind. |
strictAttributeTypes | Whether to check @Input() bindings that are made using text attributes (for example, <mat-tab label="Step 1"> vs <mat-tab [label]="'Step 1'"> ). |
strictSafeNavigationTypes | Whether the return type of safe navigation operations (for example, user?.name ) will be correctly inferred based on the type of user ). If disabled, user?.name will be of type any . |
strictDomLocalRefTypes | Whether local references to DOM elements will have the correct type. If disabled ref will be of type any for <input #ref> . |
strictOutputEventTypes | Whether $event will have the correct type for event bindings to component/directive an @Output() , or to animation events. If disabled, it will be any . |
strictDomEventTypes | Whether $event will have the correct type for event bindings to DOM events. If disabled, it will be any . |
strictContextGenerics | Whether the type parameters of generic components will be inferred correctly (including any generic bounds). If disabled, any type parameters will be any . |
strictLiteralTypes | Whether object and array literals declared in the template will have their type inferred. If disabled, the type of such literals will be any . |
If you still have issues after troubleshooting with these flags, you can fall back to full mode by disabling strictTemplates
.
If that doesn't work, an option of last resort is to turn off full mode entirely with fullTemplateTypeCheck: false
, as we've made a special effort to make Angular version 9 backwards compatible in this case.
A type-checking error that you cannot resolve with any of the recommended methods can be the result of a bug in the template type-checker itself. If you get errors that require falling back to basic mode, it is likely to be such a bug. If this happens, please file an issue so the team can address it.
In Angular version 9, the template type checker checks whether a binding expression's type is compatible with that of the corresponding directive input. As an example, consider the following component:
export interface User { name: string; } @Component({ selector: 'user-detail', template: '{{ user.name }}', }) export class UserDetailComponent { @Input() user: User; }
The AppComponent
template uses this component as follows:
@Component({ selector: 'my-app', template: '<user-detail [user]="selectedUser" />', }) export class AppComponent { selectedUser: User | null = null; }
Here, during type checking of the template for AppComponent
, the [user]="selectedUser"
binding corresponds with the UserDetailComponent.user
input. Therefore, Angular assigns the selectedUser
property to UserDetailComponent.user
, which would result in an error if their types were incompatible. TypeScript checks the assignment according to its type system, obeying flags such as strictNullChecks
as they are configured in the application.
You can avoid run-time type errors by providing more specific in-template type requirements to the template type checker. Make the input type requirements for your own directives as specific as possible by providing template-guard functions in the directive definition. See Improving template type checking for custom directives, and Input setter coercion in this guide.
When you enable strictTemplates
and the TypeScript flag strictNullChecks
, typecheck errors may occur for certain situations that may not easily be avoided. For example:
A nullable value that is bound to a directive from a library which did not have strictNullChecks
enabled.
For a library compiled without strictNullChecks
, its declaration files will not indicate whether a field can be null
or not. For situations where the library handles null
correctly, this is problematic, as the compiler will check a nullable value against the declaration files which omit the null
type. As such, the compiler produces a type-check error because it adheres to strictNullChecks
.
Using the async
pipe with an Observable which you know will emit synchronously.
The async
pipe currently assumes that the Observable it subscribes to can be asynchronous, which means that it's possible that there is no value available yet. In that case, it still has to return something—which is null
. In other words, the return type of the async
pipe includes null
, which may result in errors in situations where the Observable is known to emit a non-nullable value synchronously.
There are two potential workarounds to the above issues:
In the template, include the non-null assertion operator !
at the end of a nullable expression, such as <user-detail [user]="user!" />
.
In this example, the compiler disregards type incompatibilities in nullability, just as in TypeScript code. In the case of the async
pipe, note that the expression needs to be wrapped in parentheses, as in <user-detail [user]="(user$ | async)!" />
.
Disable strict null checks in Angular templates completely.
When strictTemplates
is enabled, it is still possible to disable certain aspects of type checking. Setting the option strictNullInputTypes
to false
disables strict null checks within Angular templates. This flag applies for all components that are part of the application.
As a library author, you can take several measures to provide an optimal experience for your users. First, enabling strictNullChecks
and including null
in an input's type, as appropriate, communicates to your consumers whether they can provide a nullable value or not. Additionally, it is possible to provide type hints that are specific to the template type checker. See Improving template type checking for custom directives, and Input setter coercion below.
Occasionally it is desirable for the @Input()
of a directive or component to alter the value bound to it, typically using a getter/setter pair for the input. As an example, consider this custom button component:
Consider the following directive:
@Component({ selector: 'submit-button', template: ` <div class="wrapper"> <button [disabled]="disabled">Submit</button>' </div> `, }) class SubmitButton { private _disabled: boolean; get disabled(): boolean { return this._disabled; } set disabled(value: boolean) { this._disabled = value; } }
Here, the disabled
input of the component is being passed on to the <button>
in the template. All of this works as expected, as long as a boolean
value is bound to the input. But, suppose a consumer uses this input in the template as an attribute:
<submit-button disabled></submit-button>
This has the same effect as the binding:
<submit-button [disabled]="''"></submit-button>
At runtime, the input will be set to the empty string, which is not a boolean
value. Angular component libraries that deal with this problem often "coerce" the value into the right type in the setter:
set disabled(value: boolean) { this._disabled = (value === '') || value; }
It would be ideal to change the type of value
here, from boolean
to boolean|''
, to match the set of values which are actually accepted by the setter. TypeScript requires that both the getter and setter have the same type, so if the getter should return a boolean
then the setter is stuck with the narrower type.
If the consumer has Angular's strictest type checking for templates enabled, this creates a problem: the empty string ''
is not actually assignable to the disabled
field, which will create a type error when the attribute form is used.
As a workaround for this problem, Angular supports checking a wider, more permissive type for @Input()
than is declared for the input field itself. Enable this by adding a static property with the ngAcceptInputType_
prefix to the component class:
class SubmitButton { private _disabled: boolean; get disabled(): boolean { return this._disabled; } set disabled(value: boolean) { this._disabled = (value === '') || value; } static ngAcceptInputType_disabled: boolean|''; }
This field does not need to have a value. Its existence communicates to the Angular type checker that the disabled
input should be considered as accepting bindings that match the type boolean|''
. The suffix should be the @Input
field name.
Care should be taken that if an ngAcceptInputType_
override is present for a given input, then the setter should be able to handle any values of the overridden type.
$any()
Disable checking of a binding expression by surrounding the expression in a call to the $any()
cast pseudo-function. The compiler treats it as a cast to the any
type just like in TypeScript when a <any>
or as any
cast is used.
In the following example, casting person
to the any
type suppresses the error Property address does not exist
.
@Component({ selector: 'my-component', template: '{{$any(person).addresss.street}}' }) class MyComponent { person?: Person; }
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https://v11.angular.io/guide/template-typecheck