Template type checking

Overview of template type checking

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 Angular's compiler options.

Basic mode

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:

• Importantly, it doesn't check embedded views, such as *ngIf, *ngFor, other <ng-template> embedded view.
• It doesn't figure out the types of #refs, the results of pipes, or the type of $event in event bindings.

In many cases, these things end up as type any, which can cause subsequent parts of the expression to go unchecked.

Full mode

If the fullTemplateTypeCheck flag is set to true, Angular is more aggressive in its type-checking within templates. In particular:

• Embedded views (such as those within an *ngIf or *ngFor) are checked
• Pipes have the correct return type
• Local references to directives and pipes have the correct type (except for any generic parameters, which will be any)

The following still have type any.

• Local references to DOM elements
• The $event object
• Safe navigation expressions

Strict mode

Angular 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 addition to the full mode behavior, Angular does the following:

• Verifies that component/directive bindings are assignable to their @Input()s
• Obeys TypeScript's strictNullChecks flag when validating the preceding mode
• Infers the correct type of components/directives, including generics
• Infers template context types where configured (for example, allowing correct type-checking of NgFor)
• Infers the correct type of $event in component/directive, DOM, and animation event bindings
• Infers the correct type of local references to DOM elements, based on the tag name (for example, the type that document.createElement would return for that tag)

Checking of *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.

Troubleshooting template errors

With strict mode, 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.

• When a library's typings are wrong or incomplete (for example, missing null | undefined if the library was not written with strictNullChecks in mind)

• When a library's input types are too narrow and the library hasn't added appropriate metadata for Angular to figure this out. This usually occurs with disabled or other common Boolean inputs used as attributes, for example, <input disabled>.

• When using $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:

• Use the $any() type-cast function in certain contexts to opt out of type-checking for a part of the expression
• Disable strict checks entirely by setting strictTemplates: false in the application's TypeScript configuration file, tsconfig.json
• Disable certain type-checking operations individually, while maintaining strictness in other aspects, by setting a strictness flag to false
• If you want to use strictTemplates and strictNullChecks together, opt out of strict null type checking specifically for input bindings using strictNullInputTypes

Unless otherwise commented, each following option is set to the value for strictTemplates (true when strictTemplates is true and conversely, the other way around).

If you still have issues after troubleshooting with these flags, 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.

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, file an issue so the team can address it.

Inputs and type-checking

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: 'app-root',  template: '<user-detail [user]="selectedUser"></user-detail>',})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.

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 in this guide.

Strict null checks

When you enable strictTemplates and the TypeScript flag strictNullChecks, typecheck errors might occur for certain situations that might 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 might result in errors in situations where the Observable is known to emit a non-nullable value synchronously.

There are two potential workarounds to the preceding issues:

• In the template, include the non-null assertion operator ! at the end of a nullable expression, such as

        
  <user-detail [user]="user!"></user-detail>
      

  In this example, the compiler disregards type incompatibilities in nullability, just as in TypeScript code. In the case of the async pipe, notice that the expression needs to be wrapped in parentheses, as in

        
  <user-detail [user]="(user$ | async)!"></user-detail>
      

• 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.

Advice for library authors

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.

Input setter coercion

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;  @Input()  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 prior to version 4.3 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 creates 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;  @Input()  get disabled(): boolean {    return this._disabled;  }  set disabled(value: boolean) {    this._disabled = (value === '') || value;  }  static ngAcceptInputType_disabled: boolean|'';}
    

Since TypeScript 4.3, the setter could have been declared to accept boolean|'' as type, making the input setter coercion field obsolete. As such, input setters coercion fields have been deprecated.

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.

Disabling type checking using $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).address.street}}'})class MyComponent {  person?: Person;}