docs: add a style recommendation for initializing inputs (angular#40698)

This practice is to better align with `strictPropertyInitialization`
which is coming by default with strict mode in v12.

PR Close angular#40698

aio/content/examples/styleguide/src/05-18/app/app.component.ts

@@ -0,0 +1,7 @@
+import { Component } from '@angular/core';
+
+@Component({
+  selector: 'sg-app',
+  template: '<toh-hero-list></toh-hero-list>'
+})
+export class AppComponent { }

aio/content/examples/styleguide/src/05-18/app/app.module.ts

@@ -0,0 +1,20 @@
+import { NgModule } from '@angular/core';
+import { BrowserModule } from '@angular/platform-browser';
+import { RouterModule } from '@angular/router';
+
+import { AppComponent } from './app.component';
+import { HeroComponent, HeroListComponent } from './heroes';
+
+@NgModule({
+  imports: [
+    BrowserModule,
+    RouterModule.forChild([{ path: '05-18', component: AppComponent }])
+  ],
+  declarations: [
+    AppComponent,
+    HeroComponent,
+    HeroListComponent
+  ],
+  exports: [AppComponent]
+})
+export class AppModule {}

aio/content/examples/styleguide/src/05-18/app/heroes/hero-list/hero-list.component.ts

@@ -0,0 +1,24 @@
+import { Component } from '@angular/core';
+
+import { Hero } from '../shared/hero.model';
+
+@Component({
+  selector: 'toh-hero-list',
+  template: `
+    <section>
+      Our list of heroes:
+      <toh-hero *ngFor="let hero of heroes">
+      </toh-hero>
+      Total powers: {{totalPowers}}<br>
+      Average power: {{avgPower}}
+    </section>
+  `
+})
+export class HeroListComponent {
+  heroes: Hero[] = [];
+  totalPowers = 1;
+
+  get avgPower() {
+    return this.totalPowers / this.heroes.length;
+  }
+}

aio/content/examples/styleguide/src/05-18/app/heroes/hero-list/index.ts

@@ -0,0 +1 @@
+export * from './hero-list.component';

aio/content/examples/styleguide/src/05-18/app/heroes/hero/hero.component.avoid.ts

@@ -0,0 +1,15 @@
+import { Component, Input } from '@angular/core';
+
+// #docregion example
+@Component({
+  selector: 'toh-hero',
+  template: `...`
+})
+export class HeroComponent {
+  // The exclamation mark suppresses errors that a property is
+  // not initialized.
+  // Ignoring this enforcement can prevent the type checker
+  // from finding potential issues.
+  @Input() id!: string;
+}
+// #enddocregion example

aio/content/examples/styleguide/src/05-18/app/heroes/hero/hero.component.optional.ts

@@ -0,0 +1,17 @@
+import { Component, Input } from '@angular/core';
+
+// #docregion example
+@Component({
+  selector: 'toh-hero',
+  template: `...`
+})
+export class HeroComponent {
+  @Input() id?: string;
+
+  process() {
+    if (this.id) {
+      // ...
+    }
+  }
+}
+// #enddocregion example

aio/content/examples/styleguide/src/05-18/app/heroes/hero/hero.component.ts

@@ -0,0 +1,11 @@
+import { Component, Input } from '@angular/core';
+
+// #docregion example
+@Component({
+  selector: 'toh-hero',
+  template: `...`
+})
+export class HeroComponent {
+  @Input() id = 'default_id';
+}
+// #enddocregion example

aio/content/examples/styleguide/src/05-18/app/heroes/hero/index.ts

@@ -0,0 +1 @@
+export * from './hero.component';

aio/content/examples/styleguide/src/05-18/app/heroes/index.ts

@@ -0,0 +1,3 @@
+export * from './hero';
+export * from './hero-list';
+export * from './shared';

aio/content/examples/styleguide/src/05-18/app/heroes/shared/hero.model.ts

@@ -0,0 +1,4 @@
+export interface Hero {
+  id: number;
+  name: string;
+}

aio/content/examples/styleguide/src/05-18/app/heroes/shared/index.ts

@@ -0,0 +1 @@
+export * from './hero.model';

aio/content/examples/styleguide/src/05-18/app/index.ts

@@ -0,0 +1,2 @@
+export * from './heroes';
+export * from './app.component';

aio/content/guide/styleguide.md

@@ -3381,6 +3381,23 @@ helps instantly identify which members of the component serve which purpose.
 
 <a href="#toc">Back to top</a>
 
+### Initialize inputs
+
+#### Style 05-18
+
+TypeScript's `--strictPropertyInitialization` compiler option ensures that a class initializes its properties during construction. When enabled, this option causes the TypeScript compiler to report an error if the class does not set a value to any property that is not explicitly marked as optional.
+
+By design, Angular treats all `@Input` properties as optional. When possible, you should satisfy `--strictPropertyInitialization` by providing a default value.
+
+<code-example path="styleguide/src/05-18/app/heroes/hero/hero.component.ts" region="example" header="app/heroes/hero/hero.component.ts"></code-example>
+
+If the property is hard to construct a default value for, use `?` to explicitly mark the property as optional.
+
+<code-example path="styleguide/src/05-18/app/heroes/hero/hero.component.optional.ts" region="example" header="app/heroes/hero/hero.component.ts"></code-example>
+
+You may want to have a required `@Input` field, meaning all your component users are required to pass that attribute. In such cases, use a default value. Just suppressing the TypeScript error with `!` is insufficient and should be avoided because it will prevent the type checker ensure the input value is provided.
+
+<code-example path="styleguide/src/05-18/app/heroes/hero/hero.component.avoid.ts" region="example" header="app/heroes/hero/hero.component.ts"></code-example>
 
 ## Directives