prevent a from input

components.form-element\.js

import Component from '@glimmer/component';

export default class extends Component {
}

components.form-input\.js

import Component from '@glimmer/component';

export default class extends Component {
}

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  @tracked
  foo;
  
  appName = 'Ember Twiddle';

	@action
	updateFoo(event){
    let value = event.target.value;
    if(value.length > 0 && 'a' === value.charAt(value.length - 1)){
    	value = value.substring(0, value.length - 1);
    }
    this.foo = value;
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>

<div>
  <label>Foo:</label>
  <FormElement @value={{this.foo}} as |el|>
  	<el.control @onChange={{this.updateFoo}}/>
  </FormElement>
</div>

<div>
  <label>Foo Control direct:</label>
  	<FormInput @value={{this.foo}} @onChange={{this.updateFoo}}/>
</div>

<div>
  <label>Foo (ctrl val): </label>
  {{this.foo}}
</div>
<br>
<br>

templates.components.form-element\.hbs

{{yield
	(hash
  	control=(component "form-input"
    	value=this.args.value
    )
  )
}}

templates.components.form-input\.hbs

<input
    type="text"
    value={{@value}}
    {{on "input" @onChange}}
  >

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}