input {{on}}

controllers.application\.js

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

export default class ApplicationController extends Controller {	
	name = "";
  @tracked nameSetInValidateName = "";

  @tracked error = "Name must be at least 3 charaters";

	@action
	validateName(event) {
    this.nameSetInValidateName = this.name
    
    this.error = "";
    if (this.name.length < 3) {
      this.error = "Name must be at least 3 charaters";
    }
  }
}

templates.application\.hbs

<p>Using this way "nameSetInValidName" will be one character after "name"</p>
<code>&lt;Input @value=\{{this.name}} \{{on "input" this.validateName}} /&gt;</code><br />
<Input
  @value={{this.name}}
  {{on "input" this.validateName}}
/>
<hr />
<br />
<p>Using this was they will both be in sync</p>
<code>&lt;Input @value=\{{this.name}} @input=\{{this.validateName}} /&gt;</code><br />
<Input
  @value={{this.name}}
  @input={{this.validateName}}
/>

<hr />
<br />
Output:
<output>
	<p>name: {{this.name}}</p>
	<p>nameSetInValidateName: {{this.nameSetInValidateName}}</p>
</output>


{{#if this.error}}
	<p style="color: red;">
		{{this.error}} <br />
    Length is {{this.name.length}}
	</p>
{{/if}}

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"
  }
}