array recompute

controllers.application\.js

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

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

  @tracked
  fruits = [
    'apples',
    'oranges',
    'cherries'
  ];

  @tracked
  fruit = '';

  @action
  updateFruit(ev) {
    this.fruit = ev.target.value;
  }

  @action
  addFruit() {
    this.fruits.addObject(this.fruit);
  }
}

helpers.len2\.js

import Helper from '@ember/component/helper';
import { observer } from '@ember/object';
import { tracked } from '@glimmer/tracking';

export default Helper.extend({
  @tracked
  param: null,
  
  onMutation: observer('param.[]', function() {
    this.param = [...this.param];
  }),
  compute([param]) {
    this.param = param;
    return param.length;
  }
});

helpers.len\.js

import Helper from '@ember/component/helper';
import { observer } from '@ember/object';

export default Helper.extend({
  param: null,
  onMutation: observer('param.[]', function() {
    this.recompute();
  }),
  compute([param]) {
    this.set('param', param);
    return param.length;
  }
});

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
{{#each fruits as |fruit|}}
	{{fruit}}<br>
{{/each}}
<br>
<br>
<input type="text" id="fruitToAdd" value={{this.fruit}} {{on "input" this.updateFruit}} />
<button {{on "click" this.addFruit}}>Add</button><br>
<br>
{{len this.fruits}}<br>
{{len2 this.fruits}}<br>

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",
    "ember-data": "3.18.0"
  }
}