replace vs setAll

components.my-component.js

import Ember from 'ember';
import { computed } from '@ember/object';

export default Ember.Component.extend({
  init() {
    this._super(...arguments);
    
    this.set('items', ['foo', 'bar', 'baz']);
  },
             
  obsers: computed('items.[]', function () {
  	return this.get('items').map((item) => item + "<<");  
  }),
  
  actions: {
    replace() {
      let tmp = this.get('items');
      tmp.replace(1, 1, ['unicorn']);
    },
    
    setAll() {
      let tmp = this.get('items');
      tmp[1] = 'unicorn';
      console.log(tmp);
    	this.set('items', tmp);
    }
	}
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component}}
<br>
<br>

templates.components.my-component.hbs

{{#each items as |item|}}
	{{item}}
{{/each}}

<hr />

{{#each obsers as |obser|}}
	{{obser}}
{{/each}}

<hr />

<button {{action "setAll"}}>
	set all
</button>

<button {{action "replace"}}>
	replace
</button>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}