I tried to use 'getelementbyclassname' in VUE methods.
The reason why I tried to use this is that
instead of attaching information as :style, 
I want to change the width of the div I applied my class
which is 'classon'.
If I tie the style to the div, it wouldn't be interactive.
So to achieve interactivity, I need to access to the DOM within VUE.
What shoudl I do if I want to access to the DOM element information in VUE?
Specifically, this part doesn't work
  methods: {
    growit(){
      let vueele=this.$el
      vueele.getElementsByClassName('classon').style.width='300px'
    },
Full code is as below.
-HTML-
<script src="https://npmcdn.com/vue/dist/vue.js"></script>
<div id="exercise">
  <!-- 1) Start the Effect with the Button. 
    The Effect should alternate the "highlight" or "shrink" class 
    on each new setInterval tick 
    (attach respective 
    class to the div 
    with id "effect" below) -->
  <div>
    <button @click="startEffect">Start Effect</button>
    <div id="effect" :class='{"highlight":classboolean, "shrink":!classboolean}'></div>
  </div>
  <!-- 2) Create a couple of CSS classes and attach them via the array syntax -->
  <div :style=[fclass,sclass]>I got no class :( but i think i just got it</div>
  <!-- 3) Let the user enter a class (create some example classes) and attach it -->
  <div>
    <label>Custom Class</label>
    <input type="text" v-model='newclass'>
    <div :style='this[newclass]'>{{newclass}}</div><br><br>
  </div>
  <!-- 4) Let the user enter a class and enter true/ false for another class (create some example classes) and attach the classes -->
  <div>
    <label>type another class</label>
    <input type="text" v-model='anotherclass'><br>
    <label>type true false for that</label>
    <input type="text" v-model='booleans'>
    <div :class='{"anotherclass":booleans}'>can't figure out this</div>
  </div>
  <!-- 5) Repeat 3) but now with values for styles (instead of class names). Attach the respective styles.  -->
  <div>
    <input type="text">
    <div></div>
  </div>
  <!-- 6) Create a simple progress bar with setInterval and style bindings. Start it by hitting the below button. -->
  <div>
    <label>What Rectangle do you want to grow?</label>
    <textarea v-model='rectselection'></textarea>
    <button @click="startProgress">Start Progress</button>
    <button class="btn-primary" @click='growit'>grow</button>
    <div class='options' :class='{classon:boolean1}'>{{rectselection}}</div>
    <div class='options' :class='{classon:boolean2}'>{{rectselection}}</div>
    <div class='options' :class='{classon:boolean3}'>{{rectselection}}</div>
    <div class='options' :class='{classon:boolean4}'>{{rectselection}}</div>
    <div class='options' :class='{classon:boolean5}'>{{rectselection}}</div>
  </div>
</div>
<script src='./app.js'></script>
<style>
  .highlight{
    background-color:yellow;
    border:solid black 1px;
    box-shadow:1px 2px 1px 1px;
    height:150px;
    width:150px;
  }
  .shrink{
    background-color:rgb(255, 255, 137);
    border:dashed grey 0.5px;
    box-shadow:0px;
    height:150px;
    width:50px;
  }
  .options{
    border:rgba(0, 0, 0, 0.564) solid 0.2px;
    width:200px;
    height:50px;
    margin:10px;
    display:block;
  }
  .classon{
    box-shadow: aquamarine 2px 2px;
    background-color: red;
  }
</style>
-JS-
new Vue({
  el: '#exercise',
  data: {
    classboolean:true,
    fclass:{
      'background-color':"red"
    },
    sclass:{
      'box-shadow':"grey 2px 2px 2px 2px",
      'margin':'50px',
    },
    newclass:'',
    anotherclass:'',
    booleans:'',
    width:'',
    rectselection:'',
    boolean1:false,
    boolean2:false,
    boolean3:false,
    boolean4:false,
    boolean5:false,
  },
  methods: {
    growit(){
      let vueele=this.$el
      vueele.getElementsByClassName('classon').style.width='300px'
    },
    startEffect() {
    this.classboolean=!this.classboolean
    },
    startProgress(){
      console.log
      if(this.rectselection==1){
        this.boolean1=true;
        this.boolean2=false; this.boolean3=false; this.boolean4=false; this.boolean5=false;
      } 
      else if(this.rectselection==2){
        this.boolean2=true;
        this.boolean1=false; this.boolean3=false; this.boolean4=false; this.boolean5=false;
      } 
      else if(this.rectselection==3){
        this.boolean3=true;
        this.boolean1=false; this.boolean2=false; this.boolean4=false; this.boolean5=false;
      } 
      else if(this.rectselection==4){
        this.boolean4=true;
        this.boolean1=false; this.boolean2=false; this.boolean3=false; this.boolean4=false;
      } 
      else if(this.rectselection==5){
        this.boolean5=true;
        this.boolean2=false; this.boolean3=false; this.boolean4=false; this.boolean1=false;
      } 
    },
  }
});
Really Appreciate always.
