title: What is `as` in the class assigning?
body: <p>I tried google and looked through the angular document, but I didn't know what I was looking for.</p>
  
  <p>Would someone explain what this "as" is? what did I do wrong in the following code?</p>
  
  <p>I have component A which is used in component B, and a variable is declared in b 'as' A. I try to use the function from A, but an error occurred here because function doesn't exist. </p>
  
  <p><strong><em>A.component.ts;</em></strong></p>
  
  <pre><code>...
  
  getValue ():Array&lt;string&gt; {
   return this.filter( elm =&gt; elm.length &gt; 5 );
  }
  
  ...
  </code></pre>
  
  <p><strong><em>B.component.ts;</em></strong></p>
  
  <pre><code>...
  
  type C = A;
  
  ...
  
  public coolArr: C;
  
  printArr(param: any):void{
   coolArr = param as A;
   console.log(coolArr.getValue);
  }
  
  ...
  </code></pre>
  
  <p>Not sure what I did wrong here.</p>