title: Typescript:Cannot invoke an expression whose type lacks a call signature. Type 'Number' has no compatible call signatures
body: <p>I know these type of questions are asked before but still it is not clear to me.</p>
  
  <p>I am a beginner and i was trying to learn import and export in typescript. So i have written the following code. Kindly have a look in to this.</p>
  
  <p>I have three files:-</p>
  
  <pre><code> 1.animal.ts  
   2.bird.ts 
   3.application.ts
  </code></pre>
  
  <p>Animal.ts:-</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>export class Animal {
      name:string
      age:number
      constructor(name:string,age:number){
          this.name=name;
          this.age=age
      }
      
      sleep(){
          console.log("I do sleep")
      }
  
      eat(){
          console.log("I do eat")
      }
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>bird.ts</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>import {Animal} from "./animal"
  
  export class Bird extends Animal{
  
      constructor(name:string,age:number){
          super(name,age)
      }
  
      fly(){
          console.log("I can fly also")
      }
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>aplication.ts</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>import { Animal } from "./animal"
  import { Bird } from "./bird"
  
  class Application {
  
      constructor() {
          console.log("Hi i am a bird !!")
      }
  }
  
  var animal = new Animal("Rhino", 10);
  var bird = new Bird("Pigeon", 3)
  
  animal.age();   //Cannot invoke an expression whose type lacks a call signature. Type 'Number' has no compatible call signatures
  animal.name();  //Cannot invoke an expression whose type lacks a call signature. Type 'String' has no compatible call signatures.
  
  bird.age();     //Cannot invoke an expression whose type lacks a call signature. Type 'Number' has no compatible call signatures
  bird.fly();
  bird.sleep();</code></pre>
  </div>
  </div>
  </p>
  
  <p>How do i solve this problem ? and what this error actually means ? </p>