title: How to extend String Prototype and use it next, in Typescript?
body: <p>I am extending String prototype chain with a new method but when I try to use it it throws me an error: <code>property 'padZero' does not exist on type 'string'</code>. Could anyone solve this for me?</p>
  
  <p>The code is below. You can also see the same error in Typescript Playground.</p>
  
  <pre><code>interface NumberConstructor {
      padZero(length: number);
  }
  interface StringConstructor {
      padZero(length: number): string;
  }
  String.padZero = (length: number) =&gt; {
      var s = this;
      while (s.length &lt; length) {
        s = '0' + s;
      }
      return s;
  };
  Number.padZero = function (length) {
      return String(this).padZero(length);
  }
  </code></pre>