title: TypeScript: Cannot read property 'push' of undefined in [null]
body: <blockquote>
    <p><code>Error: Cannot read property 'push' of undefined in [null].</code></p>
  </blockquote>
  
  <pre><code>class A implements OnInit {
      stringArr: string[];
  
      ngOnInit() {
          for(let i=0; i&lt;4; i++) {
              this.stringArr.push("aaa");
          }
      }
  }
  </code></pre>