title: What does it mean `Index signature of object type implicitly has an 'any' type.`
body: <p>I have the following code and <code>noImplicitAny:true</code> in <code>tsconfig</code>:</p>
  
  <pre><code>let o = {a: 3};
  
  // works fine
  o['a'] = 3;
  
  // reports an error 
  // Error:(4, 1) TS7017:Index signature of object type implicitly has an 'any' type.
  o['b'] = 3;
  </code></pre>
  
  <p>What does this error mean?</p>
  
  <p><a href="http://www.typescriptlang.org/play/index.html#src=let%20o%20%3D%20%7Ba%3A%203%7D%3B%0A%0A%2F%2F%20works%20fine%0Ao%5B&#39;a&#39;%5D%20%3D%203%3B%0A%0A%2F%2F%20reports%20an%20error%20%0A%2F%2F%20Error%3A(4%2C%201)%20TS7017%3AIndex%20signature%20of%20object%20type%20implicitly%20has%20an%20&#39;any&#39;%20type.%0Ao%5B&#39;b&#39;%5D%20%3D%203%3B" rel="noreferrer">Here it is on the TypeScript playground</a> - be sure to click Options and set <code>noImplicitAny</code> (doesn't seem to remember options in shareable links).</p>