title: How to set default class property value in TypeScript declaration file?
body: <p>f.e., I have</p>
  
  <pre><code>declare class Foo extends Bar {
      foo: number
  }
  </code></pre>
  
  <p>How do I declare that <code>foo</code> has a default value (or initial value) of, say, 60.</p>
  
  <p>I tried</p>
  
  <pre><code>declare class Foo extends Bar {
      foo: number = 60
  }
  </code></pre>
  
  <p>but I get an error like</p>
  
  <pre><code>4     foo: number = 60
                      ~~
  
  path/to/something.js/Foo.d.ts/(4,28): error TS1039: Initializers are not allowed in ambient contexts.
  </code></pre>