title: Colon after function declaration in javascript
body: <p>I am going through source code of Vue.js. In almost all the function declarations I find a new way of defining functions</p>
  
  <pre><code>function isStringStart (chr: number): boolean {
    return chr === 0x22 || chr === 0x27
  }
  </code></pre>
  
  <p>Can somebody explain me what is this kind of function declaration called?</p>