title: How to create instance of callable interface in TypeScript?
body: <p>I have an interface below with call signature inside it</p>
  <pre><code>type MyTest = {
    description: string;
    (x: number): number;
  };
  </code></pre>
  <p>But I don't know how to create instance of this interface in TypeScript. I have tested 3 solution below and nothing worked</p>
  <pre><code>const test1: MyTest = {
    description: 'My Name' // error: Type '{ description: string; }' provides no match for the signature '(x: number): number'.
  } 
  </code></pre>
  <pre><code>const test2: MyTest = (myNumberInput: number) =&gt; myNumberInput.toString(); // error: Property 'description' is missing in type '(myNumberInput: number) =&gt; string' but required in type 'MyTest'.
  </code></pre>
  <pre><code>const test3: MyTest = {
    description: 'My Name', // error: Type '{ description: string; }' has no call signatures.
    (myNumberInput: number) =&gt; myNumberInput.toString() // error: Cannot find name 'myNumberInput'. + 'number' only refers to a type, but is being used as a value here.
  }
  </code></pre>
  <p>Please give me an instruction for this problem. Thank a lot and have nice day!</p>