title: Typescript join specific properties of an array as string
body: <p>I have an API that gives me a response of something like this:</p>
  
  <pre><code>{
    "item": [
      {
        "_id": "5a48e0c100a5863454c0af2a",
        "name": "Maths",
        "created_by": "5a43ee3231ad5a6b0850d961",
        "__v": 0,
        "created_on": "2017-12-31T13:06:09.957Z",
        "active": 1,
        "grade": [],
        "syllabus": [
          {
            "_id": "5a47a5faed12d92d0c2449f4",
            "name": "CBSE",
            "description": "CBSE Syllabus",
            "created_by": "5a43ee3231ad5a6b0850d961",
            "__v": 0,
            "created_on": "2017-12-30T14:43:06.305Z",
            "banner": 1,
            "active": 1
          },
          {
            "_id": "5a47a615ed12d92d0c2449f5",
            "name": "State Board",
            "description": "State Board Syllabus",
            "created_by": "5a43ee3231ad5a6b0850d961",
            "__v": 0,
            "created_on": "2017-12-30T14:43:33.328Z",
            "banner": 1,
            "active": 1
          }
        ]
      }
    ]
  }
  </code></pre>
  
  <p>As you can see, there is an item named <code>maths</code> in the array of <code>item</code>. Now inside the maths, we have another array of <code>syllabus</code>. I want to join the names of all the syllabus of <code>maths</code>. In simple terms, say I want to do something like this:</p>
  
  <pre><code>array element 1 - maths - CBSE, State Board
  array element 2 - chemistry - CBSE, State Board
  </code></pre>
  
  <p>AFAIK, we can't do it without 2 forEach loops. Is there any better way to deal with this situation?</p>