title: How to extend an array with new array without declaration
body: <p>I am working on project where I need to maintain an array from json data returned from API, json can have tree, I have following code which is working fine but I wan to remove if conditions before assigning values to array elements</p>
  
  <pre><code>// data contains json 
  let newArray = []
  for(let d in data){
      for(let x in data[d]){
          if(typeof(newArray[d]) === 'undefined'){
              newArray[d] = []
          }
          if(typeof(newArray[d][data[d][x]['id']]) === 'undefined'){
              newArray[d][data[d][x]['id']] = []
          }
          newArray[d][data[d][x]['id']]['price'] = data[d][x]['price']
          newArray[d][data[d][x]['id']]['discount'] = data[d][x]['discount']
      }
  }
  </code></pre>
  
  <p>In above code I have to check the array first and declare it as array if its not otherwise it returns undefined error, is there any way to get rid of there conditions and extend array as per requirement ?</p>