title: delete specific key from arr after performing copy
body: <p>I tried to copy the original src arr into newArr and delete the object key ie header property, but unfortunately, its deleting original property from src arr,but i dont want to delete from the original arr, instead operation should perform only on copied array</p>
  
  <p>expected output described below</p>
  
  <pre><code>let arr = [[{header:"first",tail:"three"},{header:"second",tail:"four"}],[{header:"first",tail:"three"},{header:"second",tail:"four"}]];
  
  let newArr = [[{tail:"three"},{tail:"four"}],[{tail:"three"},{tail:"four"}]];
  </code></pre>
  
  <p>may i know the problem in the code
  please refer below snippet </p>
  
  <pre><code>let arr = [[{header:"first",tail:"three"},{header:"second",tail:"four"}],[{header:"first",tail:"three"},{header:"second",tail:"four"}]];
  
  let newArr = Object.assign([], arr);
  
  for(let i=0;i&lt;newArr.length;i++) {
    for(let j=0;j&lt;newArr[i].length;j++) {
      delete newArr[i][j]["header"]
    }
  }
  
  console.log(arr)
  
  </code></pre>