What is the intended solution for Copy an Array with the Spread Operator?

JavaScript
1

Basic Data Structures: Copy an Array with the Spread Operator

I got the tests to pass but I feel like i’m not supposed to do it the way I did.

At first I got all of the test to pass except the one that requires you to use the spread operator with just this:

function copyMachine(arr, num) {
  let newArr = [];
  while (num >= 1) {
    // change code below this line
    newArr.push(arr);
    // change code above this line
    num--;
  }
  return newArr;
}

And to get the last test to pass I just did a useless spread of the array in the push method

function copyMachine(arr, num) {
  let newArr = [];
  while (num >= 1) {
    // change code below this line
    newArr.push([...arr]);
    // change code above this line
    num--;
  }
  return newArr;
}

What is the intended solution for this problem that makes use of the spread syntax in a way that is actually needed and useful? Is it a lot cleaner than just appending the passed arr to the end of newArr on each iteration of the loop?

2

It looks like the intended solution is exactly what you’re doing in your second example. Probably just a bad example in the challenge, if you want open a github issue to let them know.

I came up with this:

newArr = [...newArr, arr];

It’s pretty useless in this example and it doesn’t pass the test too. But it’s good to know because you use it a lot when dealing with immutability, like in redux for example.

1 Like
3

I feel like your solution is the kind of thing I expected this challenge wanted. It surprises me that it doesn’t pass the tests and I wonder why they want the spread to be on the passed in array arr.

4

I’m just curious. Why does this work

newArr.push([…arr]);

but this does not

newArr.push([arr]); ?

Isn’t the second just appending the arr to the newArr as well?
I know … is required to complete the challenge, but I can’t pass any tests at all with the second solution from above.

Thanks for the help!

5

The 2nd option you listed above is simply wrapping around arr array inside of a new array. It’s created a new array.

The 1st option is you are spreading arr into comma separated values so this will effectively create a new array with values of arr.

1 Like