How to divide array into two arrays depending on condition?

first create a copy of the object and then with the filter javascript function for arrays, you can pass the testing function which will return the elements that comply.

Like this:

const mynullObj = {...myobj }
    mynullObj.dates = myobj.dates.filter((date)=> date===null)
    myobj.dates = myobj.dates.filter((date)=> date!==null)

EDIT:

so the filter I propposed has some caveats when used inside the forEach or map, it will modify your original array so you must make a hardcopy of the array to split it. This function below will return two objects, one with the nulls and one with the dates.

const separatenulls = (pending) => {
  const pendingCopy = JSON.parse(JSON.stringify(pending));
 
  const notNull = pending.map((item) => {
    item.dates = item.dates.filter((date) => date !== null);
    return item;
  });
  const thesearenull = pendingCopy.map((itemc)=>{
     itemc.dates = itemc.dates.filter((date)=> date===null);
     console.log("itemc", itemc)
     return itemc
    })
  return [notNull, thesearenull];
};

const [withdates, withnulls] = separatenulls(pending);