Merging contents of an array based on matching content

I am writing some Google Apps Script to get each Google Group and list it’s members then output it to a Google Sheet. Currently, I am grabbing each group and email address and pushing it to the memberArr, however I then want to ‘merge’ the relevant information.

So if for example, I have X Groups and Group 1 has 4 members (Foo, Bar, Baz and Quux) – Currently, it will output as

[ [ 'Group 1', 'Foo' ],
  [ 'Group 1', 'Bar' ],
  [ 'Group 1', 'Baz' ],
  [ 'Group 1', 'Quux' ] ]

But I want to have it output as [Group 1, Foo, Bar, Baz, Quux].
That is to say, merge the contents of the memberArr where there is a common Group

Here is my code so far:

function getAllGroupsTEST() {
  const groupArr = [];
  let gPageToken;
  let gPage;

  do {
    gPage = AdminDirectory.Groups.list({
      customer: "my_customer",
      maxResults: 100,
      gPageToken
    });
    const groups = gPage.groups;

    if (groups) {
      groups.forEach(({email}) => {
        const groupEmail = email;
        groupArr.push(groupEmail);
      });
    }

    gPageToken = gPage.nextPageToken;
  } while (gPageToken);

  console.log(`LOGGING GROUPS:nn${groupArr}`);
  const memberArr = [];
  let mPageToken;
  let mPage;

  groupArr.forEach(group => {
    mPage = AdminDirectory.Members.list(group,{
      customer: "my_customer",
      maxResults: 500,
      mPageToken
    })
    const members = mPage.members;
    if (members) {
      // console.log(`LOGGING ${members.length} MEMBERS FOR ${group}nn${members}`)
      members.forEach(member => {
        // console.log(`MEMBER: ${member.email} IS IN GROUP ${group}`)
        memberArr.push([group, member.email])
      })
    }
  })
  // console.log(`COUNTED ${groupArr.length} GROUPS`)
  // console.log(memberArr)
}

Any help would be greatly appreciated!

Edit: Updated to show the memberArr as is rather than in a table format.

Answer

I’d propose to convert the array in the object and then back to the array this way:

var arr = [
    ['group1','a'],
    ['group1','b'],
    ['group2','c'],
    ['group2','d'],
    ['group3','e']
]

var obj = {}
for (var a of arr) {
    try { obj[a[0]].push(a[1]) }
    catch(e) { obj[a[0]] = [a[1]] }
}
console.log(obj);

var new_arr = [];
for (var group in obj) {
    new_arr.push([group, ...obj[group]])
}
console.log(new_arr);

Output:

[ [ 'group1', 'a', 'b' ], [ 'group2', 'c', 'd' ], [ 'group3', 'e' ] ]