How to combine all the arrays which are inside another array and delete the last symbol in each word?

Reduce is one way, can use also the new-ish flat method; by default it flattens only to single level but possible to define depth

userGroup.flat().map(item => item.slice(0, -1));

Try this:

 console.log(userGroup.reduce((acc, curr)=> [...acc, ...curr], [])
    .map(el => el.replace('-','')));

For a one liner, consider:

const mapped = userGroup.reduce((p,c) => p.concat(c), []).map(s => s.slice(0, s.length-1));

Try this

[].concat.apply([], userGroup).toString().split('-,');

Array.prototype.flat is the one you are looking for

userGroup.flat()

and then you will have the flat array for your array of arrays.

Then user the Array.prototype.map to convert the values as you like

userGroup.flat().map(m => m.replace('-', ''));

You need few things to achieve your goal.

1. .replace()

const u = 'user1-';

console.log(u.replace('-', ''));

2. .map()

const u = ["user1-", "user2-", "user3-"];

const m = u.map(i => i.replace("-", ""));

console.log(m);

3. .flat()

const u = [
  ["user1-", "user2-"],
  ["user3-", "user4-"]
];

console.log(u.flat());

So, combing all the 3 methods in a single statement, the below is the code:

let userGroup = [
  ['user1-', 'user2-', 'user3-'],
  ['user4-', 'user5-', 'user6-'],
  ['user7-', 'user8-', 'user9-']
];

   let grouped = userGroup.flat().map(item => item.replace('-', ''));
   console.log(grouped)