Let's rewrite it a bit, and start working from inside out.

var mapCell = function (row) {
  return columns.map(
    function(column) {
      return { 
        column : column, 
        value : getColumnCell(row, column)
      }
    }
  )
}

The function(column) part is essentially a function that takes a column as a parameter, and returns a new object with two properties:

• column, that is the original value of the parameter, and
• value, that is the result of calling the getColumnCell function on the row (external variable) and column (parameter)

The columns.map() part calls the Array.map function, that takes an array and a function, and runs the function for every last item of it, and returns the results. i.e. if the input is the array [1, 2, 3, 4, 5] and the function is something like isEven, the result will be the array [false, true, false, true, false]. In your case, the input are the columns, and the output is a list of objects, each of which has a column and a value properties.

Lastly, the var mapCell = function (row) part declares that the variable mapCell will contain a function of one variable called row - and this is the same row that is used in the inner function.

In a single sentence, this line of code, declares a function that when run, will take a row and return values for all columns for that row.

The .map() function will go through the entire array, and on each step of that process it will take the current item that we are looking at and will pass it as a parameter into the function. You can then do whatever you want to that item, and whatever you return from your function will replace what is in that position in the array.

Say for example, with the array you gave in your question, we wanted to remove the name and last_name properties, and combine them into a full_name property. We can do the following:

 let people = [
   {
     id: 1,
     name: 'jhon',
     last_name: 'wilson'
   },
   {
     id: 2,
     name: 'maria',
     last_name: 'anyway'
   },
     id: 3,
     name: 'lastOne',
     last_name: 'example'
   }
];

people = people.map((person) => {
  return {
    id: person.id,
    full_name: `${person.name} ${person.last_name}`
  }
});

After this code runs, our people array would look like this:

[
   {
     id: 1,
     full_name: 'jhon wilson'
   },
   {
     id: 2,
     full_name: 'maria anyway'
   },
     id: 3,
     name: 'lastOne example'
   }
];

You can think of it as doing something very similar to this:

function transformPerson(person) {
  return {
    id: person.id,
    full_name: `${person.name} ${person.last_name}`
  }
}

let newPeople = [];
for (let i = 0; i < people.length; i++) {
  newPeople[i] = transformPerson(people[i])
}

people = newPeople;