What is the concept of Array.map?

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.