Put simply, you can't make something into a function if it's not already a function, and you can't _really_ extend arrays.

What you can do is create a wrapper function that wraps an array and provides the functionality you want, and also include the ability to get back the original array if you need it:






<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>




    var wrapper = (function() {
      function getColumnNumber(str) {
        return Array.prototype.reduce.call(str.toUpperCase(), function (t, c) {
            return 26 * t + c.charCodeAt(0) - 64;
        }, 0) - 1;
      }

      return function(arr) {
        return function(col, val) {
          if (arguments.length === 0) {
            return arr;
          }
          if (arguments.length > 1) {
            arr[getColumnNumber(col)] = val;
          }
          return arr[getColumnNumber(col)];
        };
      };
    })();

    var w = wrapper([10, 20, 30, 40, 50]);

    snippet.log(w('D')); // 40

    w('D', 33);          // set value

    snippet.log(w('D')); // 33

    w()[3] = 42;         // access underlying array directly
    w().push(60);

    snippet.log(w('D')); // 42
    snippet.log(w('F')); // 60

In google app scripts I have a one dimensional data array that I can get values from like this:

    data[0]

I'd love to be able to pass in the column name instead like this:

    data("A")

That way I don't have to convert letters into their array position.  So I'd like to extend the array object (extending isn't really risky since it's running in an isolated script environment).

I know I can add a function to the array prototype using this [letter to number function][1] and this [object extension question][2] like this:





    Array.prototype.byCol = function(colName) {
      return this[getColumnNumber(colName) - 1];
    }

    function getColumnNumber(str) {
      var out = 0, len = str.length;
      for (pos = 0; pos < len; pos++) {
        out += (str.charCodeAt(pos) - 64) * Math.pow(26, len - pos - 1);
      }
      return out;
    }

    var data = [1,2,3,4];

    document.write(data.byCol("B"));




But this is a slightly bulkier calling syntax than I wanted.  

Based on this question on [default functions][3], it looks like it's possible to assign a default function to an object, but they're doing so by just creating a function object like this:



    var test = new func(function() {
        // do something
    });

Can I get extend the array so that it will execute a default function when called as a method?

[1]: https://stackoverflow.com/a/29040784/1366033
[2]: https://stackoverflow.com/q/11337849/1366033
[3]: https://stackoverflow.com/q/10535447/1366033