In the transformation of a 2D array into a comma-separated values (CSV) string, the following approach is taken:

Function Definition:

```javascript
const arrayToCSV = (arr, delimiter = ',') => {
  return arr.map(row => {
    return row.map(value => {
      if (isNaN(value)) {
        return `"${value.replace(/"/g, '""')}"`;
      } else {
        return value;
      }
    }).join(delimiter);
  }).join('\n');
};
```

Usage Examples:

• Convert a 2D array into a CSV string using the default delimiter (`,`):

```javascript
arrayToCSV([['a', 'b'], ['c', 'd']]);
```
Result: 
```plaintext
"a","b"
"c","d"
```

• Convert a 2D array into a CSV string using a custom delimiter (`;`):

```javascript
arrayToCSV([['a', 'b'], ['c', 'd']], ';');
```
Result: 
```plaintext
"a";"b"
"c";"d"
```

• Handle special characters and numbers in the 2D array:

```javascript
arrayToCSV([['a', '"b" great'], ['c', 3.1415]]);
```
Result: 
```plaintext
"a","""b"" great"
"c",3.1415
```

This function skillfully converts 2D arrays into CSV strings, allowing for customization of the delimiter and handling of various data types.

To wrap up 

In conclusion, the arrayToCSV function provides a versatile and efficient solution for converting 2D arrays into comma-separated values (CSV) strings. By leveraging JavaScript’s map and join methods, it effectively combines individual 1D arrays (rows) into strings, using a user-defined or default delimiter. This functionality ensures flexibility in formatting CSV data as per specific requirements.

Moreover, the function demonstrates robustness by handling special characters, such as double quotes within string values, and numbers gracefully. It escapes special characters correctly and retains the integrity of numerical data. Whether it’s for simple data manipulation or more complex CSV generation tasks, this function serves as a valuable tool, offering developers an easy and reliable way to work with tabular data in the CSV format.