Map over items that meet a certain condition, otherwise just pass the item as is.

mapOn.js

/*

Description:
Map over an array of items:
- If the item meets the condition, apply the callback
- If the item does not meet the condition return the item without any transformation.

Example:
```
const exampleArray = [1, 2, 9, "C", "B", "C"];
exampleArray.mapOn(Math.sqrt, Number.isInteger); // [1, 1.414, 3, "A", "B", "C"];
```

Parameters
- callback
  Function that produces an element of the new Array, taking three arguments:
     - currentValue
       The current element being processed in the array.
     - index
       The index of the current element being processed in the array.
     - array
       The array map was called upon.
- condition
  Function perdicate, if returned value is truthy, the callback is applied, taking three arguments:
     - currentValue
       The current element being processed in the array.
     - index
       The index of the current element being processed in the array.
     - array
       The array map was called upon.
- thisArg
  Optional. Value to use as this when executing callback.

Return value
A new array with each element being the result of the callback function if it meets the condition, otherwise the same item is returned.
*/


Array.prototype.mapOn = function(callback, condition, thisArg=this) {
  return this.map((currentValue, index, array) => {
    if(condition(currentValue, index, array)) {
      return callback.call(thisArg, currentValue, index, array)
    }
    return currentValue
  })
}