From Imperative to Functional JS

functionalJS.js

/**
 * Write a simple join function in the various styles:
 * 1. Imperative
 * 2. Object-oriented
 * 3. Functional language
 */

// Imperative
function simpleJoin( stringArray ) {
  var accumulator = '';
  for ( var i = 0, l = stringArray.length; i < l; i++ ) {
    accumulator = accumulator + stringArray[i];
  }
  return accumulator;
}

// Object-oriented
Array.prototype.simpleJoin = function() {
  var accumulator = '';
  for ( var i = 0, l = this.length; i < l; i++ ) {
    accumulator = accumulator + this[i];
  }
  return accumulator;
};

// Functional (imperative)
function simpleJoin( stringArray, i, accumulator ) {
 if ( i === stringArray.length ) {
    return accumulator;
 } else {
    return simpleJoin( stringArray, i+1, accumulator+stringArray[i] );
 }
}

// Pure Functional
function simpleJoin( stringArray, i, accumulator ) {
 return ( i === stringArray.length ) ? accumulator :
   simpleJoin( stringArray, i+1, accumulator + stringArray[i] );
}

Nice post David. I was practicing this exact exercise last night too after reading about it the Functional programming article you linked to. Definitely the Imperative and OO methods come the most naturally. What is the advantage of the Pure Functional in a situation like this?