Common functional programming patterns. From Functional Programming 101 talk at nodeDC/dcjQuery meetup, 8/22/2013

filter.js

var filter = function (array, action) {
  var outbound = [];
  for (var i = 0; i < array.length; i++) {
    if (action(array[i]) {
      outbound.push(array[i]);
    }
  }
  return outbound;
};

var trueIfOdd = function(value) {
  return (value % 2 === 1);
}

var array = [1, 2, 3, 4];

filter(array, trueIfOdd); // returns [1, 3]

find.js

var find = function (array, action) {
  for (var i = 0; i < array.length; i++) {
    if (action(array[i]) {
      return array[i];
    }
  }
  return undefined;
};

var trueIfEven = function(value) {
  return (value % 2 === 0);
}

var array = [1, 2, 3, 4];

find(array, trueIfEven); // returns 2

foreach.js

var forEach = function (array, action) {
  for (var i = 0; i < array.length; i++) {
    action(array[i]);
  }
};

var annoyingAlert = function (value) {
  alert(value);
}

var array = [1, 2, 3, 4];

forEach(array, annoyingAlert); // alerts all values in sequence

group.js

var group = function (array, action) {
  var outbound = {};
  for (var i = 0; i < array.length; i++) {
    var result = action(array[i]);
    outbound[result] = outbound[result] || [];
    outbound[result].push(array[i]);
  }
  return undefined;
};

var awesomeDetector = function(value) {
  return (value.indexOf("Kyle") > -1 ? "awesome" : "still pretty cool");
}

var array = ["Kyle", "John-David", "Greg", "Asa", "Edward", "Jonathan"];

group(array, awesomeDetector); 
/* returns 
{ 
  "awesome" : ["Kyle"], 
  "still pretty cool" : ["John-David", "Greg", "Asa", "Edward", "Jonathan"]
}
*/

map.js

var map = function (array, action) {
  var outbound = [];
  for (var i = 0; i < array.length; i++) {
    outbound.push(action(array[i]));
  }
  return outbound;
};

var doubleValue = function(value) {
  return (value * 2);
}

var array = [1, 2, 3, 4];

map(array, doubleValue); // returns [2, 4, 6, 8]

reduce.js

var reduce = function (array, action, memory) {
  for (var i = 0; i < array.length; i++) {
    memory = action(memory, array[i]);
  }
  return memory;
};

var sumValues = function(memory, value) {
  return (memory + value);
}

var array = [1, 2, 3, 4];

reduce(array, sumValues, 0); // returns 10