Excercises from http://howardtang.co.uk/posts/2014-09-22-functional.html

exercises.js

// Define a function greaterThan(n), that takes a value and returns a predicate 
// which returns truthy if that value is greater than n.

var greaterThan = function(n) {
  return function(value) {
    return value > n;
  };
};

var greaterThanFour = greaterThan(4);

greaterThanFour(5); // true
greaterThanFour(3); // false

// Define the filter method using reduce.

Array.prototype.filter = function(predicate) {
  return this.reduce(function(acc, value) {
    if(predicate(value)){
      acc.push(value);
    }
    return acc;
  }, []);
};

[1,2,3,4,5,6].filter(function(v) { return v % 2 !== 0; }); // [1,3,5];

// Define a function Array.reject(predicate),
// which is like filter, but only retains values that
// fail the predicate passed.

Array.prototype.reject = function(predicate) {
  return this.reduce(function(acc, value) {
    if(!predicate(value)) {
      acc.push(value);
    }
    return acc;
  }, []);
};

[1,2,3,4,5,6].reject(function(v) { return v % 2 === 0; }); // [1,3,5];


// Define a function throttle(fn, duration),
// a function that takes a function and a duration in
// milli-seconds as input. Which returns a function that mimics the 
// function that was passed in, but ensures the function can not be run
// for the duration passed after it is called, when that duration completes
// the function should be free to be run again.

var throttle = function(fn, duration) {
  var lastRan = 0;
  return function() {
    var now = Date.now();
    var remaining = duration - (now - lastRan);
    if(remaining <= 0) {
      lastRan = now;
      return fn.apply(fn, arguments);
    } else { console.log('cant run for ', remaining, 'more ms'); }
  };
};

var sayHi = function(msg) { console.log(msg); };

var throttledSayHi = throttle(sayHi, 750);

throttledSayHi('hi mom'); // hi mom
throttledSayHi('hi mom'); // can't run for [x] more ms
// 750ms passes
throttledSayHi('hi mom'); // hi mom