[Devschool] javascript coding challenges

euler-2-revised.js

// Considering terms in Fibonnaci sequence
// whose values don't exceed 4 million
// find sum of the even-valued terms

module.exports = (function() {

  var fibonacci = function(num) {
    if (num <= 1) {
      return num;
    } else {
      return fibonacci(num - 1) + fibonacci(num - 2);
    }
  };

  var fibonacciList = function(fn, num) {
    var list = [];
    for(var i = 1; i <= num; i++) {
      list.push(fn(i));
    }

    return list;
  };

  var evenNums = function(ary) {
    return ary.filter(function(num) { return num % 2 === 0 });
  };

  var sum = function(numsAry) {
    // es6 version return numsAry.reduce((prev, curr) => prev + curr);
    return numsAry.reduce(function(acc, num) { return acc + num });
  };

  var sumEvenFibonacciNums = function(num) {
    return sum(evenNums(fibonacciList(fibonacci, num)));
  };

  return {
    call: sumEvenFibonacciNums
  };

}());

euler-2.js

// Considering terms in Fibonnaci sequence
// whose values don't exceed 4 million
// find sum of the even-valued terms

var sumEvenFibonacciNums = (function() {

  var fibonacci = function(num) {
    if (num <= 1) {
      return num;
    } else {
    return fibonacci(num - 1) + fibonacci(num - 2);
    }
  };

  var fibonacciList = function(fn, num) {
    var list = [];
    for(var i = 1; i <= num; i++) {
      list.push(fn(i));
    }

    return list;
  };

  var evenNums = function(ary) {
    var evens = [];
    ary.filter(function(num) { if (num % 2 === 0) { evens.push(num) }
    });

    return evens;
  };

  var sum = function(numsAry) {
    return numsAry.reduce((prev, curr) => prev + curr);
  };

  var sumEvenFibonacciNums = function(num) {
    return sum(evenNums(fibonacciList(fibonacci, num)));
  };

  return {
    call: sumEvenFibonacciNums
  };

}());

console.log(sumEvenFibonacciNums.call(10));

notes.md

  var fibonacciList = function(fn, num) {
    var list = [];
    for(var i = 1; i <= num; i++) {
      list.push(fn(i));
    }

    return list;
  };

• mutability is all inside the function

Revised file: analysis.md

  var evenNums = function(ary) {
    return ary.filter(function(num) { return num % 2 === 0 });
  };

vs

  var evenNums = function(ary) {
    var evens = [];
    ary.filter(function(num) { if (num % 2 === 0) { evens.push(num) }
    });

    return evens;
  };

They both produce the same outcome.

If I were to pass in an array, say [1,2,3,4,5,6], both would return [2,4,6]

The revised is functional programming, and a much better way to solve the same problem the original snippet is trying to solve.

The filter essentially is for what the "if (num % 2 === 0) { evens.push(num) }" does.

The revised returns an immutable version and it just returns the values that meet the requirements of the filter, versus the original, a mutable version where items are being added to an array.

In the original one, the var evens = []; is a side effect, in that a variable is being created and changing it.