Functional JS

IDEAS AND PRACTICE

Paradigms


  • Imperative
  • Declarative
  • OOP - imperative, focuses on nouns
  • Functional - declarative, focuses on verbs
  • Logic, agent-oriented, component-based

Imperative

  • Todo list
  • Fixed execution order
  • HOW it should be done
  • function is a subroutine (repeatable)

Declarative

  • Expresses logic
  • No fixed execution order
  • expresses WHAT gets done
  • defines "the world", data passes through


The State of OOP




Functional Programming


  • Based in math
  • Return an output based on input
  • Functions are the PRIMARY abstractions vs. objects
  • Iterate data structures and apply functions to return new structure
  • Can still use objects
  • OOP = collection of interacting, changing objects
  • FP = stateless function evaluations that can be sequenced in various ways

Purity


A "pure" function is a function that takes an input and returns a value
This is also called "referentially transparent"



        var f = function(x) { return x + 1; }
        f(3);
        //=> 4

f(3) will ALWAYS = 4

Mutability


  • Mutable: can change
  • Immutable: cannot change, new version constructed

Purity has it's regrets..


In basic math, when you define a function there is no notions of a persistent "world" outside of it


f(x) = x + 1 #=> f(3) = 4

functions have no state...

Whenever we modify program state or talk to something outside of a function this is called a Side Effect - and should be a first class consideration

Functional Idioms



Functional Iteration


  • map 
  • reduce 
  • reduceRight
  • filter
  • forEach 
  • every 
  • some

Map

Transform a collection into a new one


        var arr = [1,2,3];
        for (var i=0; i < arr.length; i++) {
          arr[i] += 1;
        }
    

    
        // Arrays (native js)    
        [1, 2, 3].map(function(num) { return num + 1 });
        //=> [2, 3, 4]
    
        // Objects (lodash)
        _.map({ 'one': 1, 'two': 2, 'three': 3 }, function(num) { return num * 3; });
        //=> [3, 6, 9]

Reduce

Reduce a collection into a single value


        var arr = [1,2,3];
        var results = 0;
        for (var i=0; i < arr.length; i++) {
          results += arr[i];
        }
        
    

        // Array (native JS)
        
        [1, 2, 3].reduce(function(memo, num) { return memo + num });
        //=> 6
        
        // Objects (lodash)
        
        _.reduce({ 'a': 1, 'b': 2, 'c': 3 }, function(result, num, key) {
            result[key] = num * 3;
            return result;}, {});
        //=> { 'a': 3, 'b': 6, 'c': 9 }

Reduce Right




        var list = [[0, 1], [2, 3], [4, 5]];
        list.reduceRight(function(a, b){ return a.concat(b); });
        //=> [4, 5, 2, 3, 0, 1]

Filter

Filter items out of an array 


        var arr = [1,2,3,4,5];
        var evens = []
        for (var i=0; i<arr.length; i++) {
          if (arr[i] & 2) {
            evens.push(arr[i]); 
          }
        }
    

        // get evens        
        var arr = [1,2,3,4,5];                
        arr.filter(function(num){ return (num % 2 == 0) });

Foreach

Iterate each item



        
        var arr = [1, 2, 3];
        
        arr.forEach(function(x){
            // do something outside of me
            console.log(x);
        });

Every

Something must be true for every item


        var isOdd = true;
        var arr = [1,2,5,7,9];
        for (var i=0; i<arr.length; i++) {
          if ((arr[i] & 1) === 0) {
            isOdd = false;
            break;
          }
        }
    

        [1, 2, 3, 4, 5].every(function(item){ return item >= 3; });
        //=> false

Some

Function is true for some items


        var hasOdd = false;
        var arr = [1,2,5,7,9];
        for (var i=0; i<arr.length; i++) {
          if ((arr[i] & 1) === 1) {
            hasOdd = true
            break;
          }
        }
    

        [1, 2, 3, 4].some(function(item){
            return item >= 3;
        });

Lodash

Has everything you could want






Functions



Compose

Combine 2 functions

f1(f2())


        var hello = function(name){ return "hi" + name; }
        var place = function(place){ return place + "!"; }
        var hi = _.compose(hello, place);
        hi("world");
        //=> hi world!

Bind/Curry

http://repl.it/Mv3/5 


        var book = function(name) { 
            console.log(name);
        }

        angular = book.bind(null, "AngularJS In Action");
        angular();

Bind with scope



        
        var book = function(review) { 
            console.log(this.name+" is "+review);
        }   

        var mybook = book.bind({name:'foo'}, "good!");
        mybook();

Some code


    var flow = [

        getUrls,
        getDocs,
        clearUnlessES,
        findUrls,
        filterDupes,
        saveUrls
      
    ].map(function(task){ return task.bind(null, stats); });
    
    waterfall(flow, function(err) {
    
        stats.end      = Date.now();
        stats.duration = stats.end-stats.start;
        cb(err, stats);
    
    });

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