Functional programming

side effects

shared state

data mutation

declarative programming

var myNumbers = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];

 

const evenNumbers = () => {

 

 

 

 

 

 

 

}

 

 

 

 

    var evenNumbers = [ ] ;

    for ( let i=0 ;  i < myNumbers.length ;  i++ ) {

        if ( myNumbers[ i ] % 2 == 0 ) {

            evenNumbers.push ( myNumbers [ i ] );

        }

    };

 

}

 

 

 

 

 

 

 

 

 

 

 

 

 

    myNumbers = evenNumbers;

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

evenNumbers ( );

console.log ( myNumbers );

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

// output: [2, 4, 6, 8, 10]

var myNumbers = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];

 

 

const filter = ( predicate, items ) => {

 

 

   

   

     

     

     

}

 

 

 

    let filteredItems = [ ];

    for ( let i = 0;  i < items.length ;  i++ ) {

 

 

 

    }

 

 

 

 

 

 

 

 

 

 

 

 

 

 

const isEven = num => num % 2 == 0;

 

 

 

 

 

 

 

 

 

        if ( predicate (  items [ i ]  ) ) {

            filteredItems.push ( items [ i ] );

        }

 

 

 

 

 

 

 

 

 

 

 

 

    return filteredItems;

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

console.log("even - ", filter( isEven, myNumbers ) );

 

// output: even - [2, 4, 6, 8, 10]

// Array.filter

 

var myArray = [ 1, 2, 3, 4, 5 ];

let output = myArray.filter( x => x > 3 );

//output = [ 4, 5 ]

//Array.map

 

var myArray = [ 1, 2, 3, 4, 5 ];

let output = myArray.map( x => x * 2 );

//output = [ 2, 4, 6, 8, 10 ]

//Array.reduce

 

var myArray = [ 1, 2, 3, 4, 5 ];

let output = myArray

      .reduce( ( accum , x ) => { accum + x  } );

 //output = 15

//declarative usage

 

var myArray = [ 1, 2, 3, 4, 5 ];

let output = myArray.filter( x => x > 3 )

                                     .reduce( ( x , y ) => x + y );

//output = 9