javascript 函数式 Functional

* Array functions

find, filter, map, reduce, every, some

let meetups = [
    {name: 'js', isActive: true, members: 450},
    {name: 'Angular', isActive: true, members: 900},
    {name: 'Node', isActive: false, members: 900}
];
// filter
let activeMeetupsFP = meetups.filter((m) => {return m.isActive});
console.log(activeMeetupsFP);
// [{"name":"js","isActive":true,"members":450},
// {"name":"Angular","isActive":true,"members":900}]

// find
let js = meetups.find((m) => {
    return m.name === 'js';
});
console.log(js);

// map, reduce
let sumFPChain = meetups.filter((m) => {
	return m.isActive;
}).map((m)=> {
	return m.members - 0.1 * m.members;
}).reduce((acc, m) => {
	return acc + m;
}, 0);
console.log(sumFPChain); // 1215

           

Libraries to support FP

RamdaJS, UnderscoreJS, lodash

* First Class Functions

var isActive = (meetup) => {
    return meetup.isActive;
}
meetups.filter(isActive);           

* Currying

Currying is the technique of

translating the evaluation of a function that

takes multiple arguments into evaluating a sequence of functions, 

each with a single argument.

let byName = (name) => {
    return (meetup) => {
        return meetup.name === name
    }
}
meetups.find(byName('js'))           

* Function Composition

let byName = (name) => {
    return (meetup) => {
        return meetup.name === name
    }
}
let isValid = isFinite(meetups.find(byName('js')).members)           

* Side Effects

const scheduleMeetup = (date, place) => {
    meetup.date = date;
    meetup.place = place;
    if (meetup.members < 5) {
        meetup.isActive = false;
    }
}

const publishMeetup = () => {
    if (meetup.isActive)
        meetup.publish = true;
}           

* Immutability

change the original data / return a new copy of the data ?

 Immutable Library

 Seamless-immutable, ImmutableJS

reduce方法用法示例：

Grouping objects by a property:

var people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    var key = obj[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}

var groupedPeople = groupBy(people, 'age');
// groupedPeople is:
// { 
//   20: [
//     { name: 'Max', age: 20 }, 
//     { name: 'Jane', age: 20 }
//   ], 
//   21: [{ name: 'Alice', age: 21 }] 
// }           

// friends - an array of objects 
// where object field "books" - list of favorite books 
var friends = [{
  name: 'Anna',
  books: ['Bible', 'Harry Potter'],
  age: 21
}, {
  name: 'Bob',
  books: ['War and peace', 'Romeo and Juliet'],
  age: 26
}, {
  name: 'Alice',
  books: ['The Lord of the Rings', 'The Shining'],
  age: 18
}];

// allbooks - list which will contain all friends' books +  
// additional list contained in initialValue
var allbooks = friends.reduce(function(accumulator, currentValue) {
  return [...accumulator, ...currentValue.books];
}, ['Alphabet']);

// allbooks = [
//   'Alphabet', 'Bible', 'Harry Potter', 'War and peace', 
//   'Romeo and Juliet', 'The Lord of the Rings',
//   'The Shining'
// ]