JS Part3

1.  filter (concise): 

const number = [1,2,3,3,4,4,5,6,7,8,9]

const unique = number.filter((item,index)=>(number.indexOf(item)= = = index) );

filter will always return new array. It will not modify exixting array. isBiggerThanFive is the inline callback function 

number.filter(isBiggerThanFive);

function isBiggerThanFive(n){

 return n>5;

}

In callback function we can test the conditions and return the callback.

2. map (ES6 --includes)

We can use any type of key and values

const myMap = new Map() // empty map object

console.log(myMap)

const key1 = 'mystr'

const key2 = {}

const key3 = function(){}

string map values

myMap.set(key1, 'this is a string');

myMap.set(key2, 'This is a blank object');

myMap.set(key3, 'this is empty function')

console.log(myMap);

// getting a values from map

let value1 = myMap.get(key1);

console.log(value1);

//get the size of the map

console.log(myMap.size);

you can loop using for of to get keys and values

for(let[key,value] of myMap){

console.log(key,value)

}

get only keys

for (let key of myMap.keys()){

console.log(key);

console.log('key is', key);

}

// get only values

for(let values of myMap.values()){

console.log('value is', value);

}

// you can loop through a map using for each loop

myMap.forEach((v,k)=>{
console.log('key is',k);

console.log('value is',v)

)}

// convert map to an array

let myArray = Array.from(myMap);

console.log('map to array is ', myArray)

// converting mapkey to an array 

let myKeyArray = array.from(myMap.key())

console.log('map to array is', mykeyArray);

converting map values to an array

let myValuesArray= Array.from(myMap.values);

console.log('map to array is', myValuesArray);

merge Map-- flattering operator

observable ---- (subscrive =>(res))

                                 (subscribe => (res)=>res.data)

source.subscribe(rs=>{console.log(rs)})

source.subscribe(res=>res.subscribe(res2=>{console.log(res)}))

array to observable we use from operator

const source = ['tech', 'comedy', 'news']

const source= from(source)

source.subscribe((res:any)=>{
    console.log(res);
}

of() used to create observable

const source = of(['tech','comdey', 'news'])

MergeMap is the combination of map & mergeAll