Function program of the Javascript

use map()，forEach() method

let arr:number[]=[1,2,3];
let b:number=arr.map((item:number)=>item*item);
arr.forEach((item:number)=>console.log(item));

reference

curried function

let add=(m:number)=>(n:number)=>m*n;
add(4)(3) //12;
let add3=add(4);
add3(4);// 16;

reference

compose function

let compose=(f,g)=>x=>f(g(x));
let f=x=>x.toUpperCase();
let g=x=>x+'!';
let useCompose=compose(f,g);
console.log(useCompose('xixi')); //XIXI!

reference **recommend **

this article is too difficult to read ,so that I will update in the future when I understand it.

Question

solution

const extname = (filename) => {
    let string = Array.from(filename.toString());
    let index = string.findIndex((val, index, arr) => val == '.');
    let strL = string.length;
    let index2 = string.reverse().findIndex((val, index, arr) => val == '.');
    if (index < 1) {
        return filename.substr(strL, 0);
    } else {
        index == strL - index2 ? index = index : index = strL - index2;
        return filename.substring(index - 1, strL)
    }
}
extname('.hello'); //''
extname('fadfa.jpg'); //.jpg
extname('fda.gad.jj'); //.jj
extname('gafgad,jpg'); //''

update 11.16.2017

methods

1. map()

let a:Array=[1,2,3,4,5]
let b:number[]=a.map(item=>item*item)

2. reduce()

let a:Array=[1,2,3,4,5];
let b:number=a.reduce((sum,val)=>sum+=val);

3. filter()
   let a:Array=[1,2,3,4,5];
   let b:number[]=a.filter(val=>val>2)