How to remove repeated characters from a string?

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Com reduces

var s1 = 'xyaabbbccccdefww';
var s2 = 'yestheyarehere';
   let res2 = s1.concat(s2);

const remDup= s=> s.split("").sort().reduce((a,b)=>(a[a.length-1]!=b)?(a+b):a,"")
console.log(remDup(res2))

Com filter:

    var s1 = 'xyaabbbccccdefww';
    var s2 = 'yestheyarehere';
       let res2 = s1.concat(s2);

const remDup= s=> s.split("").filter((e,i,f)=>f.indexOf(e)==i).sort().join("")
console.log(remDup(res2))

With map

var s1 = 'xyaabbbccccdefww';
var s2 = 'yestheyarehere';
   let res2 = s1.concat(s2);

const remDup= s=> s.split("").map((c,i,o)=>(o.indexOf(c)==i)?c:"").sort().join("")
console.log(remDup(res2))

  

You can use the new spread operator of JavaScript with Set to get an array of unique values.

     

The Set object allows that you store unique values of any type

var s1 = 'xyaabbbccccdefww';
var s2 = 'yestheyarehere';
   let res2 = s1.concat(s2);

   const remDup= e => [...new Set(e)].sort().join("");
   console.log(remDup(res2))

 

Spread syntax

The Spread Operator basically converts an array to arguments, it is very useful when you need to break an array to pass its values to a function or constructor of an object as separate value arguments. To illustrate in practice, let's create a simple addition function, which needs 2 arguments as an input parameter in its function:

 function soma(a, b) {
     return a + b;
   }

If you intend to use this function you can simply do

 soma(1, 2); // retorna: 3

What if you plan to use an array? How to pass 2 values from an array as an argument? The most obvious way would be:

 var arr = [1, 2];
 soma(arr[0], arr[1]); // retorna: 3

Are you more elegant? Has! You can use soma.apply(null, arr) to invoke this function:

 var arr = [1, 2];
 soma.apply(null, arr); // retorna: 3

With the Spread operator

 var arr = [1, 2];
 soma(...arr); // retorna: 3

Basically the regex you need is this: replace(/(.)(?=.*)/g, "")

Your code looks like this:

let longest = (s1, s2) => {
  var s1 = 'xyaabbbccccdefww';
  var s2 = 'yestheyarehere';
  let res2 = s1.concat(s2);
  let str =  res2.replace(/(.)(?=.*)/g, "");
  console.log(str);
};