how to concat

// how to concat
let string = 'hello';
let string2 = 'world';

string + string2; // ehh

let cc = '';
cc += string;
cc += string2; // unsure

[string, string2].join(''); // maybe

for(var cc = string, i = 0; i < string2.length; i++) cc += string2.charAt(i); // is this pro code?

[...string].concat([...string2]).join(''); // ooouh, the es6 way, always better..?

string.split('').concat(string2.split('')).join(''); // 4 of 5 stars, better browser support is a plus

string2.split('').reverse().concat(string.split('').reverse()).reverse().join(''); // pro code, for sure.