[문자열 합치기]

1. ‘+’연산자

2. concat()

3. join()

4. ‘+’연산자

let a='Hello';
let b='World';

document.writeln(a+b);
//HelloWorld

1. concat()

const a='Hello'.concat('','World');

document.write(a);
//HelloWorld

1. join()

const a=['Hello','World'].join();

document.write(a);
//Hello,World