Build a conditional String in JavaScript

conditionalString.js

var foo = 'Hello'
var bar = 'World'

function clean (str) {
  return str.trim().replace(/\s\s+/g, ' ');
}

// With ES6
function buildES6 (foo, bar) {
  return clean(`${foo || ''} ${bar || ''}`);
}

// Without ES6
function buildWithoutES6 (foo, bar) {
  return clean((foo || '') + ' ' + (bar || ''));
}

// With array
function buildArray (foo, bar) {
  var arr = [];
  arr.push( foo || '', bar || '' );
  return arr.join(' ');
}

// With string concatenation operator
function buildConcat (foo, bar) {
  return clean(""
    + ( foo || '' )
    + ' '
    + ( bar || '' ));

}

console.log(buildES6(foo, bar));
// "Hello World"
console.log(buildWithoutES6(foo, bar));
// "Hello World"
console.log(buildArray(foo, bar));
// "Hello World"
console.log(buildConcat(foo, bar));
// "Hello World"