Concat multiple arrays into one array without duplicates [duplicate]

Answer 1

Here is another version:

var d = b.concat(c);
  d.forEach(function(el) {
    if (a.indexOf(el) === -1) {
    a.push(el)
  }
})

ES6 version:

let d = b.concat(c);
d.forEach(el => {
  if (a.indexOf(el) === -1) {
    a.push(el)
  }
})