This a very simple question. I want to map and form an array given another array. then I would like to remove the duplicated values.

this is what I did:

let status = listHotels.map(hotel => {
  return hotel.status
})

const x = status.filter((v, i) => (
  status.indexOf(v) === i
));

It works. But I would like a solution that doesn't involve writing two blocks of code. I tried this:

let status = listHotels.map(hotel => {
  return hotel.status
}).filter((v, i) => (
  status.indexOf(v) === i
));

But it didnt work. It says

Cannot read property 'indexOf' of undefined

Does anyone know a workaround this?

This a very simple question. I want to map and form an array given another array. then I would like to remove the duplicated values.

this is what I did:

let status = listHotels.map(hotel => {
  return hotel.status
})

const x = status.filter((v, i) => (
  status.indexOf(v) === i
));

It works. But I would like a solution that doesn't involve writing two blocks of code. I tried this:

let status = listHotels.map(hotel => {
  return hotel.status
}).filter((v, i) => (
  status.indexOf(v) === i
));

But it didnt work. It says

Cannot read property 'indexOf' of undefined

Does anyone know a workaround this?

• javascript

• status is not available inside the chained method – David Casanellas Commented Dec 17, 2018 at 14:36
• status will only be assigned once all chained methods have executed, with the result of the last chained method. So you attempting to access status inside a chained method before it has been assigned. – ste2425 Commented Dec 17, 2018 at 14:41
• 1 Why would you filter ? Because your statement array.indexOf(v) === i will always return true... – ChrisR Commented Dec 17, 2018 at 14:41
• No it wont @ChrisR. It's a mon way to remove duplicates from the array. – Blue Commented Dec 17, 2018 at 14:42

4 Answers 4

status is still not defined when you call the .filter method.

change your code to:

const listHotels = [{status:'s1'}, {status:'s1'}, {status:'s2'}]
let status = listHotels.map(hotel => {
  return hotel.status
}).filter((v, i, currentStatus) => (
  currentStatus.indexOf(v) === i
));
console.log(status);

You can do this using findIndex, passing a function to pare statuses, and swapping filter/map around:

const listHotels = [{status: 'bad'}, {status: 'good'}, {status: 'bad'}];

let status = listHotels.filter((v, i) =>
  listHotels.findIndex(v2 => v.status === v2.status) === i
).map(hotel => hotel.status);

console.log(status);

Since you are using let and const I'm assuming you can also use ES6 Set:

const data = [{
  status: 'one',
  id: 1
}, {
  status: 'one',
  id: 2
}, {
  status: 'two',
  id: 3
}, {
  status: 'two',
  id: 4
}]

const uniqueStatus = [...new Set(data.map(({
  status
}) => status))]

console.log(uniqueStatus)

Passing the mapped array into Set will ensure uniqueness. You can destructure the Set back into an array.

status is not ready yet (undefined) to access inside the filter(). But in the first solution, map() returns and stores result in status which makes the variable available in the later statements.

You can pass the array itself as the third parameter to use in the filter(). You also do not need to use unnecessary return statement here:

var listHotels = [{status:'a'}, {status:'b'}, {status:'a'}]

let status = listHotels.map(hotel => hotel.status)
                       .filter((v, i, arr) => arr.indexOf(v) === i);
console.log(status);