Arrow function with return assignment not working even with parenthesis

Question

I have the following filter:

const auxHash = {};
myArray.filter(house =>
  house.members === 3 &&
  auxHash[house.id] ? false : auxHash[house.id] = true
)

And I get a lint error saying that an arrow function should not return an assignment. I have tried this too:

const auxHash = {};
myArray.filter(house =>
  house.members === 3 &&
  auxHash[house.id] ? false : (auxHash[house.id] = true)
)

But same problem. How can I solve this?

To clarify. I am trying to filter out elements of an array which attribute members is different than 3, and also I am trying to remove duplicate elements based on the attribute id (that's why I am using the hash).

It's valid JavaScript, but is not good code when used as the return value of an arrow function, as the error says... — Heretic Monkey, Mar 19 '18 at 19:36
Updated, I wasn't using `{}` even though I tried that too with explicit `return`. — Hommer Smith, Mar 19 '18 at 19:36

Suren Srapyan · Accepted Answer · 2018-03-19T19:45:44.933

2

Before question edit.

If you use {} body you need explicitly return a result from that function.

const auxHash = {};
myArray.filter(house => {
    return house.members === 3 && auxHash[house.id] ? false : auxHash[house.id] = true;
})

Also, if your statement is one line, you can omit the {} part and remove return

const auxHash = {};
myArray.filter(house => house.members === 3 && auxHash[house.id] ? false : auxHash[house.id] = true)

edited Mar 19 '18 at 19:45

answered Mar 19 '18 at 19:35

Suren Srapyan

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The linter is complaining if I do what you suggest. Still saying that should not return an assignment. – Hommer Smith Mar 19 '18 at 19:39
The linter is right, generally you're not supposed to modify array elements inside a filter function – frodo2975 Mar 19 '18 at 19:40
I'm not modifying an array element. I'm modifying a hash that I am not filtering. – Hommer Smith Mar 19 '18 at 19:41

score 0 · Answer 2 · answered Mar 19 '18 at 20:18

You're reformatting the exact same statement and ignoring the linter's error. You are returning an assignment. That is your problem. You're doing it right here - auxHash[house.id] = true

Your ternary resolves to false or auxHash[house.id] = true

So, refactor to return an actual value -

const auxHash = {};
myArray.filter(house => {
  if(house.members === 3 && auxHash[house.id]) {
      return false;
  } else {
      auxHash[house.id] = true;
      return true;
  }
});

You can write it more concisely but I stretched it out to make it clear.

Arrow function with return assignment not working even with parenthesis

2 Answers2