How can I fix ES6 Eslint errors while trying to return a ternary operator in arrow function?

Question

I understand that () => {} does not need return, however if it is not there then Eslint complains about unused expressions.

export const isInInterval = (from, to, target) => {
  if (isNaN(Date.parse(to)) && isNaN(Date.parse(from)) === true) {
    return
  }
  const toUnixTimestamp = time => new Date(time).getTime()
  toUnixTimestamp(to) - target > toUnixTimestamp(from) ? true : false
}

Here is the function: it tries to find out whether some specified date(to) minus specified period(target) is later in time than from. It should return true if so and false in the opposite case. I keep bumping into eslint error expected assignment to a function call and instead saw expression.

I tried to rewrite it several times, but in most iterations I got `arrow function expects no return error, ex.:

return (toUnixTimestamp(to) - target > toUnixTimestamp(from)) ? true : false

Answer 1

I understand that () => {} does not need return

That's not the case. Arrow functions only implicitly return when what followed the => is a single expression. If you use => {, the opening bracket { indicates the start of a function block, and you do indeed need to explicitly return at the end of the block (or wherever else you want to return something).

At the moment, your code isn't returning anything at all - that's what the linting error is trying to tell you - the true : false is going unused at the moment, it's just an orphaned expression.

So, just add the return statement to the beginning of your conditional:

export const isInInterval = (from, to, target) => {
  if (isNaN(Date.parse(to)) && isNaN(Date.parse(from)) === true) {
    return
  }
  const toUnixTimestamp = time => new Date(time).getTime()
  return toUnixTimestamp(to) - target > toUnixTimestamp(from)
    ? true
    : false
}

Or, because > evaluates to a boolean already, you might omit the conditional operator entirely:

return toUnixTimestamp(to) - target > toUnixTimestamp(from)

Here's an example of how you would write an arrow function that does use implicit return:

export const isEarlyTimestamp = (timestamp) => (
  timestamp < 4000000
  ? true
  : false
);

Answer 2

try this:

export const isInInterval = (from, to, target) => {
    if (isNaN(Date.parse(to)) && isNaN(Date.parse(from)) === true) {
    return false
    }
    const toUnixTimestamp = time => new Date(time).getTime()
    return toUnixTimestamp(to) - target > toUnixTimestamp(from);
}