That depends entirely on what the return value means. Returning Boolean from
`add` doesn’t mean, “does the value now exist in the set”, but rather means
“was the set modified as a result of this operation”.
To avoid any possible performance cost for calling `has` before `add`, I
usually have to do something like:
```
function setAdd(set, value) {
const size = set.size;
set.add(value);
return size !== set.size;
}
```
From: es-discuss <[email protected]> On Behalf Of Jordan Harband
Sent: Wednesday, October 10, 2018 7:19 PM
To: [email protected]
Cc: es-discuss <[email protected]>
Subject: Re: Array.prototype.remove(item)
Man: `add` doesn't need to return a boolean, because it always results in the
item being in the collection after the fact. You could subclass Set, and make
`.add` do that, though, if you like! Alternatively, you could use `.has` prior
to calling `.add`, to get your boolean value.
On Wed, Oct 10, 2018 at 1:01 AM Man Hoang
<[email protected]<mailto:[email protected]>> wrote:
The problem with `Set` is that its `add` method returns `this` instead of
`boolean`. If `Set.prototype.add` returned `boolean`, I would have used `Set`.
That’s why in the `select` method of my sample code, I use a custom defined
method named `pushIfAbsent`. The actual type of `_values` is not `Array` but a
subclass of `Array`.
``` js
export class MyArray<E> extends Array<E> {
/**
* Adds [item] to the end of this array if it's not already in this array.
*
* Returns `true` is [item] was added, `false` otherwise.
*/
pushIfAbsent(item: E): boolean {
if (!this.includes(item)) {
this.push(item);
return true;
}
return false;
}
/**
* Removes the first occurrence of [item] from this array.
*
* Returns `true` if [item] was in this array, `false` otherwise.
*/
remove(item: E): boolean {
const i = this.indexOf(item);
if (i >= 0) {
this.splice(i, 1);
return true;
}
return false;
}
}
```
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