Seems reasonable but it is too late to make any changes to ES6.
On Thu, Oct 16, 2014 at 2:11 PM, Ron Buckton <[email protected]>
wrote:
> I recall from earlier discussions on this list that the reason
> `Set.prototype.add` returns `this` is to support chained calls to the set,
> to add multiple items, similar to how some libraries like jQuery operate,
> for example:
>
>
> ```
>
> var s = new Set();
>
> s.add(1).add(2).add(3);
>
> ```
>
>
> I have found myself using Set more and more and have found that there
> are more instances where I would rather it return a boolean value, where
> `true` means the value was added to the set, and `false` when the value was
> not added as it already exists. Without this, I only have two options to
> detect whether an item was added to the set: (a) test using
> `Set.prototype.has` before adding the value, or (b) test using
> `Set.prototype.size` after adding the value.
>
>
> # (a) Test using `Set.prototype.has`
>
>
> ```
>
> var s = new Set();
>
> ...
>
> if (!s.has(value)) {
>
> s.add(value);
> // code that executes only for unique values...
>
> }
>
> ```
>
>
> The problem with this solution is that we have to look up the value in
> the Set instance twice. As the Set grows, the cost of this algorithm will
> always be double the cost of the lookup. I imagine implementations can (and
> likely will) attempt to cache recent lookups for performance to mitigate
> this cost, however to a consumer of Set it could appear as if I'm still
> performing the same lookup twice.
>
>
> # (b) Test using `Set.prototype.size`
>
>
> ```
>
> var s = new Set();
>
> ...
>
> var size = s.size;
>
> s.add(value);
>
> if (size < s.size) {
> // code that executes only for unique values...
>
> }
>
> ```
>
>
> This solution removes the double lookup, but feels unnecessarily wordy
> and unintuitive.
>
>
> # Proposal: `Set.prototype.add` returns Boolean
>
>
> ```
>
> var s = new Set();
>
> ...
>
> if (s.add(value)) {
>
> // code that executes only for unique values
> }
>
> ```
>
>
> This seems the most concise and intuitive, and mirrors the
> `Set.prototype.delete` method's return behavior. From the consumer's
> perspective it has the appearance that the lookup only needs to be
> performed once.
>
>
> Best regards,
>
> Ron
>
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>
>
--
erik
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