??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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