> The example code isn't very compelling either; something more real-world
> would be good
I recently ran across a usage of `reduce(..)` that could have benefitted from
an early return. Figured I'd just drop it here for posterity sake, in case
anything ever comes of this idea.
I had an array of arrays (2-dimensional array), where each inner array had a
list of simple string/number elements (none of them falsy) that could have some
repetition within the inner array. I wanted to filter the outer array (the list
of the inner arrays) based on whether an inner array had any adjacent
duplicates. That is, `[1,4,2,4]` is fine to keep but `[2,4,4,5]` should be
filtered out.
Since `reduce(..)` conveniently can compare two adjacent elements if you always
return the current value, I decided to model the inner check as a `reduce(..)`
that reduces from the original array value to either a `false` or a truthy
value (the last element of the inner array element). This reduction result then
is how `filter(..)` decides to keep or discard. The reason an early exit would
be nice is that as soon as you run across an adjacency-duplication, no more
reduction is necessary -- you can immediately "reduce" to `false`.
Here's how I did it, which worked but which was slightly less appealing:
```js
var outer = [
// [1,2,1,3,4,2]
// ["foo","bar","bar",10,"foo"]
// ..
];
outer = outer.filter(function filterer(inner){
return inner.reduce(function reducer(prev,current){
if (prev === false || prev === current) return false;
return current;
});
});
```
The reduction initial-value is omitted, so it's `undefined`, which never
matches any of the `inner` contents.
The `prev === false` check is the way that I fake the "early exit", by which
once the reduction value is tripped to `false`, that's always the result for
the rest of the reduction.
There's lots of other ways to "slice" that problem, I know. But reduction was
attractive except for its lack of early exit.
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