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// Replace items in `as` with a matching `id` with the | |
// respective item in `bs`, otherwise append new items. | |
const abs = Object.fromEntries([ | |
...as.map(a => [a.id, a]), | |
...bs.map(b => [b.id, b]) | |
]); | |
// The 'trick' is that if you have two entries with the same id | |
// (in this case, implying that the value was updated within `bs`) | |
// then with this method the value of the right-most entry will be used. |
Another idea, might be to group by values other than id
in order to "select" more aggressively (e.g. instead of removing items with the same id
, we could key by something else like a hash of some of the internal values).
As for grouping:
function append(previousItems = [], element, index, array) {
return previousItems.concat([element]);
}
function groupToMap(items, callbackFn, reduceFn = append) {
if (!Array.isArray(items)) {
throw new Error(`groupToMap must receive an array as its first argument.`);
}
if (typeof callbackFn !== "function") {
throw new Error(
`groupToMap must receive a function as its second argument.`
);
}
if (typeof reduceFn !== "function") {
throw new Error(
`groupToMap must receive a function as its third argument.`
);
}
return items.reduce((map, element, index, array) => {
const key = callbackFn(element, index, array);
return map.set(
key,
map.has(key) ? reduceFn(map.get(key), element, index, array) : reduceFn(undefined, element, index, array)
);
}, new Map());
}
function group(items, callbackFn, appendFn = append) {
return Object.fromEntries(groupToMap(items, callbackFn, appendFn).entries());
}
Create two partitions for odd and even values containing a count of the number of items belonging to these partitions:
groupToMap(
[1, 2, 3, 4, 5],
(v) => (v % 2 === 0 ? "even" : "odd"),
(previous = 0) => previous + 1
);
// Map {"odd" => 3, "even" => 2}
Create two partitions for odd and even values containing the sum of the values within these partitions:
groupToMap(
[1, 2, 3, 4, 5],
(v) => (v % 2 === 0 ? "even" : "odd"),
(previous = 0, v) => previous + v
);
// Map(2) {"odd" => 9, "even" => 6}
If I get time I will implement an immutable version of Counter
.
function total(map) {
return map.values().reduce((acc, count) => acc + count, 0);
}
function mostCommon(map, n) {
// ...
// Not trivial to do well, so I'll come back to this... :)
}
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In general,
Object.fromEntries
seems useful in situations in which you want to deduplicate while considering items that are right-most as superseding those to the left. We couldsort
with this in mind...