What is the way of producing an array of unique values out of another array using functional programming in JavaScript?
This should do: toUnique([1,1,2,3,4,4]) => [1,2,3,4]
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3 Answers
Take a look at the uniq function of the Ramda functional javascript libriary.
R.uniq([1, 1, 2, 1]); //=> [1, 2]
R.uniq([{}, {}]); //=> [{}, {}]
R.uniq([1, '1']); //=> [1, '1']
You can use the function from libriary or check the source code...
function uniq(list) {
var idx = -1, len = list.length;
var result = [], item;
while (++idx < len) {
item = list[idx];
if (!_contains(item, result)) {
result[result.length] = item;
}
}
return result;
};
1 Comment
Roman Liutikov
This one is great! Is there any FRP lib with same amount of functions?
This has been asked and answered 1000 times before, but since you're asking for a functional programming solution, here you go:
head = function(ls) { return ls[0] };
tail = function(ls) { return ls.slice(1) };
empty = function(ls) { return ls.length == 0 };
cons = function(a, b) { return [a].concat(b) };
has = function(x, ls) {
return empty(ls) ? false : head(ls) == x || has(x, tail(ls));
};
_uniq = function(ls, seen) {
return empty(ls) ? [] :
has(head(ls), seen) ?
_uniq(tail(ls), seen) :
cons(head(ls),
_uniq(tail(ls),
cons(head(ls), seen)));
};
uniq = function(ls) {
return _uniq(ls, []);
};
console.log(uniq([1,1,2,3,1,2,5])); // [1,2,3,5]
This is pure functional solution, as requested (in fact, a straight port of nub). For a practical one, consider one of the answers over here.
Comments
Well, if you are not worried about the performance, I would use Array.prototype.filter and Array.prototype.indexOf, like this
function toUnique(array) {
return array.filter(function(currentItem, index) {
return (index === array.indexOf(currentItem));
});
}
console.log(toUnique([1, 1, 2, 3, 4, 4]));
# [ 1, 2, 3, 4 ]
If you can use any other libraries, you can use lodash's uniq function, like this
_.uniq([1, 1, 2, 3, 4, 4]);
// → [1, 2, 3, 4]
It can also take advantage of the fact that the input array is already sorted. So, you might want to invoke it like this
_.uniq([1, 1, 2, 3, 4, 4], true);
// → [1, 2, 3, 4]
