jgonggrijp
commented
3 years ago Before I answer your question, I should emphasize that Underscore will probably never implement eachRight. The distinction between "forward" and "backward" iteration really only applies to arrays; in objects, any order over the keys is arbitrary, and in maps, sets and iterables (which we will add support for in Underscore 2.0), there is only one direction to choose from. In fact, for this same reason, I'm considering to drop _.reduceRight in Underscore 2.0. "Right-first" iteration has no future for any collection type other than arrays, for which you can already use _.findLastIndex and _.lastIndexOf.
That said, you can patch an eachRight into Underscore with just a few lines of code. I think the following is the most functional, future-proof and backwards-compatible way to do it:
// Demonstrating monolithic import here. Use modular imports if you're
// building a custom Underscore. In that case, make sure to also
// import 'underscore/modules/iteratee.js' for its side effects.
import _, { find, findLastIndex, findKey, isFunction, isNumber, compose, noop } from 'underscore';
// export default only needed if you're building a custom Underscore.
// In that case, this would be your eachRight.js.
export default function eachRight(collection, iteratee, context) {
// Pass the result of the iteratee to _.noop to ensure that nothing is "found"
// so that iteration continues all the way to the end.
iteratee = compose(noop, _.iteratee(iteratee, context));
// Optionally, prevent functions from being iterated as arrays. Currently,
// Underscore doesn't do this but Lodash does.
if (isFunction(collection)) {
// Force treatment as a plain object.
findKey(collection, iteratee);
} else if (collection && isNumber(collection.length)) {
// Scan the array-like collection from right to left.
findLastIndex(collection, iteratee);
} else {
// Treat the collection as a plain object (or in the future, map, set or
// iterable), just like in forward _.each.
find(collection, iteratee);
}
return collection;
}
// If you just want to extend Underscore:
_.mixin({
eachRight: eachRight,
forEachRight: eachRight
});
// (now you can use the function in chains under both names)
// Alternatively, if you are building a custom Underscore:
// In your custom `index.js`:
export { default as eachRight, default as forEachRight } from './eachRight.js';Note that I didn't bother to reverse the order of the keys of a plain object. This is a difference from Lodash _.eachRight and also from Underscore's _.reduceRight, but as I said before, the order of the keys doesn't really mean anything, anyway.
Lodash also has a procedural break like construction where the iteratee can return false in order to stop iteration. If you want to copy that behavior as well, you can modify the iteratee a bit:
var internalIteratee = _.iteratee(iteratee, context);
iteratee = function(value, key, collection) {
return internalIteratee(value, key, collection) === false;
};Final note: the code I've written above supports the context argument like in all Underscore iteration functions. Lodash used to have that as well, but dropped it in version 4.
I'll close this now on the assumption that your question is answered, but please feel free to reopen if you feel the need.
sivaprabug
How to implement forEachRight lodash feature in underscorejs ?