lazy-iter

lazy-iter provides a suite of generator methods which all share a common prototype (named lazy)

const lazy = require("lazy-iter");

The lazy method is used to wrap any iterable into the base class, transforming it into a lazy-iter.

const enumerable = lazy([1, 2, 3, 4]); // iter:[1, 2, 3, 4]

The returned object now provides access to several lazy methods, most of which are nearly identical in usage to Array methods. These methods also, when appropriate, return lazy-iters, allowing you to compose iterator chains, similar to linq.

enumerable.filter(x => x < 4).map(y => y * 2).slice(1); // iter:[4, 6]

lazy-iter implements nearly every method of Arrays, with a handful of caveats, and a few additional methods which are useful.

Array methods not present in lazy-iter

Array.isArray - has no practical application

Array.prototype.copyWithin - this method assumes the user knows values before enumeration, which is unlikely to actually be the case

findIndex, indexOf, lastIndexOf - methods dealing with indices are unusual in an iterator context

toString, toSource, toLocaleString - a lazy-iter can simply be flattened to an array first

keys, values, entries - returning an iterator from an iterator is redundant

Additional methods

lazy.keys - similar to lazy, but takes Object.keys of the input

lazy.values - similar to lazy, but takes Object.values of the input

lazy.entries - similar to lazy, but takes Object.entries of the input

lazy.prototype.toArray - receives an optional value selector, then consumes the entire sequence into an array, applying the selector to each item

lazy.prototype.toObject - receives a key selector, and optional value selector, consumes the sequence to produce an object mapping keys to values.

lazy.prototype.toSet - identical to toArray, but returns a set

lazy.prototype.toMap - identical to toObject, but returns a map

Method Caveats

All array-like methods which deal with indices do not use indices in lazy-iter. For example, map takes in a callback method which recieves three arguments: (item, index, array). In lazy-iter, the index and reference to the array are, for obvious reasons, omitted from callbacks.

In addition, array-like methods which take positions as input (such as slice and splice) do not treat negative indices as values starting from the end of the sequence, as that would require knowing the length of the sequence ahead of time. Instead, negative numbers will simply behave like 0.

lazy-iter instances are also immutable: the underlying sequence is never mutated; rather, a new instance with the new sequence is returned. This is only relevant in a handful of cases, as this maps to most array methods naturally.

splice is one such method which does have a noticeable change. The array version mutates the array, and returns deleted items. lazy-iter's version simply returns the new sequence with the specified range deleted/replaced with new values.

There are additionally four methods which one must keep aware of, as they are not perfectly lazy. The first two must consume the full sequence on the first call to next, but are afterwards as lazy as possible. The other two are simply not as similar to each other as they would appear to be.

reverse must consume the entire sequence in order to reach the end, and return an iterator which walks back through in reverse order.

sort must similarly consume the sequence first. However, it still lazily sorts the consumed list by performing a heapSort; heapifying the full list, and returning items by popping them from the resulting heap.

shift is an odd case, not necessarily unlazy, but it returns the first item of the sequence, as opposed a new sequence, leaving the initial sequence only partially-consumed.

pop is similar to shift, but worse, as it must consume the entire sequence to be able to return the last value. One should typically only use these two methods as the final operation on a sequence.

The search methods can potentially also partially consume sequences, so they also should be the final operation.

The reduction methods (reduce, join, toArray...) will of course all wholly consume the sequence and return a single, non-iterator value.

List of methods

const lazy = require("lazy-iter");
//static methods
lazy(iterable);             //returns an instance of lazy-iter
lazy.from(iterable);        //alias of lazy()
lazy.of(...args);           //shorthand for lazy([...args])
lazy.keys(object);          //shorthand for lazy(Object.keys(object))
lazy.values(object);        //shorthand for lazy(Object.values(object))
lazy.entries(object);       //shorthand for lazy(Object.entries(object))

//chainable iterator methods
lazy.prototype.filter(filterMethod);
lazy.prototype.map(mappingMethod);
lazy.prototype.slice(start = 0, end = Infinity);
lazy.prototype.splice(start = 0, deleteCount = 0, ...items);
lazy.prototype.fill(value, start = 0, end = 0);
lazy.prototype.push(...items);
lazy.prototype.unshift(...items);
lazy.prototype.concat(otherIterable);
lazy.prototype.forEach(someMethod);

//chainable methods which must consume the full sequence up front (but are still lazy)
lazy.prototype.sort(sortMethod = (a, b) => b - a);
lazy.prototype.reverse();

//methods for retrieving first or last item
lazy.prototype.shift();
lazy.prototype.pop();

//conditional search methods
lazy.prototype.every(conditionMethod);
lazy.prototype.some(conditionMethod);
lazy.prototype.find(conditionMethod);
lazy.prototype.includes(item);

//methods which consume the sequence to return a singular item
lazy.prototype.reduce(reduceMethod);
lazy.prototype.reduceRight(reduceMethod);
lazy.prototype.join(separator = ",");
lazy.prototype.toArray(valueSelector = null);
lazy.prototype.toObject(keySelector, valueSelector = null);
lazy.prototype.toSet(valueSelector = null);
lazy.prototype.toMap(keySelector, valueSelector = null);