extra-iterator

An extension of JavaScript's built-in Iterator class, with several utility functions for transforming and aggregating sequences of values.

Contributions are welcome. 🤗

Features

• 30+ utility methods
• Lazy operations
• Zero dependencies 🎉
• Fully typed with TypeScript
• 100% test coverage

Installation

npm install extra-iterator

Runtime requirements

This package extends the built-in Iterator class and uses the Iterator Helpers API.

Use it in a runtime that already supports iterator helpers, or load an appropriate polyfill.

Usage Examples

Work with ranges

import { ExtraIterator } from 'extra-iterator';

const range = ExtraIterator.range(1, 6)
    .map(n => n * 2)
    .toArray();

console.log(range); // [1, 2, 3, 4, 5]

Manipulate the sequence

import { ExtraIterator } from 'extra-iterator';

const result = ExtraIterator.concat([1, 2, 3], [4, 5, 6])
    .dropWhile(n => n < 3)
    .take(3)
    .loop(4)
    .toArray();

console.log(result); // [3, 4, 5, 3, 4, 5, 3, 4, 5, 3, 4, 5]

Map an object's key-values pairs

import { ExtraIterator } from 'extra-iterator';

const example = {
    first: 1,
    second: 2,
    third: 3,
};

const result = ExtraIterator.from(Object.entries(example))
    .append(['fourth', 4])
    .interposeWith(([leftKey, leftValue], [rightKey, rightValue]) =>
        [`${leftKey}_${rightKey}`, (leftValue + rightValue) / 2]
    )
    .collect(Object.fromEntries);

console.log(result);
// {
//   first: 1,
//   first_second: 1.5,
//   second: 2,
//   second_third: 2.5,
//   third: 3,
//   third_fourth: 3.5,
//   fourth: 4
// }

Use ExtraIterator.from() to wrap an iterable, iterator, or array-like object

import { ExtraIterator } from 'extra-iterator';

const fromIterable = ExtraIterator.from(new Set([1, 2, 3]));
const fromGenerator = ExtraIterator.from(function*() {
    yield 'a';
    yield 'b';
}());
const fromArrayLike = ExtraIterator.from({ 0: 'x', 1: 'y', length: 2 });

console.log(fromGenerator.toArray()); // ['a', 'b']
console.log(fromArrayLike.toArray()); // ['x', 'y']

Laziness and consumption

Transformation methods are lazy. Nothing happens until you consume the iterator.

const iter = ExtraIterator.from([1, 2, 3])
    .map(value => value * 2)
    .withEach(value => console.log('seen:', value));

// No output yet.

console.log(iter.take(2).toArray());
// seen: 2
// seen: 4
// [2, 4]

Like native iterators, ExtraIterator instances are generally one-shot. Once consumed, the values are gone unless you create a new iterator.

Work with infinite sequences

import { ExtraIterator } from 'extra-iterator';

const evens = ExtraIterator.count({ start: 0, increment: 2 })
    .take(5)
    .toArray();

console.log(evens); // [0, 2, 4, 6, 8]

Flatten nested arrays

import { ExtraIterator } from 'extra-iterator';

const values = ExtraIterator.from([0, [1, [2, [3, 4]]], [5, [6]], 7])
    .flat()
    .toArray();

console.log(values); // [0, 1, 2, 3, 4, 5, 6, 7]

Group adjacent values into chunks

import { ExtraIterator } from 'extra-iterator';

const chunks = ExtraIterator.from([1, 1, 2, 3, 3, 3, 2, 2])
    .chunkWith((left, right) => left === right)
    .toArray();

console.log(chunks); // [[1, 1], [2], [3, 3, 3], [2, 2]]

Find the nearest common ancestor between all DOM paragraph elements

import { ExtraIterator } from 'extra-iterator';

const commonAncestor = ExtraIterator.from(document.querySelectorAll('p'))
    .map(element => ExtraIterator.from(function*() {
        for (let node = element.parentNode; node; node = node.parentNode) {
            yield node;
        }
    }()))
    .map(ancestorList => ancestorList.toArray().reverse())
    .zip()
    .takeWhile(nodes => new Set(nodes).size === 1)
    .last()
    .first();

API overview

See full API documentation at: leonardoraele.github.io/extra-iterator

Static constructors

• ExtraIterator.from(source) - Iterates over any iterable object or array-like object.
• ExtraIterator.empty() - Creates an empty iterator.
• ExtraIterator.single(value) - Creates an iterator with a single value.
• ExtraIterator.count() - An infinite incremental number iterator.
• ExtraIterator.range(start, end) - Iterates over a specific range.
• ExtraIterator.repeat(value) - An iterator that repeats a single value infinitely.
• ExtraIterator.random() - An iterator that yields infinite random numbers.
• ExtraIterator.randomBytes() - An iterator that yields infinite chunks of random bytes.
• ExtraIterator.zip(...sources) - Iterates over multiple sequences simultaneously.

Standard iterator helpers that remain chainable

These native iterator-helper methods work just like the built-in methods, but return chainable ExtraIterator instances instead of built-in Iterator objects.

• filter()
• flatMap()
• map()
• take()
• drop()

take() and drop() also support negative counts:

• take(n) keeps the first n elements of the iterator (or the last ones if n is negative), and discard the rest.
• drop(n) discards the first n elements (or the last ones if n is negative), and keep the rest.

Transformation helpers

• flat() - Similar to Array.prototype.flat(), but recursive.
• unique() - Filters repeated elements out of the iterator.
• compact() - Filters null and undefined values out of the iterator.
• append(value) - Appends one value to the end of the iterator.
• prepend(value) - Prepends one value to the beginning of the iterator.
• concat(iterable) - Concatenates multiple values to the end of the iterator.
• prependAll(iterable) - Prepends multiple values to the beginning of the iterator.
• takeWhile(predicate) - Similar to Iterator.prototype.take(), but with a condition instead of a fixed count.
• dropWhile(predicate) - Similar to Iterator.prototype.drop(), but with a condition instead of a fixed count.
• chunk(size) - Groups the elements into chunks of fixed size.
• chunkWith(predicate) - Groups adjacent elements based on a predicate function.
• zip(others) - Creates a new iterator that yields the values of this and the other iterators in tuples.
• interpose(separator) - Adds a separator value between each pair of adjacent elements.
• interposeWith(separatorProvider) - Adds a separate value between each pair of elements based on a function.
• interleave(other) - Interleave the values of this iterator with those of another iterator.
• splice(startIndex, deleteCount, ...newItems) - Similar to Array.prototype.splice().
• defaultIfEmpty(provider) - If the iterator is empty, adds a default value to it.
• loop(n = Infinity) - Expands the iterator by repeating its values n times.
• withEach(callback) - Similar to Iterator.prototype.forEach(), but returns this iterator.

Aggregation helpers

These methods consume the iterator and return a final value instead of another iterator:

• first() - Returns the first value or undefined if empty.
• last() - Returns the last value or undefined if empty.
• at(index) - Returns the value at the index, or undefined.
• groupBy(callback) - Similar to Object.groupBy().
• toMap(callback) - Similar to Map.groupBy().
• toSet() - Creates a Set object with the elements of this iterator.
• toChainOfResponsibilityFunction(invokeHandler)
• collect(callback) - Reduce the iterator to a single value by calling a callback function.
• sum() - Sum all numbers in the iterator.
• count() - Returns the number of elements in the iterator.
• testUnique() - Checks whether the iterator has duplicated elements.

You also still have the native consuming helpers provided by iterator helpers, such as reduce(), some(), every(), find(), forEach(), and toArray().

Optional [toExtra]() helper

An optional, chainable helper method that transforms any iterable object into an ExtraIterator.

import { toExtra } from 'extra-iterator/to-extra';

const result = [1, 2, 3]
    [toExtra]()    // The `[toExtra]()` method transforms the built-in array into an `ExtraIterator` object...
    .prepend(0)    // Allowing you to chain into the new  helper methods:
    .splice(2, 0, 0.5)
    .drop(-2)
    .loop(3)
    .toArray();

console.log(result);
// [0, 0.5, 1, 0, 0.5, 1, 0, 0.5, 1]

By importing the optional extra-iterator/to-extra submodule, you opt-in to expanding the global Object prototype with the [toExtra]() method as a side-effect.

This side-effect is optional. If you don't want to "polute" the global prototypes, you can still use this package as normal. Simply don't import extra-iterator/to-extra and the global prototypes will remain unchanged.

License

This project is licensed under the MIT License. See the LICENSE.txt file for the full license text.