Tools for working with iterators, collections, and data.

The its-it project is a collection of utility functions intended to make it easier to work with data in JavaScript. Its design is loosely modeled on ideas from the Swift collections API and it follows the function-oriented design of date-fns.

API

All functionality provided by its-it comes in the form of functions which are broken down into small submodules. This structure is intended to keep the project lean and make it highly compatible with tree shaking.

The iterators created by its-it are all lazy. They do no work until they are used in a for...of loop or converted into a collection like an Array.

Generator Iterators

These iterators generate a sequence of values based on their input parameters. They're useful for when you need some data as a starting point and can't conveniently write a C-style for-loop.

fill

An iterator whose items are provided by a function which receives an offset and the previously generated item.

import { fill } from 'its-it/fill';

const squares = fill(8, (offset, prev) => Math.pow(offset, 2));
// => 0, 1, 4, 9, 16, 25, 36, 49

maybe

An iterator whose only item is a non-null, defined value.

import { maybe } from 'its-it/fill';

const hello = maybe("hello");
// => "hello"

When passed a null or undefined value, the iterator has no items.

import { maybe } from 'its-it/fill';

const nothing = maybe();
// => 

Useful when combined with flatMap.

range

An iterator whose items are a sequence of numbers in a specified exclusive range.

import { range } from 'its-it/range';

const numbers = range(0, 10);
// => 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

const tens = range(0, 100, 10);
// => 0, 10, 20, 30, 40, 50, 60, 70, 80, 90

A more modern feeling away to build indexing for loops.

import { range } from 'its-it/range';

for (const n of range(0, 10)) {
    console.log(n);
}

repeat

An iterator which repeats a given value the specified number of times.

import { repeat } from 'its-it/repeat';

const emptyStrings = repeat(10, '');
// => '', '', '', '', '', '', '', '', '', ''

Handy for populating arrays in a certain shape.

import { repeat } from 'its-it/repeat';

const zeros = [...repeat(10, 0)];

Transforming Iterators

filter

An iterator whose items are the items of another iterator that pass a test.

import { filter } from 'its-it/filter';

const numbers = [1, 2, 3, 4, 5, 6];
const evenNumbers = filter(numbers, n => (n % 2) === 0);
// => 2, 4, 6

flatMap

An iterator whose items are the items of another iterator that have been transformed by a mapping function which returns another nested iterator.

import { flatMap } from 'its-it/flat-map';

const numbers = [1, 2, 3, 4];
const pairs = flatMap(numbers, (n) => [n, String(n)]);
// => 1, '1', 2, '2', 3, '3'

Handy for combining filter and map operations into a single mapping function.

import { flatMap } from 'its-it/flat-map';
import { maybe } from 'its-it/maybe';
import { range } from 'its-it/range';

const numbers = range(0, 9);
const oddNumberStrings = flatMap(numbers, (n) => {
    if ((n % 2) !== 0) {
        return maybe(String(n));
    } else {
        return maybe(null);
    }
});
// => '1', '3', '5', '7', '9'

map

An iterator whose items are the items of another iterator that have been transformed by a mapping function.

import { map } from 'its-it/map';

const conditions = ['true', 'false'];
const bools = map(conditions, s => Boolean(s));

Iterator Tests

every

Check whether every item from an iterator passes a test.

import { every } from 'its-it/every';

const ages = [13, 19, 57, 39];
const copaCompliant = every(ages, age => age >= 13);

some

Check whether at least one item from an passes a test.

import { some } from 'its-it/some';

const ages = [13, 19, 57, 39];
const isAdultInTheRoom = some(ages, age => age >= 18);

Utility Functions

arraySegmentBy

Split an array into groups containing the specified number of items.

import { arraySegmentBy } from 'its-it/segment';

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const segments = arraySegmentBy(numbers, 3);
// => [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

cast

Check that a value has a specified type at runtime returning undefined if it does not.

Specify a primitive type by passing its typeof name as the type.

import { cast } from 'its-it/cast';

function getName(json: object | undefined) {
    return cast(json?.age, 'string');
}

Specify a class/constructor function by passing it as the type.

import { cast } from 'its-it/cast';

function getComments(json: object | undefined) {
    return cast(json?.comments, Array);
}

Combine with unwrap to easily write JSON parsing code.

import { cast } from 'its-it/cast';

function hydratePerson(json: unknown) {
    const data = unwrap(cast(json, 'object'), 'json');
    const name = unwrap(cast(data.name, 'string'), 'data.name');
    const age = unwrap(cast(data.age, 'number'), 'data.age');
    const comments = unwrap(cast(data.age, Array), 'data.comments');
    return { name, age, comments };
}

ifNotUndef

Apply a mapping function to a value if it is not null and not undefined, returning undefined otherwise.

import { ifNotUndef } from 'its-it/nullable';

const number = Math.random() > 0.5 ? Math.random() : null;
const numberString = ifNotUndef(possiblyNumber, n => new Intl.NumberFormat().format(n));
// => e.g. "54" or undefined

Very handy for JSX props.

<li>
    <b>Age:</b> {ifNotUndef(data?.age, age => new Intl.NumberFormat().format(age)) ?? '--'}
</li>

isEmpty

Check whether a value is an empty string, array, map, or set.

import { isEmpty } from 'its-it/empty';

isEmpth('');
// => true

isEmpty([]);
// => true

isEmpty(new Map());
// => true

isEmpth(new Set());
// => true

onlyOne

Extract the only item of an array-like collection, throwing a RangeError if there are no items or more than one item.

import { onlyOne } from 'its-it/only-one';

onlyOne([42]);
// => 42

onlyOne([]);
// => throw RangeError

onlyOne([99, 4]);
// => throw RangeError

unwrap

Assert that a value is not null and not undefined.

import { unwrap } from 'its-it/nullable';

unwrap(42);
// => 42

unwrap(undefined);
// => throw RangeError

unwrap(null);
// => throw RangeError

Provide an optional name to get a more descriptive error message.

import { unwrap } from 'its-it/nullable';

const name = unwrap(data?.name, 'data.name');