Lazy Collections

Fast and lazy collection operations.

---

Working with methods like .map(), .filter() and .reduce() is nice, however they create new arrays and everything is eagerly done before going to the next step.

This is where lazy collections come in, under the hood we use iterators and async iterators so that your data flows like a stream to have the optimal speed.

All functions should work with both iterator and asyncIterator, if one of the functions uses an asyncIterator (for example when you introduce delay(100)), don't forget to await the result!

let program = pipe(
  map((x) => x * 2),
  filter((x) => x % 4 === 0),
  filter((x) => x % 100 === 0),
  filter((x) => x % 400 === 0),
  toArray()
)

program(range(0, 1000000))

Table of Contents

• Benchmark
• API
  • Composing functions
    • compose
    • pipe
  • Known array functions
    • at
    • concat
    • every
    • filter
    • find
    • findIndex
    • flatMap
    • includes
    • join
    • map
    • reduce
    • replace
    • reverse
    • some
    • sort
  • Math / Statistics
    • average
    • max
    • min
    • sum
    • product
  • Utilities
    • batch
    • chunk
    • compact
    • delay
    • flatten
    • generate
    • groupBy
    • head
    • partition
    • range
    • skip
    • slice
    • take
    • takeWhile
    • tap
    • toArray
    • toLength
    • toSet
    • unique
    • wait
    • where
    • windows
    • zip

Benchmark

⚠️ This is not a scientific benchmark, there are flaws with this. This is just meant to showcase the power of lazy-collections.

	Lazy	Eager
Duration	2.19ms	1.29s	589x faster
Memory heapTotal	9.48 MB	297.96 MB	31x less memory
Memory heapUsed	5.89 MB	265.46 MB	45x less memory

Memory data collected using: http://nodejs.org/api/process.html#process_process_memoryusage

import { pipe, range, filter, takeWhile, slice, toArray } from 'lazy-collections'

// Lazy example
let program = pipe(
  range(0, 10_000_000),
  filter((x) => x % 100 === 0),
  filter((x) => x % 4 === 0),
  filter((x) => x % 400 === 0),
  takeWhile((x) => x < 1_000),
  slice(0, 1_000),
  toArray()
)

program() // [ 0, 400, 800 ]

// Eager example
function program() {
  return (
    // Equivalent of the range()
    [...new Array(10_000_000).keys()]
      .filter((x) => x % 100 === 0)
      .filter((x) => x % 4 === 0)
      .filter((x) => x % 400 === 0)

      // Equivalent of the takeWhile
      .reduce((acc, current) => {
        return current < 1_000 ? (acc.push(current), acc) : acc
      }, [])
      .slice(0, 1_000)
  )
}

program() // [ 0, 400, 800 ]

---

This is actually a stupid non-real-world example. However, it is way more efficient at doing things. That said, yes you can optimize the eager example way more if you want to. You can combine the filter / reduce / .... However, what I want to achieve is that we can have separated logic in different filter or map steps without thinking about performance bottlenecks.

API

Composing functions

compose

Table of contents

We can use compose to compose functions together and return a new function which combines all other functions.

import { compose } from 'lazy-collections'

// Create a program (or a combination of functions)
let program = compose(fn1, fn2, fn3)

program()
// fn1(fn2(fn3()))

pipe

Table of contents

We can use pipe to compose functions together and return a new function which combines all other functions.

The difference between pipe and compose is the order of execution of the functions.

import { pipe } from 'lazy-collections'

// Create a program (or a combination of functions)
let program = pipe(fn1, fn2, fn3)

program()
// fn3(fn2(fn1()))

Known array functions

at

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Returns the value at the given index.

import { pipe, at } from 'lazy-collections'

let program = pipe(at(2))

program([1, 2, 3, 4])

// 3

You can also pass a negative index to at to count back from the end of the array or iterator.

Warning: Performance may be degraded because it has to exhaust the full iterator before it can count backwards!

import { pipe, at } from 'lazy-collections'

let program = pipe(at(-2))

program([1, 2, 3, 4])

// 3

If a value can not be found at the given index, then undefined will be returned.

import { pipe, at } from 'lazy-collections'

let program = pipe(at(12))

program([1, 2, 3, 4])

// undefined

concat

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Concat multiple iterators or arrays into a single iterator.

import { pipe, concat, toArray } from 'lazy-collections'

let program = pipe(concat([0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10]), toArray())

program()
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

every

Table of contents

Should return true if all values match the predicate.

import { pipe, every } from 'lazy-collections'

let program = pipe(every((x) => x === 2))

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// false

filter

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Filter out values that do not meet the condition.

import { pipe, filter, toArray } from 'lazy-collections'

let program = pipe(
  filter((x) => x % 2 === 0),
  toArray()
)

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// [ 2, 4, 6, 8, 10 ]

find

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Find a value based on the given predicate.

import { pipe, find } from 'lazy-collections'

let program = pipe(find((x) => x === 2))

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// 2

findIndex

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Find an index based on the given predicate.

import { pipe, findIndex } from 'lazy-collections'

let program = pipe(findIndex((x) => x === 2))

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// 1

flatMap

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Map a value from A to B and flattens it afterwards.

import { pipe, flatMap, toArray } from 'lazy-collections'

let program = pipe(
  flatMap((x) => [x * 2, x * 4]),
  toArray()
)

program([1, 2, 3])
// [ 2, 4, 4, 8, 6, 12 ]

includes

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Check if a value is included in an array or iterator.

import { pipe, includes } from 'lazy-collections'

let program = pipe(includes(1))

program([1, 2, 3, 4])

// true

Each value is compared using Object.is. This will guarantee that edge cases with NaN also work the same as Array.prototype.includes.

Optionally, you can start searching from a positive index:

import { pipe, includes } from 'lazy-collections'

let program = pipe(includes(1, 1))

program([1, 2, 3, 4])

// false

join

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Join an array or iterator of strings.

import { pipe, join } from 'lazy-collections'

let program = pipe(join())

program(['foo', 'bar', 'baz'])
// 'foo,bar,baz'

Optionally, you can join with a separator string:

import { pipe, join } from 'lazy-collections'

let program = pipe(join(' '))

program(['foo', 'bar', 'baz'])
// 'foo bar baz'

toLength

Table of contents

Warning: Performance warning, it has to exhaust the full iterator before it can calculate length!

Get the length of an array or iterator.

import { pipe, toLength, filter } from 'lazy-collections'

let program = pipe(
  filter((x) => x % 2 === 0),
  toLength()
)

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// 5

map

Table of contents

Map a value from A to B.

import { pipe, map, toArray } from 'lazy-collections'

let program = pipe(
  map((x) => x * 2),
  toArray()
)

program([1, 2, 3])
// [ 2, 4, 6 ]

reduce

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Reduce the data to a single value.

import { pipe, reduce } from 'lazy-collections'

let program = pipe(reduce((total, current) => total + current, 0))

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// 55

replace

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Replace an item at a given index with a new value.

import { pipe, replace } from 'lazy-collections'

let program = pipe(replace(2, 42))

program([1, 2, 3, 4])
// [ 1, 2, 42, 4 ]

reverse

Warning: Performance may be degraded because it has to exhaust the full iterator before it can reverse it!

Table of contents

Reverses the iterator.

import { pipe, reverse, toArray } from 'lazy-collections'

let program = pipe(range(0, 5), reverse(), toArray())

program()
// [ 5, 4, 3, 2, 1, 0 ]

some

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Should return true if some of the values match the predicate.

import { pipe, some } from 'lazy-collections'

let program = pipe(some((x) => x === 2))

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// true

sort

Warning: Performance may be degraded because it has to exhaust the full iterator before it can sort it!

Table of contents

Should sort the data. You can also provide a comparator function to the sort function.

import { pipe, generate, take, sort, toArray } from 'lazy-collections'

let program = pipe(
  generate(() => (Math.random() * 100) | 0),
  take(5),
  sort(),
  toArray()
)

program()
// [ 11, 18, 24, 27, 83 ]

Math / Statistics

average

Table of contents

Alias: mean

Gets the average of number of values.

import { pipe, average, toArray } from 'lazy-collections'

let program = pipe(average())

program([6, 7, 8, 9, 10])
// 8

max

Table of contents

Find the maximum value of the given list

import { pipe, range, max } from 'lazy-collections'

let program = pipe(range(0, 5), max())

program()
// 5

min

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Find the minimum value of the given list

import { pipe, range, min } from 'lazy-collections'

let program = pipe(range(5, 10), min())

program()
// 5

sum

Table of contents

Should sum an array or iterator.

import { pipe, sum } from 'lazy-collections'

let program = pipe(sum())

program([1, 1, 2, 3, 2, 4, 5])
// 18

product

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Should multiply an array or iterator.

import { pipe, product } from 'lazy-collections'

let program = pipe(product())

program([1, 1, 2, 3, 2, 4, 5])
// 240

Utilities

batch

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This will call up to N amount of items in the stream immediately and wait for them in the correct order. If you have a list of API calls, then you can use this method to start calling the API in batches of N instead of waiting for each API call sequentially.

import { pipe, range, map, batch, toArray } from 'lazy-collections'

let program = pipe(
  range(0, 9),
  map(() => fetch(`/users/${id}`)),
  batch(5), // Will create 2 "batches" of 5 API calls
  toArray()
)

await program()
// [ User1, User2, User3, User4, User5, User6, User7, User8, User9, User10 ];

chunk

Table of contents

Chunk the data into pieces of a certain size.

import { pipe, chunk, toArray } from 'lazy-collections'

let program = pipe(chunk(3), toArray())

program([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
// [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ], [ 10 ] ];

compact

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Filters out all falsey values.

import { pipe, compact, toArray } from 'lazy-collections'

let program = pipe(compact(), toArray())

program([0, 1, true, false, null, undefined, '', 'test', NaN])
// [ 1, true, 'test' ];

delay

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Will make he whole program async. It will add a delay of x milliseconds when an item goes through the stream.

import { pipe, range, delay, map, toArray } from 'lazy-collections'

let program = pipe(
  range(0, 4),
  delay(5000), // 5 seconds
  map(() => new Date().toLocaleTimeString()),
  toArray()
)

await program()
// [ '10:00:00', '10:00:05', '10:00:10', '10:00:15', '10:00:20' ];

flatten

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By default we will flatten recursively deep.

import { pipe, flatten, toArray } from 'lazy-collections'

let program = pipe(flatten(), toArray())

program([1, 2, 3, [4, 5, 6, [7, 8], 9, 10]])
// [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

But you can also just flatten shallowly

import { pipe, flatten, toArray } from 'lazy-collections'

let program = pipe(flatten({ shallow: true }), toArray())

program([1, 2, 3, [4, 5, 6, [7, 8], 9, 10]])
// [ 1, 2, 3, 4, 5, 6, [ 7, 8 ], 9, 10 ]

generate

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Generate accepts a function that function will be called over and over again. Don't forget to combine this with a function that ensures that the data stream will end. For example, you can use take, takeWhile or slice.

import { pipe, generate, take, toArray } from 'lazy-collections'

let program = pipe(generate(Math.random), take(3), toArray())

program()
// [ 0.7495421596380878, 0.09819118640607383, 0.2453718461872143 ]

groupBy

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Groups the iterator to an object, using the keySelector function.

import { pipe, groupBy, range } from 'lazy-collections'

// A function that will map the value to the nearest multitude. In this example
// we will map values to the nearest multitude of 5. So that we can group by
// this value.
function snap(multitude: number, value: number) {
  return Math.ceil(value / multitude) * multitude
}

let program = pipe(
  range(0, 10),
  groupBy((x: number) => snap(5, x))
)

program()
// {
//   0: [0],
//   5: [1, 2, 3, 4, 5],
//   10: [6, 7, 8, 9, 10],
// }

head

Table of contents

Alias: first

Gets the first value of the array / iterator. Returns undefined if there is no value.

import { pipe, chunk, toArray } from 'lazy-collections'

let program = pipe(head())

program([6, 7, 8, 9, 10])
// 6

partition

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Partition data into 2 groups based on the predicate.

import { pipe, partition, range, toArray } from 'lazy-collections'

let program = pipe(
  range(1, 4),
  partition((x) => x % 2 !== 0),
  toArray()
)

program()
// [ [ 1, 3 ], [ 2, 4 ] ]

range

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Create a range of data using a lowerbound, upperbound and step. The step is optional and defaults to 1.

import { pipe, range, toArray } from 'lazy-collections'

let program = pipe(range(5, 20, 5), toArray())

program()
// [ 5, 10, 15, 20 ]

skip

Table of contents

Allows you to skip X values of the input.

import { pipe, range, skip, toArray } from 'lazy-collections'

let program = pipe(range(0, 10), skip(3), toArray())

program()
// [ 3, 4, 5, 6, 7, 8, 9, 10 ]

slice

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Slice a certain portion from your data set. It accepts a start index and an end index.

import { pipe, range, slice, toArray } from 'lazy-collections'

let program = pipe(range(0, 10), slice(3, 5), toArray())

program()
// [ 3, 4, 5 ]

// Without the slice this would have generated
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

take

Table of contents

Allows you to take X values of the input.

import { pipe, range, take, toArray } from 'lazy-collections'

let program = pipe(range(0, 10), take(3), toArray())

program()
// [ 0, 1, 2 ]

takeWhile

Table of contents

This is similar to take, but instead of a number as a value it takes a function as a condition.

import { pipe, range, takeWhile, toArray } from 'lazy-collections'

let program = pipe(
  range(0, 10),
  takeWhile((x) => x < 5),
  toArray()
)

program()
// [ 0, 1, 2, 3, 4 ]

tap

Table of contents

Allows you to tap into the stream, this way you can intercept each value.

import { pipe, range, tap, toArray } from 'lazy-collections'

let program = pipe(
  range(0, 5),
  tap((x) => {
    console.log('x:', x)
  }),
  toArray()
)

program()
// x: 0
// x: 1
// x: 2
// x: 3
// x: 4
// x: 5
// [ 0, 1, 2, 3, 4, 5 ]

toArray

Table of contents

Converts an array or an iterator to an actual array.

import { pipe, range, toArray } from 'lazy-collections'

let program = pipe(range(0, 10), toArray())

program()
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

toSet

Table of contents

Converts an array or an iterator to Set.

import { pipe, range, toSet } from 'lazy-collections'

let program = pipe(range(0, 10), toSet())

program()
// Set (11) { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }

unique

Table of contents

Make your data unique.

import { pipe, unique, toArray } from 'lazy-collections'

let program = pipe(unique(), toArray())

program([1, 1, 2, 3, 2, 4, 5])
// [ 1, 2, 3, 4, 5 ]

wait

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Will make he whole program async. It is similar to delay, but there is no actual delay involved. If your stream contains promises it will resolve those promises instead of possibly resolving to an array of pending promises.

Note: This will execute the fetch calls sequentially, it will go to the next call once the first call is done. To prevent this you can use the batch function to help with this.

import { pipe, range, map, wait, toArray } from 'lazy-collections'

let program = pipe(
  range(0, 4),
  map((id) => fetch(`/my-api/users/${id}`)),
  wait(),
  toArray()
)

await program()
// [ User1, User2, User3, User4, User5 ];

where

Table of contents

Filter out values based on the given properties.

import { pipe, where, range, map, where, toArray } from 'lazy-collections'

let program = pipe(
  range(15, 20),
  map((age) => ({ age })),
  where({ age: 18 }),
  toArray()
)

program()
// [ { age: 18 } ]

windows

Table of contents

Get a sliding window of a certain size, for the given input.

import { pipe, windows, toArray } from 'lazy-collections'

let program = pipe(windows(2), toArray())

program(['l', 'a', 'z', 'y'])
// [ [ 'l', 'a' ], [ 'a', 'z' ], [ 'z', 'y' ] ]

zip

Table of contents

Zips multiple arrays / iterators together.

import { pipe, zip, toArray } from 'lazy-collections'

let program = pipe(zip(), toArray())

program([
  [0, 1, 2],
  ['A', 'B', 'C'],
])
// [ [ 0, 'A' ], [ 1, 'B' ], [ 2, 'C' ] ]