laziness

Lazy evaluation utilities for JavaScript powered by ES6 iterators and generators.

Well-tested and with Flow type definitions included.

Compatible with Node v6.11.2 LTS or later.

Installation

npm install --save laziness

Usage example

const { default: Laziness, range } = require('laziness');

// logs numbers 0, 1, ..., 9 one per line
for (const x of range(0, 10)) {
  console.log(x);
}

// the same
Laziness.from(range(0, 10))
  .forEach(x => console.log(x));

function* fib() {
  yield 1;
  yield 1;
  const [fib1, fib2] = Laziness.from(fib()).tee();
  yield* fib1.map2((x, y) => x + y, fib2.tail());
}
fib() // 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

API

Table of Contents

• INFINITE ITERATORS
• cycle
• range
• repeat
• UTILITIES
• filter
• forEach
• map
• map2
• slice
• tail
• tee
• LAZINESS WRAPPER
• Laziness
  • filter
  • forEach
  • map
  • map2
  • slice
  • tail
  • tee
  • toArray
  • from

INFINITE ITERATORS

Infinite Iterators

cycle

Generates all elements of the iterable. Once the original iterable is exhausted, yields all elements again. Repeats indefinitely.

Parameters

• iter Iterable<T>

Examples

cycle('ABC') // 'A', 'B', 'C', 'A', 'B', 'C', ...

Returns Generator<T, void, void>

range

Parameters

• from number
• to number
• step number (optional, default 1)

Examples

range(0, Infinity) // 0, 1, 2, ...

Returns Generator<number, void, void>

repeat

Repeats value endlessly or up to limit times.

Parameters

• value T
• limit number (optional, default Infinity)

Examples

repeat(5) // 5, 5, 5, 5, ...

repeat(10, 3) // 10, 10, 10

Returns Generator<T, void, void>

UTILITIES

Basic functions

filter

Analogical to Array.prototype.filter

Parameters

• iter Iterable<T>
• callback function (T): boolean

Returns Generator<T, void, void>

forEach

Analogical to Array.prototype.forEach

Parameters

• iter Iterable<T>
• callback function (T): void

Returns void

map

Analogical to Array.prototype.map

Parameters

• iter Iterable<T>
• callback function (T): U

Returns Generator<U, void, void>

map2

Generates values of the function when applied arguments from each of the iterables. Stops when the shortest iterable is exhausted.

Parameters

• callback function (U, V): T
• iter1 Iterable<U>
• iter2 Iterable<V>

Examples

map2((x, y) => x + y, range(1, 4), range(10, 40, 10)) // 11, 22, 33

Returns Generator<T, void, void>

slice

Analogical to Array.prototype.slice

Parameters

• iter Iterable<T>
• begin number
• end number

Returns Generator<T, void, void>

tail

Skips first element of iterable

Parameters

• iter Iterable<T>

Returns Generator<T, void, void>

tee

Clone interable into n independent instances

Parameters

• iter Iterable<T>
• n number (optional, default 2)

Returns Array<Generator<T, void, void>>

LAZINESS WRAPPER

Handy wrapper for common operations on iterables.

Laziness

Convenient wrapper for chaining calls to library functions.

You can also use it as iterable.

Parameters

• iter Iterable<T>

Examples

Array.from(new Laziness([1, 2, 3])) // 1, 2, 3

function* fib() {
  yield 1;
  yield 1;
  const [fib1, fib2] = Laziness.from(fib()).tee();
  yield* fib1.map2((x, y) => x + y, fib2.tail());
}
fib() // 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

filter

See filter

Parameters

• callback function (T): boolean

Returns Laziness<T>

forEach

See forEach

Parameters

• callback function (T): void

map

See map

Parameters

• callback function (T): U

Returns Laziness<U>

map2

See map2

Parameters

• callback function (T): U
• iter2 Iterable<V>

Returns Laziness<U>

slice

See slice

Parameters

• begin number
• end number

Returns Laziness<T>

tail

See tail

Returns Laziness<T>

tee

See tee

Parameters

• n number (optional, default 2)

Returns Array<Laziness<T>>

toArray

Returns Array<T>

from

Same as new Laziness(iter)

Parameters

• iter Iterable<T>

Returns Laziness<T>