itiriri

A library built for ES6 iteration protocol.

function* numbers() {
  let n = 1;

  while (true) {
    yield n++;
  }
}

const s = query(numbers()).map(n => 1 / (n * n)).take(1000).sum();
console.log(Math.sqrt(6 * s));
// 3.1406380562059946

itiriri provides similar functions as the natives for arrays: filter, slice, map, reduce, every, some etc. and more. The functions are optimized for ES6 iterators and can be chained to write simple but powerful queries over iterables.

Installation

Using npm:

$ npm install 'itiriri' --save

Importing:

import { query } from 'itiriri';

Support

The itiriri library can be used with any ES6 compatible runtime.

Iterators

An iterator is a structured pattern for pulling information from a source in one-at-a-time fashion (Y-D-N-JS).

Starting with ES6, built-in types like arrays, Map, Set are iterables and can all be used with itiriri. A generator is also an iterable.

import { query } from 'itiriri';

function* values() {
  yield 2;
  yield 0;
  yield 4;
  yield 8;
}

const s = query(values()).map(n => n / 2).reverse();
console.log(s.toString()); // 4,2,0,1

Deferred execution

JavaScript's array methods like filter, slice and others that return an array create a shallow copy for the result and are executed once called.

itiriri functions that return iterables are not executed unless chained with a function that reduces a value or transforms to a built-in type.

Let's see what happens in the below example.

import { query } from 'itiriri';

function* fibonacci() {
  let [a, b] = [0, 1];

  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}

// Finding first 3 Fibonacci numbers that contain 42
const result = query(fibonacci())
  .filter(x => x.toString().indexOf('42') !== -1)
  .take(3);

for (const e of result) {
  console.log(e);
}

// outputs: 514229, 267914296, 7778742049

Step by step:

1. result is assigned to a query. At this point numbers array is not iterated, the execution is deferred until the result is being iterated.

2. filter method creates an iterator to pipe only numbers passing the predicate. filter does not buffer elements and only pipes them one-by-one to take as it is iterated.

3. take pipes only first three elements as it is iterated and breaks.

4. for...of instruction starts iteration and requests elements one at a time.

Due to deferred execution, most of the functions that don't need entire sequence of elements to build an iterator (like filter, map, concat etc.) can be used with infinite iterables (like Fibonacci in the above example). These functions are also optimized to pass elements through and do not buffer them resulting in a more optimized memory usage.

Functions like sort, reverse, shuffle etc. that require entire sequence of elements in order to build an iterator expect to receive a finite iterable.

Benchmarks

Using itiriri is considerable faster than using array methods when processing large inputs.

In filter-map-slice example arrays of different size are used to filter and map a result of 100 elements:

Array size (N)	array	itiriri
1000	111,611 ops/sec ±9.63% (86 runs)	44,213 ops/sec ±1.92% (88 runs)
5000	18,507 ops/sec ±0.67% (90 runs)	42,103 ops/sec ±2.63% (84 runs)
10000	8,655 ops/sec ±0.70% (91 runs)	42,803 ops/sec ±2.20% (86 runs)
50000	1,640 ops/sec ±0.79% (88 runs)	43,446 ops/sec ±2.17% (88 runs)
100000	848 ops/sec ±0.93% (87 runs)	43,137 ops/sec ±2.15% (87 runs)
200000	46.38 ops/sec ±0.74% (59 runs)	42,445 ops/sec ±2.48% (90 runs)

Using array methods, performance drops significantly for large inputs due to creation of intermediary states for filter and map.
Using itiriri iteration always stops after 100 elements are found, therefore the size of the input doesn't affect the performance.

There are examples in /benchmark.

Running Tests

$ npm install
$ npm test

Complete list of methods

• average
• concat
• distinct
• entries
• every
• exclude
• fill
• filter
• find
• findIndex
• findLast
• findLastIndex
• first
• flat
• forEach
• groupBy
• groupJoin
• includes
• indexOf
• intersect
• join
• keys
• last
• lastIndexOf
• leftJoin
• length
• map
• max
• min
• nth
• prepend
• reduce
• reduceRight
• reverse
• rightJoin
• shuffle
• skip
• skipWhile
• slice
• some
• sort
• splice
• sum
• take
• takeWhile
• toArray
• toGroups
• toMap
• toSet
• toString
• union
• values

average

Returns the average value.

Syntax

average(): number;
average(selector: (element: T, index: number) => number): number;

Parameters

• selector - (optional) a value transformer that accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns a number

For a sequence with no elements returns undefined.

Example

import { query } from 'itiriri';

query([41, 42, 43]).average()  // returns 42
query([{value: 1}, {value: 2}]).average(elem => elem.value) // returns 1.5
query([]).average() // returns undefined

concat

Concatenates the sequence with another one.

Syntax

concat(other: Iterable<T>): IterableQuery<T>;

Parameters

• other - (required) sequence to concatenate

Example

import { query } from 'itiriri';

query([1, 2, 3]).concat([4, 5]).toArray()  // returns [1, 2, 3, 4, 5]

concat is a deferred method and is executed only when the result sequence is iterated.

distinct

Returns a sequence of unique elements.

Syntax

distinct(): IterableQuery<T>;
distinct<S>(selector: (element: T) => S): IterableQuery<T>;

Parameters

• selector - (optional) a function to get element's value for comparison. Accepts one argument:
  • element - current element
  • returns a value to be used for comparison

Example

import { query } from 'itiriri';

query([1, 42, 3, 4, 1]).distinct().toArray();  // returns [1, 42, 3, 4]
query([{value: 1}, {value: 2}, {value: 1}])
  .distinct(elem => elem.value)
  .toArray(); // returns [{value: 1}, {value: 2}]

distinct is a deferred method and is executed only when the result sequence is iterated.

entries

Returns a sequence of key/value pair for each element and its index.

Syntax

entries(): IterableQuery<[number, T]>;

Example

import { query } from 'itiriri';

query(['Alice', 'Bob', 'David']).entries().toArray();
// returns [[0, 'Alice'], [1, 'Bob'], [2, 'David']]

entries is a deferred method and is executed only when the result sequence is iterated.

every

Tests whether all the elements pass the predicate.

Syntax

every(predicate: (element: T, index: number) => boolean): boolean;

Parameters

• predicate - (required) function to test for each element
  • element - the current element
  • index - the index of the current element
  • returns true or false

Example

import { query } from 'itiriri';

query([2, 4, 9]).every(elem => elem > 0); // returns true
query([7, 23, 3]).every(elem => elem % 3 === 0); // returns false

exclude

Returns a sequence of elements not contained in a given sequence.

Syntax

exclude<S>(others: Iterable<T>): IterableQuery<T>;
exclude<S>(others: Iterable<T>, selector: (element: T) => S): IterableQuery<T>;

Parameters

• others - (required) a sequence of elements to be excluded
• selector - (optional) a function to get element's value for comparison, accepts one argument:
  • element - current element
  • returns a value to be used for comparison

Example

import { query } from 'itiriri';

query([2, 0, 1, 8, 2]).exclude([0, 1]).toArray(); // returns [2, 8, 2]
query([{id: 1}, {id: 2}])
  .exclude([{id: 2}, elem => elem.id])
  .toArray(); // returns [{id: 1}]

exclude is a deferred method and is executed only when the result sequence is iterated.

fill

Returns a sequence filled from a start index to an end index with a static value. The end index is not included.

Syntax

fill(value: T): IterableQuery<T>;
fill(value: T, start: number): IterableQuery<T>;
fill(value: T, start: number, end: number): IterableQuery<T>;

Parameters

• value - (required) value to fill
• start - (optional) start index, defaults to 0
• end - (optional) end index, defaults to sequence length

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).fill([7]).toArray(); // returns [7, 7, 7, 7, 7]
query([1, 2, 3, 4, 5]).fill([7, 3]).toArray(); // returns [1, 2, 3, 7, 7]
query([1, 2, 3, 4, 5]).fill([7, 1, 3]).toArray(); // returns [1, 7, 7, 4, 5]

fill is a deferred method and is executed only when the result sequence is iterated.

filter

Returns a sequence of elements that pass the predicate.

Syntax

filter(predicate: (element: T, index: number) => boolean): IterableQuery<T>;

Parameters

• predicate - (required) function to test for each element that accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).filter(elem => elem < 3).toArray(); // returns [1, 2]
query([1, 2, 3]).filter(elem > 10).toArray(); // returns []

filter is a deferred method and is executed only when the result sequence is iterated.

find

Finds the first element that satisfies the specified predicate.

Syntax

find(predicate: (element: T, index: number) => boolean): T;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

If no element satisfies the predicate, returns undefined.

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).find(elem => elem % 2 === 0); // returns 2
query([1, 2, 3]).find(elem > 10); // returns undefined

findIndex

Finds the first index at which a given element satisfies the specified predicate.

Syntax

findIndex(predicate: (element: T, index: number) => boolean): number;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

If no element satisfies the predicate, returns -1.

Example

import { query } from 'itiriri';

query([7, 12, 15]).findIndex(elem => elem > 10 && elem < 15); // returns 1
query([1, 2, 3]).findIndex(elem > 10); // returns -1

findLast

Finds the last element that satisfies the specified predicate.

Syntax

findLast(predicate: (element: T, index: number) => boolean): T;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

If no element satisfies the predicate, returns undefined.

Example

import { query } from 'itiriri';

query([11, 7, 21]).findLast(elem => elem > 10); // returns 21
query([1, 2, 3]).findLast(elem > 10); // returns undefined

findLastIndex

Finds the last index at which a given element satisfies the specified predicate.

Syntax

findLastIndex(predicate: (element: T, index: number) => boolean): number;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

If not present, returns -1.

Example

import { query } from 'itiriri';

query([11, 7, 21]).findLastIndex(elem => elem > 10); // returns 2
query([1, 2, 3]).findLastIndex(elem > 10); // returns -1

first

Returns the first element in a sequence.

Syntax

first(): T;

For an empty sequence returns undefined.

Example

import { query } from 'itiriri';

query(['a', 'b', 'c']).first(); // returns 'a'
query([]).first(); // returns undefined

flat

Returns a sequence with all sub-sequences concatenated.

Syntax

flat<S>(selector: (element: T, index: number) => Iterable<S>): IterableQuery<S>;

Parameters

• selector - (required) a transformation function to map each element to a sequence, accepts two arguments
  • element - the current element
  • index - the index of the current element
  • returns an iterable

Example

import { query } from 'itiriri';

query([{value: [1, 2], {values: [7, 9]}]).flat(elem => elem.value).toArray();
// returns [1, 2, 7, 9]

flat is a deferred method and is executed only when the result sequence is iterated.

forEach

Runs through every element and applies a given function.

Syntax

forEach(action: (element: T, index: number) => void): void;

Parameters

• action - (required) function to apply on each element, accepts two arguments
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]).forEach(elem => console.log(elem));
// 1
// 2
// 3

groupBy

Groups elements by a given key, optionally applying a transformation over each element.

Syntax

groupBy<K>(
  keySelector: (element: T, index: number) => K): IterableQuery<[K, IterableQuery<T>]>;

groupBy<K, E>(
  keySelector: (element: T, index: number) => K,
  valueSelector: (element: T, index: number) => E): IterableQuery<[K, IterableQuery<E>]>;

Parameters

• keySelector - (required) function that provides group's key, accepts two arguments
  • element - the current element
  • index - the index of the current element
  • returns the group key of current element
• valueSelector - (optional) function to transform values, accepts two arguments
  • element - the current element
  • index - the index of the current element
  • returns a transformation of current element

Example

import { query } from 'itiriri';

const students = [
  {name: 'Alice', gender: 'female'},
  {name: 'Bob', gender: 'male'},
  {name: 'David', gender: 'male'},
];

query(students).groupBy(elem => elem.gender, elem => elem.name).toArray();
// [['female', ['Alice']], ['male', ['Bob', 'David']]]

groupBy is a deferred method and is executed only when the result sequence is iterated.

groupJoin

Returns a sequence of correlated elements where each element from the current sequence is matched with zero or more elements from the other sequence.

Syntax

groupJoin<TKey, TRight, TResult>(
    other: Iterable<TRight>,
    leftKeySelector: (element: T, index: number) => TKey,
    rightKeySelector: (element: TRight, index: number) => TKey,
    joinSelector: (left: T, right: TRight[]) => TResult,
  ): IterableQuery<TResult>;

Parameters

• other - (required) sequence to join
• leftKeySelector - (required) function that provides the key of each element from source sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• rightKeySelector - (required) function that provides the key of each element from joined sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• joinSelector - (required) a transformation function to apply on each joined element with group, accepts two arguments:
  • left - element from the original source
  • right - array of elements from the joined source that have the same key as left element's key

The joinSelector function is called on each element from the source sequence and the array of matched elements from the joined sequence.
When an element from the source sequence doesn't match with any of the elements from the joined sequence, the joinSelector function will be called with an empty array.

Example

import { query } from 'itiriri';

const books = [
  {title: 'Clean code', categoryId: 1 },
  {title: 'Code complete', categoryId: 1},
  {title: 'Scrum', categoryId: 2},
];

const categories = [
  {id: 1, name: 'CS'},
  {id: 2, name: 'Agile'},
];

query(categories).groupJoin(
  books,
  category => category.id,
  book => book.categoryId,
  (category, books) => ({ category: category.name, books: books.map(b => b.title) })
).toArray();
// [
//   {category: 'CS', books: ['Clean code', 'Code complete']},
//   {category: 'Agile'}, books: ['Scrum']
// ]

groupJoin is a deferred method and is executed only when the result sequence is iterated.

includes

Determines whether the sequence includes a certain element.

Syntax

includes(element: T): boolean;
includes(element: T, fromIndex: number): boolean;

Parameters

• element - (required) the element to search for
• fromIndex - (optional) starting index, defaults to 0

includes uses triple equals === to compare elements.

Example

import { query } from 'itiriri';

query([1, 2, 3]).includes(2); // returns true
query([1, 2, 3]).includes(0); // returns false

indexOf

Returns the first (zero-based) index at which a given element can be found.

Syntax

indexOf(element: T): number;
indexOf(element: T, fromIndex: number): number;

Parameters

• element - (required) the element to search for
• fromIndex - (optional) starting index, defaults to 0

When an element is not found, returns -1.
indexOf uses triple equals === to compare elements.

Example

import { query } from 'itiriri';

query(['a', 'b', 'c']).indexOf('c'); // returns 2
query(['a', 'b', 'c']).indexOf('x'); // returns -1

intersect

Returns a set intersection with a given sequence.

Syntax

intersect(others: Iterable<T>): IterableQuery<T>;
intersect<S>(other: Iterable<T>, selector: (element: T) => S): IterableQuery<T>;

Parameters

• other - (required) the sequence to intersect with
• selector - (optional) a value transformer function to be used for comparisons, accepts one argument:
  • element - the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]]).intersect([2, 3, 4]).toArray(); // returns [2, 3]
query([{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'})
  .intersect([{id: 3, name: 'David'}, {id: 1, name: 'Alice'}], elem => elem.id)
  .toArray(); // returns [{id: 1, name: 'Alice'}]

intersect is a deferred method and is executed only when the result sequence is iterated.

join

Returns a sequence of correlated elements transformation that match a given key.

Syntax

join<TKey, TRight, TResult>(
    other: Iterable<TRight>,
    leftKeySelector: (element: T, index: number) => TKey,
    rightKeySelector: (element: TRight, index: number) => TKey,
    joinSelector: (left: T, right: TRight) => TResult,
  ): IterableQuery<TResult>;

Parameters

• other - (required) sequence to join
• leftKeySelector - (required) function that provides the key of each element from source sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• rightKeySelector - (required) function that provides the key of each element from joined sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• joinSelector - (required) a transformation function to apply on each matched tuple, accepts two arguments:
  • left - element from the source sequence
  • right - element from the joined sequence
  • returns a new result

The join method works as an sql inner join.

Example

import { query } from 'itiriri';

query([1, 2, 3])
  .join([2, 3, 4], n => n, n => n, (a, b) => `${a}-${b}`)
  .toArray();
// returns ['2-2', '3-3']

query([{countryId: 1, code: '+1'}, {countryId: 2, code: '+44'}]])
  .join(
    [{ id: 1, country: 'US' }, {id: 3, country: 'MD'}],
    left => left.countryId,
    right => right.id,
    (left, right) => ({country: right.country, code: left.code}))
  .toArray();
// returns [{country: 'US', code: '+1'}]

join is a deferred method and is executed only when the result sequence is iterated.

keys

Returns a sequence of keys for each index in the source sequence.

Syntax

keys(): IterableQuery<number>;

Example

import { query } from 'itiriri';

query(['a', 'b', 'c']).keys().toArray(); // returns [0, 1, 2]

keys is a deferred method and is executed only when the result sequence is iterated.

last

Returns the last element in a sequence.

Syntax

last(): T;

For an empty sequence returns undefined.

Example

import { query } from 'itiriri';

query(['a', 'b', 'c']).last(); // returns 'c'
query([]).last(); // returns undefined

lastIndexOf

Returns the last index at which a given element can be found.

Syntax

lastIndexOf(element: T): number;
lastIndexOf(element: T, fromIndex: number): number;

Parameters

• element - (required) the element to search for
• fromIndex - (optional) starting index, defaults to 0

When an element is not found, returns -1.
lastIndexOf uses triple equals === to compare elements.

Example

import { query } from 'itiriri';

query(['a', 'c', 'c']).lastIndexOf('c'); // returns 2
query(['a', 'b', 'c']).lastIndexOf('x'); // returns -1

leftJoin

Returns a sequence of correlated elements transformation that match a given key.

Syntax

leftJoin<TKey, TRight, TResult>(
    other: Iterable<TRight>,
    leftKeySelector: (element: T, index: number) => TKey,
    rightKeySelector: (element: TRight, index: number) => TKey,
    joinSelector: (left: T, right?: TRight) => TResult,
  ): IterableQuery<TResult>;

Parameters

• other - (required) sequence to join
• leftKeySelector - (required) function that provides the key of each element from source sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• rightKeySelector - (required) function that provides the key of each element from joined sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• joinSelector - (required) a transformation function to apply on each matched tuple, accepts two arguments:
  • left - element from the source sequence
  • right - element from the joined sequence, or undefined if no match was found
  • returns element's key

The leftJoin method works as an sql left join. When an element from the left sequence doesn't match with any of the elements from the right sequence, the joinSelector function is called with an undefined right value.

Example

import { query } from 'itiriri';

query([1, 2, 3])
  .leftJoin([2, 3, 4, 2], n => n, n => n, (a, b) => `${a}-${b || '#'}`)
  .toArray();
// returns ['1-#', '2-2', '2-2', '3-3']

query([{book: 'History', owner: 3}, {book: 'Math', owner: 2}, {book: 'Art'}]])
  .leftJoin(
    [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}, {id: 3, name: 'Eve'}],
    left => left.owner,
    right => right.id,
    (left, right) => ({book: left.book, owner: right && right.owner || '--'}))
  .toArray();
// returns [
//   {book: 'History', owner: 'Eve'},
//   {book: 'Math', owner: 'Bob'},
//   {book: 'Art', owner: '--'}]

leftJoin is a deferred method and is executed only when the result sequence is iterated.

length

Returns the number of elements in a sequence.

Syntax

length(): number;
length(predicate: (element: T, index: number) => boolean): number;

Parameters

• predicate - (optional) a function to count only the elements that match the predicate, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).length();  // returns 5
query([1, 2, 3, 4, 5]).length(elem => elem > 2);  // returns 3

map

Returns a sequence of transformed values.

Syntax

map<S>(selector: (element: T, index: number) => S): IterableQuery<S>;

Parameters

• selector - (required) a value transformer function to apply to each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]).map(elem => elem * 10).toArray(); // returns [10, 20, 30]

map is a deferred method and is executed only when the result sequence is iterated.

max

Returns the maximum element in a sequence.

Syntax

max(): T;
max(compareFn: (a: T, b: T) => number): T;

Parameters

• compareFn - (optional) a comparer function that compares two elements from a sequence and returns:
  • -1 when a is less than b
  • 1 when a is greater b
  • 0 when a equals to b

If sequence is empty, returns undefined.

Example

import { query } from 'itiriri';

query([1, 2, 3]).max(); // returns 3
query([]).max(); // returns undefined
query([7, 3, 11, 5]).max((a, b) => (1 / a) - (1 / b)); // returns 3

min

Returns the minimum element in a sequence.

Syntax

min(): number;
min(compareFn: (a: T, b: T) => number): T;

Parameters

• compareFn - (optional) a comparer function that compares two elements from a sequence and returns:
  • -1 when a is less than b
  • 1 when a is greater b
  • 0 when a equals to b

If sequence is empty, returns undefined.

Example

import { query } from 'itiriri';

query([1, 2, 3]).min(); // returns 1
query([]).min(); // returns undefined
query([7, 3, 11, 5]).min((a, b) => (1 / a) - (1 / b)); // returns 11

nth

Returns the element at a specified index.

Syntax

nth(index: number): T;

Parameters

• index - (required) zero based index at which to get the element

For a negative index returns the element from the end of the sequence.
If index is out of the range, returns undefined .

Example

import { query } from 'itiriri';

query(['a', 'b', 'c', 'd']).nth(2)  // returns 'c'
query(['a', 'b', 'c', 'd']).nth(-1) // returns 'd'
query(['a', 'b', 'c', 'd']).nth(10) // returns undefined

prepend

Returns a sequence with given elements at the beginning.

Syntax

prepend(other: Iterable<T>): IterableQuery<T>;

Parameters

• other - (required) the sequence to be added at the beginning

Example

import { query } from 'itiriri';

query([1, 2, 3]).prepend([9, 10]).toArray(); // returns [1, 2, 3, 9, 10]

prepend is a deferred method and is executed only when the result sequence is iterated.

reduce

Applies a function against an accumulator and each element (from left to right) to reduce it to a single value.

Syntax

reduce(
    callback: (accumulator: T, current: T, index: number) => T,
  ): T;

reduce<S>(
    callback: (accumulator: S, current: T, index: number) => S,
    initialValue: S,
  ): S;

Parameters

• callback - (required) function to execute on each element in the sequence, taking three arguments
  • accumulator the accumulator accumulates the callback's return values;
  • current the current element being processed;
  • currentIndex the index of the current element being processed;
• initialValue - (optional) value to use as the first argument to the first call of the callback

Calling reduce on an empty sequence without an initial value throws an error.

Example

import { query } from 'itiriri';

query([ 1, 2, 42, 0 ]).reduce((acc, elem) => Math.max(acc, elem)); // returns 42
query([ 1, 2, 3 ]).reduce((acc, elem) => acc + elem, 10); // returns 16

reduceRight

Applies a function against an accumulator and each element (from right to left) to reduce it to a single value.

Syntax

reduceRight(
    callback: (accumulator: T, current: T, index: number) => T,
  ): T;

reduceRight<S>(
    callback: (accumulator: S, current: T, index: number) => S,
    initialValue: S,
  ): S;

Parameters

• callback - (required) function to execute on each element in the sequence, taking three arguments
  • accumulator the accumulator accumulates the callback's return values;
  • current the current element being processed;
  • currentIndex the index of the current element being processed;
• initialValue - (optional) value to use as the first argument to the first call of the callback

Calling reduceRight on an empty sequence without an initial value throws an error.

Example

import { query } from 'itiriri';

query([ 1, 2, 42, 0 ]).reduceRight((acc, elem) => Math.max(acc, elem)); // returns 42
query([ 1, 2, 3]).reduceRight((acc, elem) => acc.concat(elem), []); // returns [3, 2, 1]

reverse

Returns a sequence of elements in a reversed order.

Syntax

reverse(): IterableQuery<T>;

Example

import { query } from 'itiriri';

query([1, 2, 3]).reverse().toArray(); // returns [3, 2, 1]

reverse is a deferred method and is executed only when the result sequence is iterated.

rightJoin

Returns a sequence of correlated elements transformation that match a given key.

Syntax

rightJoin<TKey, TRight, TResult>(
    other: Iterable<TRight>,
    rightKeySelector: (element: TRight, index: number) => TKey,
    leftKeySelector: (element: T, index: number) => TKey,
    joinSelector: (right: TRight, left?: T) => TResult,
  ): IterableQuery<TResult>;

Parameters

• other - (required) sequence to join
• rightKeySelector - (required) function that provides the key of each element from joined sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• leftKeySelector - (required) function that provides the key of each element from source sequence, accepts two arguments:
  • element - the current element
  • index - the index of the current element
  • returns element's key
• joinSelector - (required) a transformation function to apply on each matched tuple, accepts two arguments:
  • right - element from the joined sequence
  • left - element from the source sequence, or undefined if no match found
  • returns new result

The rightJoin method works as an sql right join. When an element from the right sequence doesn't match with any of the elements from the left sequence, the rightJoin function is called with an undefined left value.

Example

import { query } from 'itiriri';

query([1, 2, 3])
  .rightJoin([2, 3, 4, 2], n => n, n => n, (a, b) => `${a || '#'}-${b}`)
  .toArray();
// returns ['2-2', '3-3', '#-4', '2-2']

query([{book: 'History', owner: 3}, {book: 'Math', owner: 2}]])
  .rightJoin(
    [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}, {id: 3, name: 'Eve'}],
    right => right.id,
    left => left.owner,
    (right, left) => ({student: right.name, book: left && left.book || '--'}))
  .toArray();
// returns [
//   {student: 'Alice', book: '--'},
//   {student: 'Bob', book: 'Math'},
//   {student: 'Eve', book: 'History'}]

rightJoin is a deferred method and is executed only when the result sequence is iterated.

shuffle

Returns the sequence of elements in a random order.

Syntax

shuffle(): IterableQuery<T>;

This method is implemented using Fisher–Yates algorithm for generating the random permutation. Math.rand() is used to generate random numbers.

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).shuffle().toArray();
// returns a random permutation of the same elements
// like: [2, 5, 3, 1, 4]

shuffle is a deferred method and is executed only when the result sequence is iterated.

skip

Skips the specified number of elements from the beginning of sequence and returns the remaining ones.

Syntax

skip(count: number): IterableQuery<T>;

Parameters

• count - (required) number of elements to skip

When count is greater than actual number of elements, results in an empty sequence.
Accepts also a negative count, in which case skips the elements from the end of the sequence.

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).skip(2).toArray(); // [3, 4, 5]
query([1, 2, 3, 4, 5]).skip(10).toArray(); // []
query([1, 2, 3, 4, 5]).skip(-2).toArray(); // [1, 2, 3]

skip is a deferred method and is executed only when the result sequence is iterated.

skipWhile

Skip elements while they satisfy the predicate.

Syntax

skipWhile<T>(predicate: (element: T, index: number) => boolean): IterableQuery<T>;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]).skipWhile(() => true); // returns []
query([1, 2, 3]).skipWhile(() => false); // returns [1, 2, 3]
query([1, 2, 3]).skipWhile(e => e < 3); // returns [3]
query([1, 2, 3]).skipWhile(e => e % 2 === 0); // returns [1, 2, 3]

skipWhile is a deferred method and is executed only when the result sequence is iterated.

slice

Returns a sequence that represents the range of elements from start to end.

Syntax

slice(start: number): IterableQuery<T>;
slice(start: number, end: number): IterableQuery<T>;

Parameters

• start - (required) zero-based index at which to begin extraction
• end - (optional) zero-based index before which to end extraction

The end index is not included in the result.

Example

import { query } from 'itiriri';

query([1, 2, 3, 4, 5]).slice(1, 3).toArray(); // returns [2, 3]

slice is a deferred method and is executed only when the result sequence is iterated.

some

Tests whether at least one element passes the predicate.

Syntax

some(predicate: (element: T, index: number) => boolean): boolean;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3, 42, 5]).some(elem => elem > 40); // returns true
query([1, 2, 3, 42, 5]).some(elem => elem < 0); // returns false

sort

Returns a sequence of sorted elements.

Syntax

sort(): IterableQuery<T>;
sort(compareFn: (a: T, b: T) => number): IterableQuery<T>;

Parameters

• compareFn - (optional) a comparer function that compares two elements from a sequence and returns:
  • -1 when a is less than b
  • 1 when a is greater b
  • 0 when a equals to b

This method fallbacks to native JavaScript array sort.

Example

import { query } from 'itiriri';

query([7, 9, 0, 4, 12]).sort().toArray(); // returns [0, 4, 7, 9, 12]
query([
  {score: 1, value: 'a'},
  {score: 0, value: 'b'},
  {score: 2, value: 'c'}])
  .sort((a, b)) => a.score - b.score);
// returns [
//  {score: 0, value: 'b'},
//  {score: 1, value: 'a'},
//  {score: 2, value: 'c'}]

sort is a deferred method and is executed only when the result sequence is iterated.

splice

Returns a sequence that skips elements and/or adds new elements.

Syntax

splice(start: number, deleteCount: number, ...items: T[]): IterableQuery<T>;

Parameters

• start - (required) index at which to start changing the sequence
• deleteCount - (optional) an integer indicating the number of original elements to skip
• items - (optional) elements to add to the sequence

Example

import { query } from 'itiriri';

query(['angel', 'clown', 'mandarin', 'sturgeon'])
  .splice(2, 0, 'drum').toArray();
// returns ['angel', 'clown', 'drum', 'mandarin', 'sturgeon']

query(['angel', 'clown', 'drum', 'mandarin', 'sturgeon'])
  .splice(3, 1).toArray();
// returns ['angel', 'clown', 'drum', 'sturgeon']

splice is a deferred method and is executed only when the result sequence is iterated.

sum

Returns the sum of all elements.

Syntax

sum(): number;
sum(selector: (element: T, index: number) => number): number;

Parameters

• selector - (optional) a value transformer function to apply to each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Optionally, a function can be provided to apply a transformation and map each element to a value.

Example

import { query } from 'itiriri';

query([1, 2, 3]).sum(); // returns 6
query([{value: 1}, {value: 2}]).sum(elem => elem.value); // returns 3

take

Returns a specified number of elements from the beginning of sequence.

Syntax

take(count: number): IterableQuery<T>;

Parameters

• count - (required) number of elements to take

If a negative count is specified, returns elements from the end of the sequence.

Example

import { query } from 'itiriri';

query([1, 2, 3]).take(2); // returns [1, 2]
query([1, 2, 3]).take(-2); // returns [2, 3]
query([1, 2, 3]).take(10); // returns [1, 2, 3]

take is a deferred method and is executed only when the result sequence is iterated.

takeWhile

Returns elements while they satisfy the predicate.

Syntax

takeWhile<T>(predicate: (element: T, index: number) => boolean): IterableQuery<T>;

Parameters

• predicate - (required) function to test for each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]).takeWhile(() => true); // returns [1, 2, 3]
query([1, 2, 3]).takeWhile(() => false); // returns []
query([1, 2, 3]).takeWhile(e => e < 3); // returns [1, 2]
query([1, 2, 3]).takeWhile(e => e % 2 === 0); // returns []

takeWhile is a deferred method and is executed only when the result sequence is iterated.

toArray

Creates an array copy of the sequence.

Syntax

toArray(): T[];
toArray<S>(selector: (element: T, index: number) => S): S[];

Parameters

• selector - (optional) a value transformer function to apply to each element

When providing a selector function, creates an array of values returned by applying the function on each element.

Example

import { query } from 'itiriri';

query([1, 2, 3]).toArray(); // returns [1, 2, 3]
query([{value: 1}, {value: 2}]).toArray(elem => elem.value); // returns [1, 2]

toGroups

Creates a map of element groups by a given key.

Syntax

toGroups<M>(
  keySelector: (element: T, index: number) => M): Map<M, T[]>;
  
toGroups<M, N>(
  keySelector: (element: T, index: number) => M,
  valueSelector: (element: T, index: number) => N): Map<M, N[]>;

Parameters

• keySelector - (required) a transformer function to apply to each element to get its key, accepts two arguments:
  • element - the current element
  • index - the index of the current element
• valueSelector - (optional) a transformer function to apply to each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Method toGroups creates a JavaScript Map of key-value pairs where each key is the result from keySelector and value is an array of elements (or the result of applying valueSelector on each element) from the original sequence for which the key is the same.

Example

import { query } from 'itiriri';

query([1, 7, 14, 4, 9]).toGroups(elem => elem % 2 === 0);
// returns Map {0 => [14, 4], 1 => [1, 7, 9]}

query([
    {name: 'Alice', gender: 'female'},
    {name: 'Bob', gender: 'male'},
    {name: 'David', gender: 'male'}
  ])
  .toGroups(elem => elem.gender, elem => elem.name);
// returns Map {'female' => ['Alice'], 'male' => ['Bob', 'David']}

toMap

Creates a map of elements by a given key.

Syntax

toMap<M>(
    keySelector: (element: T, index: number) => M): Map<M, T>;

toMap<M, N>(
    keySelector: (element: T, index: number) => M,
    valueSelector: (element: T, index: number) => N): Map<M, N>;

Parameters

• keySelector - (required) a transformer function to apply to each element to get its key, accepts two arguments:
  • element - the current element
  • index - the index of the current element
• valueSelector - (optional) a transformer function to apply to each element, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Method toMap returns a JavaScript Map of key-value pairs where each key is the result from keySelector and value is the element (or the result of applying valueSelector on the element) that corresponds to the key.

If the sequence contains two elements with the same key, method toMap throws an error.

Example

import { query } from 'itiriri';

query(['a', 'b', 'c']).toMap(elem => elem.charCodeAt(0));
// returns Map {97 => 'a', 98 => 'b', 99 => 'c'}

query(['a', 'b', 'c']).toMap(elem => elem.charCodeAt(0), elem => elem.toUpperCase());
// returns Map {97 => 'A', 98 => 'B', 99 => 'C'}

query([1, 1]).toMap(elem => elem);
// throws an Error

toSet

Creates a set of elements.

Syntax

toSet(): Set<T>;
toSet<S>(selector: (element: T, index: number) => S): Set<S>;

Parameters

• selector - (optional) a transformer function to apply to each element to get its value, accepts two arguments:
  • element - the current element
  • index - the index of the current element

Method toSet returns a JavaScript Set of the original elements in the sequence, or their transformation when a selector is provided.

Example

import { query } from 'itiriri';

query([1, 2, 3, 1, 3]).toSet(); // returns Set {1, 2, 3}
query([{value: 1}, {value: 2}, {value: 1}])
  .toSet(elem => elem.value); // returns Set {1, 2}

toString

Returns a string representing the specified sequence and its elements.

Syntax

toString(): string;

Method toString calls .toString() function on each element and joins the result by ,.

Example

import { query } from 'itiriri';

query([1, 2, 3]).toString(); // returns 1,2,3
query([1, null, 3]).toString(); // returns 1,,3
query([{value: 1}, {value: 2}]).toString(); // returns [object Object],[object Object]

union

Returns a set union with a given sequence.

Syntax

union(other: Iterable<T>): IterableQuery<T>;
union<S>(other: Iterable<T>, selector: (element: T) => S): IterableQuery<T>;

Parameters

• other - (required) the sequence to join with
• selector - (optional) a value transformer function to be used for comparisons, accepts one argument:
  • element - the current element

Example

import { query } from 'itiriri';

query([1, 2, 3]]).union([2, 3, 4]).toArray(); // returns [1, 2, 3, 4]

query([{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'})
  .union([{id: 3, name: 'David'}, {id: 1, name: 'Alice'}], elem => elem.id)
  .toArray();
// returns [
//  {id: 1, name: 'Alice'},
//  {id: 2, name: 'Bob'},
//  {id: 3, name: 'David'}]

union is a deferred method and is executed only when the result sequence is iterated.

values

Returns a sequence of values for each index in the source sequence.

Syntax

values(): IterableQuery<T>;

Example

import { query } from 'itiriri';

query([1, 2, 3]]).values().toArray(); // returns [1, 2, 3]

values is a deferred method and is executed only when the result sequence is iterated.

License

MIT

Keywords

iterator iterable filter map query collections deferred lazy