Streams

JavaScript/TypeScript library for easy collections manipulation via stream, inspired by Java.

Installation

yarn add @jeyz/streams

Usage

Given the following example array of data:

const cars: CarData[] = [
        { brand: "Audi", model: "A4", production: 2017, specs: { doors: 4, horsePower: 200 }},
        { brand: "Audi", model: "A6", production: 2021, specs: { doors: 4, horsePower: 235 }},
        { brand: "Audi", model: "A6", production: 2021, specs: { doors: 4, horsePower: 235 }},
        { brand: "BMW", model: "X5", production: 2016, specs: { doors: 5, horsePower: 240 }},
        { brand: "Mercedes", model: "C W205", production: 2017, specs: { doors: 4, horsePower: 190 }},
        { brand: "Tesla", model: "3", production: 2021, specs: { doors: 4, horsePower: 360 }},
    ];

We can transform the classic array instance to the stream form, pretty similar to this, known from Java - Stream:

const carStream = Stream.of(cars);

Stream might be used in the following example processing:

const threeMostPowerfulCars = carStream
    .filter(car => car.production > 2016)
    .distinct()
    .map(car => ({ name: `${car.brand} ${car.model}`, ...car.specs }))
    .sorted((car1, car2) => car1.horsePower < car2.horsePower ? 1 : -1)
    .limit(3)
    .collect(Collectors.toArray());

/** Gives Array(3)
 * 0: {name: "Mercedes C W205", doors: 4, horsePower: 190}
 * 1: {name: "Audi A4", doors: 4, horsePower: 200}
 * 2: {name: "Audi A6", doors: 4, horsePower: 235}
 */

You can also call use different collectors or extend Collector class and implement your own:

const carsGroupedByBrand = carStream
    .map(car => ({ car.brand, car.model, car.production }))
    .collect(Collectors.groupingBy('brand'));

/** Gives Map(4)
 * "Audi" => [ "A4", "A6", "A6" ],
 * "BMW" => ["X5"],
 * "Mercedes" => ["C W205"],
 * "Tesla" => ["3"]
 */

Stream

static builder()

Returns a new StreamBuilder, which allows to create Stream, by appending single elements manually or append whole collection of elements. See below for more details.

static concat(aStream, bStream)

Returns a new Stream, which consists of two streams given as arguments, concatenated in order. Both streams must use the same type of elements.

static empty()

Returns a new Stream without any elements inside.

static of(collection)

Returns a new Stream, built from collection.

allMatch(predicate)

Returns true when all the elements in stream match given predicate. Otherwise returns false.

anyMatch(predicate)

Returns true when at least one of the elements in stream match given predicate.

collect(collector)

Collects stream elements using the collector. See below for more details.

count()

Gives the number of elements in stream.

distinct()

Returns a new Stream having no duplicate elements. All elements are compared via deep equality verification, which enables the usage with object values.

filter(filterFn)

Returns a new Stream with only those elements, that match filterFn argument.

findFirst()

Gives an Optional with first element in stream. For Optional, see below for more details.

flatMap(flatMapFn)

Returns a new Stream with elements mapped by a given function and then flattened with depth 1.

forEach(forEachFn)

Performs a given function on each element of stream. Does not return any value.

limit(maxSize)

Returns a new Stream having only requested number of elements, starting from beginning of the stream.

map(mapFn)

Returns a new Stream with elements mapped by a given function.

max(comparator)

Gives an Optional with the maximum value in stream, based on a given comparator function, that decides about order. The function should decide between elements a and b and return one of them.

const mostModernCar = carStream
    .max((a, b) => a.production > b.production ? a : b);

/** Gives Optional of
 * { brand: "Tesla", model: "3", production: 2021, specs: {...}}

min(comparator)

Gives an Optional with the minimum value in stream, based on a given comparator function.

noneMatch(predicate)

Returns true when none of the elements in stream match given predicate. Otherwise returns false.

peek(peekFn)

Performs a given function on each element of stream. Returns this.

reduce(initValue, reduceFn)

Returns an Optional with the reduced value, based on given reducer. The initValue argument specifies the initial value for accumulator.

const averageHorsePower = carStream
    .map(car => car.specs.horsePower)
    .reduce(0, (prev, next) => prev + next)
    .orElse(0) / carStream.count();

skip(n)

Returns a new Stream having only those elements of stream starting from n index.

sorted(sortFn)

Returns a new Stream with all elements sorted by a given argument comparator:

• If the result is negative a is sorted before b,
• If the result is positive b is sorted before a,
• If the result is 0 no changes are done with the sort order of the two values.

StreamBuilder

add(value)

Returns a new StreamBuilder enhanced by a single value.

addAll(collection)

Returns a new StreamBuilder enhanced by a collection.

build()

Returns a Stream based on stream builder data.

Collectors

static toArray()

Returns a new array of elements from stream.

static groupingBy(key)

Returns a new map of elements from stream, grouped by a given key.

Optional

static empty()

Returns an empty Optional.

static of(value)

Returns an Optional from value.

get()

Returns the value of optional.

isPresent()

Returns true if value exists in optional. Otherwise returns false.

orElse(other)

Returns the value of optional if exists. Otherwise returns other.

Advanced usage

For easier process of creating stream from Array source, you can define a custom extension for Array prototype:

// in topmost file of project
Array.prototype.stream = function() {
    return new Stream.of(this);
}

// in types declaration file (TypeScript only)
declare global {
    interface Array<T> {
        stream(): Stream<T>;
    }
}

From now, you can transform Array into Stream as below:

const carStream = cars.stream();

References

Java Stream Documentation