Iterable for Typescript

Similar to what we know from C#, Dart or any other language which supports them, we use Iterables to stream over collections.

Why using the Iterable?

Iterables are useful when you want to chain several operations on a collection such as

• filter
• filterNotNull
• group
• sort
• map
• mapNotNull
• take
• skip
• every
• none
• some
• etc ...

For example, lets consider a case. We need to work with a collection to filter the numbers greater than 20, map to the string only the 3'rd value.

• Without the Iterable

const data = [1, 10, 20, 30, 40, 50, ...];
const filteredData = data.filter(x => x > 20);
const value = filteredData[3];
const mappedValue = value.toString();

// result "50"

• Using the Iterable

const data = [1, 10, 20, 30, 40, 50, ...];
const mappedValue = asIterable(data)
    .filter(x => x > 20)
    .skip(2)
    .map(x => x.toString())
    .first()

// result "50"

The Iterable code would be similar as

let skipped = 0;
for (let i = 0; i < data.length; i++) {
    const element = data[i];
    if (element > 20 && ++skipped < 2) return element.toString();
}
throw new NoElementError();

Not only the difference stays that we have written it differently, but also how much data was processed.

On the example without using the Iterable

• All elements of the collection are visited and filtered
• The third element is retrieved
• The retrieved element is mapped to a string

Now, if the collection is really huge, this will take time to process.

While, using the Iterable, that is not necessarily as we know we do not need all the elements. Because we call first() at the end, that means that the operation will stop as soon this condition is meet.

• Find from collection only the first value that is greater than 20
• Map the value to a string

Installation

npm i @xeinebiu/ts-iterable

Examples

Convert a list to iterable

const data = [1, 2, 3, 4, 5];
const iterable = asIterable(data);

Filter

// without the Iterable
const filtered = data.filter(x => x < 4);

// with iterable
const filtered = asIterable(data)
    .filter(x => x < 4)
    .toList();

// result [1, 2, 3]

Filter Not Null

Filter undefined|null values out

const data = [1, 2, null, 3, undefiend, 4];
const filtered = asIterable(data)
    .filterNotNull();

// result [1, 2, 3, 4]

Take

Take specific amount of elements

// without iterable
const taken = data.slice(0, 3);

// with iterable
const taken = asIterable(data)
    .take(3)
    .toList();

// result [1, 2, 3]

Every

Return true if all elements match the predicate.

const result = asIterable(data)
    .every(x => x.toString() !== "hello world");

// result true

Some

Return true if any of the elements match the predicate

const result = asIterable(data)
    .some(x => x.toString() !== "1");

// result true

None

Return true if all the elements do not match the predicate

const result = asIterable(data)
    .none(x => x <= -1);

// result true

First

Return the first element if available, otherwise throw NoElementError

const result = asIterable(data)
    .filter(x => x > 4)
    .first();

// result 5

First Or Null

Return the first element if available, otherwise null.

const result = asIterable(data)
    .filter(x => x > 100)
    .firstOrNull();

// result null

Map

Map the elements using a mapper

const result = asIterable(data)
    .filter(x => x < 3)
    .map(x => x.toString())
    .toList();

// result ["1", "2"]

Map Not Null

Map the elements using the mapper and avoid inserting null|undefined values in the list

const data = [1, null, 2, undefined, 3];

const result = asIterable(data)
    .filter(x => x < 3)
    .mapNotNull(x => x?.toString())
    .toList();

// result ["1", "2", "3"]

Skip

Offset the elements cursor starting from index 0

const result = asIterable(data)
    .skip(1)
    .toList();

// result ["2", "3", "4", "5"]

Take

Take specific amount of elements

const result = asIterable(data)
    .take(2)
    .toList();

// result ["1", "2"]

Sort

Sort all elements and return new [ExtendedIterable]

const sorted = asIterable(data)
    .sort((a, b) => b - a)
    .toList();

// result [5, 4, 3, 2, 1]

Group

Group all elements and return new [ExtendedIterable]

const data = [-9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

const groupedData = asIterable(data)
    .group(x => {
        if (x < 0) return "negative";
        return "positive";
    })
    .toList();

// result
// [
//     ["negative", [-9, -8, -7, -6, -5, -4, -3, -2, -1]],
//     ["positive", [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]
// ];

To List

Convert the Iterable to a collection.

const list = asIterable(data)
    .toList();

// result [1, 2, 3, 4, 5]

MIT

The MIT License