pipeline-linq

sync/async generators and helpers to form typed linq-like iterable queries

this is an experimental package and subject to change on minors versions

Install

install with either npm or yarn:

• npm install pipeline-linq
• yarn add pipeline-linq

Dependencies

For versions pipeline-linq @ < 1.0, this package, when imported, will polyfill the Symbol.asyncIterator using core-js.

Basic Usage

linq is a helper function used to wrap and chain pipeline methods together. Simply pass through an object that implements [Symbol.iterator], this includes [] or any typed-array.

import {linq} from 'pipeline-linq'
 
let data = {
  *[Symbol.iterator](){
    yield 1; 
    yield 2; 
    yield 3; 
    yield 4;
  }
}
 
let result = 
  linq(data)
    .where(x=>x > 2)
    .select(x=>x * 2)
    .first();
 
console.log(result);

By default the call to linq returns the synchronous query, you can switch to the async equivalents by passing through true at the end of most supporting methods.

Converting to an async query will cause all subsequent method calls to be evaluated using the Symbol.asyncIterator and you must await the resulting promise or use for-await to iterate the query.

import {linq} from 'pipeline-linq'
 
let data = [1, 2, 3, 4];
 
async function main() {
  let query = linq(data)
    .where(x => x > 2)
    .select(async x => x * 2, true);
 
  for await (let value of query) {
    console.log(value);
  }
}
main();

note: when using linq the evaluation of the iterators is lazy, thus you can iterate over query multiple times.

Generators

You can use the sync generators directly by importing them:

import { where, select, first } from 'pipeline-linq';
 
let data = [1, 2, 3, 4];
 
let where_ = where(data, x => x > 2);
let select_ = select(where_, x => x * 2);
let result = first(select_);

Advanced Usage

The Linq and LinqAsync interfaces have a method chain that can be used to insert custom iterators into the iterator sequence.

A custom filter method:

let isNumber = function* (source) {
  for (let item of source) {
    if (typeof item === "number") {
      yield item as number;
    }
  }
};
 
let results = linq([10, "hello", true])
  .chain(isNumber)
  .toArray();

It is also possible to insert complex queries:

let query = function (source: Iterable<number>) {
  return linq(source)
    .where(x => x >= 2)
    .select(x => x * 2)
};
 
let results = linq([1, 2, 5, "hello", true])
  .chain(isNumber)
  .chain(query)
  .skip(1)
  .toArray();

Pipeline Operator

see proposal pipeline-operator

Subject to change. All generators and helper methods take the source as the first parameter and so will be readily available to use with the pipeline operator. For now, you could use the babel transformer and use:

import { where, select, first } from 'pipeline-linq';
 
let result = [1, 2, 3, 4] 
  |> _ => where(_, x => x > 2)
  |> _ => select(_, x => x * 2)
  |> first();

Acknowledgements