coseq

Compose chains of functions to manipulate data via synchronous and asynchronous iterators. Data iteration is lazy, which means that items are not read/evaluated until the data is needed. This allows for a really nice integration with infinite and/or really large data sets where eager evaluation and itermediate data storage is prohibitively expensive or impossible.

async iterators are a thing in JavaScript.

The first use case is around generators, which can create asynchronous and synchrnous iterators. You can craft generators to yield anything you want (more than once), in which case they are referred to as coroutines. Combining the coroutine nature of generators and the ability to iterate over the data they yield is why the name coseq came about. Not very creative - I know.

subroutines return one value. coroutines can yield multiple values before running to completion.

You can take a look at this codepen if you want to play with coseq.

features

• Lazy data evaluation
  • infinite data sequence
  • large data sets
• No intermediate arrays
• Asynchronous and synchronous iterators
  • Compatibility with for await (var x of iterator) and for (var x of iterator)

usage

install

$ npm install coseq --save

api

coseq

Method that wraps an iterator to create chains of high order functions to process your data.

The contract with coseq is the iterator protocol, which is why anything that can create an interator can be used with coseq. This includes array, Set, Map... etc.

Below is an example of filtering even numbers and multiplying the values by 2.

import coseq from 'coseq'
 
const set = new Set([1, 2, 3, 4]);
 
const sequence = coseq(set.values())
  .filter(value => value % 2)
  .map(value => value * 2);
 
function runSequence() {
  for (var value of sequence) {
    console.log(value);
  }
}
 
runSequence();

The potential of coseq can be more easily experienced with infinite or really large data sets. Generators are a good example of this.

function* getItemsSync() {
  for (var i = 1; i <= 5000000; i++) {
    yield i;
  }
 
  return "YES!!";
}
 
const sequence = coseq(getItemsSync())
  .skip(500)
  .filter(value => value % 2)
  .map(value => value * 2);
 
function runSequence(sequence) {
  for (var value of sequence) {
    console.log(value);
  }
}
 
runSequence(sequence);

Below is a contrived example with async generators.

async function* getItemsAsync() {
  for (var i = 1; i <= 5000000; i++) {
    yield await i;
  }
 
  return "YES!!";
}
 
const sequence = coseq(getItemsAsync())
  .filter(value => value % 2)
  .map(value => value * 2);
 
async function runSequence(sequence) {
  for await (var value of sequence) {
    console.log(value);
  }
}
 
runSequence(sequence);

You can also await values in the sequence with the helper method awaitValue.

async function* getItemsAsync() {
  for (var i = 1; i <= 5000000; i++) {
    yield await Promise.resolve(i); // <== This yields promises that can be awaited with awaitValue
  }
 
  return "YES!!";
}
 
const sequence = coseq(getItemsAsync())
  .awaitValue() // <== This will await the promise yielded by the iterator
  .filter(value => value % 2)
  .map(value => value * 2);
  
async function runSequence(sequence) {
  for await (var value of sequence) {
    console.log(value);
  }
}
 
runSequence(sequence);

filter

Method to skip items in a sequence when the provided predicate returns falsy values.

alias where

coseq(getItemsAsync()).filter(value => value % 2);

map

Method to transform the current value in the sequence.

alias select

coseq(getItemsAsync()).map(value => value * 2);

forEach

Method to iterate through the items in a sequence. You can use this instead of the for of and for await of constructs.

forEach runs asynchronously and returns a promise, which is particularly useful for accessing to the value returned by generator functions.

In the example below, all values are printed out in the forEach. And when the generator is all done generating values, the last then method in the chain will print YES!!;

async function* getItemsAsync() {
  for (var i = 1; i <= 5000000; i++) {
    yield await i;
  }
 
  return "YES!!";
}
 
coseq(getItemsAsync())
  .filter(value => value % 2)
  .map(value => value * 2)
  .forEach(value => console.log(result.value))
  .then(result => console.log(result.value));

take

Method to read a specific number of items from a sequence. This is a one time operation so once the specified number of items are read, no further values will be processed.

The example below will pull the first 3 items

coseq(getItemsAsync()).take(3);

takeUntil

Method to read items from a sequence until the predicate function returns true. This is a one time operation so once the condition is true, the sequence will no longer return values.

The example below will pull items until the value is equal to 2, including 2.

coseq(getItemsAsync()).takeUntil(value => value === 2);

takeWhile

Method to read items from a sequence while the predicate function returns true. This is a one time operation so once the condition is no longer satisfied, the sequence will no longer return values.

The example below will pull items while the value is smaller or equal to 2

coseq(getItemsAsync()).takeWhile(value => value <= 2);

skip

Method to skip (discard) a specific number of items in a sequence. This is a one time operation so once the specified number of items are read, no more items will be skipped.

The example below skips the first 3 items.

coseq(getItemsAsync()).skip(3);

skipUntil

Method to skip (discard) items until the provided predicate function returns true. This is a one time operation so once the condition is true, skipUntil will no longer skip items.

The example below skips until the value is 3.

coseq(getItemsAsync()).skipUntil(value => value === 3);

skipWhile

Method to skip (discard) items while the provided predicate function return true. This is a one time operation so once the condition is true, skipWhile will no longer skip items.

The example below skips while the value is 1.

coseq(getItemsAsync()).skipWhile(value => value === 1);

awaitValue (async sequence only)

Method to await the current value. The primary use case for this is when a sequence yields a promise which needs to be awaited.

coseq(getItemsAsync())
  .awaitValue()
  .skipUntil(value => value === 2);

delay (async sequence only)

Method to add a delay before continuing to process the current item in the sequence. The delay is sepecified in milliseconds.

coseq(getItemsAsync())
  .skipUntil(value => value === 2)
  .delay(3000);