npm package: @pipes/utils
Description: Basic utilities for web streams
Author: Diwank Singh
Latest version: 0.3.1

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### Table of Contents

-   [Introducing pipes/utils](#introducing-pipesutils)
-   [About](#about)
-   [Installing](#installing)
-   [API Reference](#api-reference)
-   [batch](#batch)
-   [compact](#compact)
-   [Pipe](#pipe)
-   [debounce](#debounce)
-   [drop](#drop)
-   [filter](#filter)
-   [head](#head)
-   [intersperse](#intersperse)
-   [last](#last)
-   [pick](#pick)
-   [pluck](#pluck)
-   [repeat](#repeat)
-   [scan](#scan)
-   [slice](#slice)
-   [take](#take)
-   [tap](#tap)
-   [throttle](#throttle)
-   [uniq](#uniq)

## Introducing pipes/utils

<a href='http://www.recurse.com' title='Made with love at the Recurse Center'><img src='https://cloud.githubusercontent.com/assets/2883345/11325206/336ea5f4-9150-11e5-9e90-d86ad31993d8.png' height='20px'/></a>

The `pipesjs/utils` module is to `web streams` what `highland.js` is to `node streams`.
It contains utility functions to make working with `web streams` a lot easier.
For more about `Web Streams`, refer to the [spec](https://streams.spec.whatwg.org).

* * *


## About

The `utils` module is to `web streams` what `highland.js` is to `node streams`. It contains utility functions to make working with `web streams` a lot easier. Here's more about `Web Streams` from the [spec](https://streams.spec.whatwg.org) itself:

> Large swathes of the web platform are built on streaming data: that is, data that is created, processed, and consumed in an incremental fashion, without ever reading all of it into memory. The Streams Standard provides a common set of APIs for creating and interfacing with such streaming data, embodied in readable streams, writable streams, and transform streams.

The spec is still evolving but has reached a fairly stable stage with a [reference implementation](https://github.com/whatwg/streams/tree/master/reference-implementation) as well. The API has almost been finalized and `Stream`s are coming to the web very soon!

At it's core, the API exposes three major components:

-   `ReadableStream` encapsulates a source producing values and emits them.
-   -   `TransformStream` are essentially `{ readable, writable}` pairs that take a function which can be used to transform the values flowing through it.
-   -   `WritableStream` encapsulates a sink that receives values and writes to it.

`Stream`s are essentially data structures that handle sequential flow of values. You can split streams, merge them and connect them together in various ways. What's amazing is that, in most cases, they can handle [backpressure](https://streams.spec.whatwg.org/#pipe-chains) automatically, so you don't have to mess with the underlying details.

For further information, the spec is quite informative and easy to read. [Jake Archibald](https://github.com/jakearchibald) also wrote a great [blog post](https://jakearchibald.com/2016/streams-ftw/) on them.

**Heads up:** If you're coming from `node` land, `web streams` are quite a lot different from `node streams` and incompatible with each other.


## Installing

### For browsers

The library depends on [@pipes/core](https://github.com/pipesjs/core), so make sure you include it in before including the library.

You can use either of the builds from the `dist` folder:

```html
    <script src="path/to/web-streams-polyfill.js"></script>
    <script src="path/to/pipes.utils.js"></script>
```

And in your code, all the functions will be available on the `window.Pipes.utils` variable.

```javascript
    let { uniq, compact } = window.Pipes.utils;
```

### For browserify users

The library has a [peer-dependency](https://nodejs.org/en/blog/npm/peer-dependencies/) on [@pipes/core](https://github.com/pipesjs/core), so to install it:

```bash
    npm install @pipes/core @pipes/utils
```

The library is split up into modules, so you can both require the whole library or only parts of it:

```javascript
    let { compact } = require("@pipes/utils");
    let compact = require("@pipes/utils/compact");
```

### For ES6 and Rollup users

If you want, you can directly import the es6 modules like so:

```javascript
    import pipesUtils from "@pipes/utils/src";
    import { compact } from "@pipes/utils/src";
    import compact from "@pipes/utils/src/compact";
```


## API Reference

The `utils` library only consists of the following functions:

-   [batch](#batch)
-   [compact](#compact)
-   [cycle](#cycle)
-   [debounce](#debounce)
-   [drop](#drop)
-   [filter](#filter)
-   [head](#head)
-   [intersperse](#intersperse)
-   [last](#last)
-   [pick](#pick)
-   [pluck](#pluck)
-   [repeat](#repeat)
-   [scan](#scan)
-   [slice](#slice)
-   [take](#take)
-   [tap](#tap)
-   [throttle](#throttle)
-   [uniq](#uniq)

**Set up code for examples**

```javascript
  // Setup
  let createReadable = data => new ReadableStream({
      start (controller) {
      this.data = data || [1,2,3];

      // Kickstart stream
      controller.enqueue( this.data.pop() );
      },
      pull (controller) {
      if ( !this.data.length )
          return controller.close()

      controller.enqueue( this.data.pop() );
      }
  }),
  createWritable = () => new WritableStream({
      write (chunk) {
      console.log( chunk );
      }
  });
```


## batch

This function takes an int `n` and returns a transform stream
that batches the incoming values in arrays of lengths no
more than `n`.

**Parameters**

-   `size` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** 

**Examples**

```javascript
let
  input = [1,2,3,4,5],
  expected = [[1,2],[3,4],[5]];

let readable, writable, res=[];

// Create test streams
readable = createTestReadable( input );
writable = createTestWritable( c => res.push( c ));

// Connect the streams
connect(
  readable,
  batch( 2 ),
  writable
); // res == expected
```

Returns **TransformStream** 

## compact

This function returns a transform stream that spits out only truthy
values from the input stream.

**Examples**

```javascript
let readable, writable,
  count = 0;

// Create test streams
readable = createTestReadable( [true, false, 0, "", "hello", 1] );
writable = createTestWritable( () => count++ );

// Connect the streams
connect(
  readable,
  compact(),
  writable
); // count == 3
```

## Pipe

This function takes an `iterable` as argument and returns
a readable stream that repeatedly emits values generated by the emitter.

**Examples**

```javascript
let readable, writable, values=[1,2,3], sum=0;

// Create test streams
  readable = cycle( values );
  writable = createTestWritable( c => { sum+=c });

// Connect the streams
const expected = 2 * values.reduce( (a, b) => a+b );

connect(
  readable,
  take( 2 * values.length ),
  writable
); // sum == expected
```

## debounce

This function takes an int `n` and returns a
`transform stream` that debounces the incoming values by `n` ms,
only producing values with `n` ms delay between them and dropping the rest.

**Parameters**

-   `wait` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)**  (optional, default `0`)
-   `head` **[boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)**  (optional, default `true`)

Returns **ReadableWritable** 

## drop

This function takes an int `n` and returns a `transform stream`
that drops the first `n` values from the input stream.

**Parameters**

-   `count` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** 

**Examples**

```javascript
let readable, writable,
  count = 0;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( () => count++ );

// Connect the streams
connect(
  readable,
  drop(3),
  writable
); // count == 3
```

Returns **ReadableWritable** 

## filter

This function takes a predicate function as argument and returns
a `transform stream` that only emits values that satisfy the predicate.

**Parameters**

-   `pred` **function (any?): [boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)** 

**Examples**

```javascript
let readable, writable;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( c => assert( c > 3 ) );

// Connect the streams
connect(
  readable,
  filter( a => a > 3 ),
  writable
);
```

Returns **ReadableWritable** 

## head

This function returns a `transform stream` that takes the first value
from the input stream and enqueues it on the output stream.

**Examples**

```javascript
let readable, writable, el;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( e => { el = e; });

// Connect the streams
connect(
  readable,
  head(),
  writable
); // el == 1
```

Returns **ReadableWritable** 

## intersperse

This function takes any value `a` and returns a `transform stream`
that intersperses the values from the input stream with the `a`.

**Parameters**

-   `val` **any** 

**Examples**

```javascript
let readable, writable,
  res = [];

// Create test streams
readable = createTestReadable( [1,2,3] );
writable = createTestWritable( c => res.push(c) );

// Connect the streams
connect(
  readable,
  intersperse(0),
  writable
); // res == [1,0,2,0,3]
```

Returns **ReadableWritable** 

## last

This function returns a `transform stream` that takes
the last value from the input stream and enqueues it on the output stream.

**Examples**

```javascript
let readable, writable, el;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( e => { el = e; });

// Connect the streams
connect(
  readable,
  last(),
  writable
); // el == 6
```

Returns **ReadableWritable** 

## pick

This function takes any number of `string`s as arguments and
returns a `transform stream` that extracts the passed property names from
incoming values.

**Parameters**

-   `props` **...[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)&lt;[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)>** 

**Examples**

```javascript
let readable, writable,
  o = {
    'a': 1,
    'b': 2
  };

// Create test streams
readable = createTestReadable( [o, o, o] );
writable = createTestWritable( c => assert( c.a ) && assert( !c.b ) );

// Connect the streams
connect(
  readable,
  pick('a'),
  writable
);
```

Returns **ReadableWritable** 

## pluck

This function takes a `string` as argument and returns
a `transform stream` that extracts the passed property from
incoming values.

**Parameters**

-   `prop` **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** 

**Examples**

```javascript
let readable, writable,
  o = {
    'a': 1,
    'b': 2
  };

// Create test streams
readable = createTestReadable( [o, o, o] );
writable = createTestWritable( c => assert( c == 1 ) );

// Connect the streams
connect(
  readable,
  pluck('a'),
  writable
);
```

Returns **ReadableWritable** 

## repeat

This function takes a value as argument and returns a `readable stream`
that repeatedly emits that value.

**Parameters**

-   `value` **any** 

**Examples**

```javascript
let readable, writable, val=1, len=6, sum=0;

// Create test streams
readable = repeat(val);
writable = createTestWritable( c => { sum+=c });

// Connect the streams
connect(
  readable,
  take( len ),
  writable
); // sum == (val * len)
```

Returns **ReadableWritable** 

## scan

This function takes a reducer function and an optional `init` value
as arguments and returns a `transform stream` that applies the
function to the incoming values and enqueues the accumulation of the results.

If an `init` value is not passed, the first incoming value is treated as one.

**Parameters**

-   `func` **function ((T2? | T1?), T1?): T2** 
-   `init` **T1?** 

**Examples**

```javascript
let readable, writable, res;

// Create test streams
readable = createTestReadable( [1,2,3] );
writable = createTestWritable( c => {
  // Check last element is number
  let n = c[c.length-1];
  assert( n === +n );

  res = c;
});

// Connect the streams
connect(
  readable,
  scan( add, 0 ),
  writable
); // res[res.length-1], [1,2,3].reduce( add )
```

Returns **ReadableWritable** 

## slice

This function takes an int `m` and an int `n` and returns
a `transform stream` that drops the first `m` values and
takes the next `(m-n)` values from the input stream.

**Parameters**

-   `beg` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** 
-   `end` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** 

**Examples**

```javascript
let readable, writable,
  count = 0;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( () => count++ );

// Connect the streams
connect(
  readable,
  slice(2,5),
  writable
); // count == 3
```

Returns **ReadableWritable** 

## take

This function takes an int `n` and returns a `transform stream`
that takes the first `n` values from the input stream.

**Parameters**

-   `count` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** 

**Examples**

```javascript
let readable, writable,
  count = 0;

// Create test streams
readable = createTestReadable( [1,2,3,4,5,6] );
writable = createTestWritable( () => count++ );

// Connect the streams
connect(
  readable,
  take(3),
  writable
); // count == 3
```

Returns **ReadableWritable** 

## tap

This function takes a function as rgument and returns
a `transform stream` that applies the function to the incoming
values before re-emitting them.

**Parameters**

-   `func` **anyFn** 

**Examples**

```javascript
let readable, writable;

// Create test streams
readable = createTestReadable( [1,2,3] );
writable = createTestWritable( c => assert( !Number.isNaN( c*1 ) ) );

// Connect the streams
connect(
  readable,
  tap( console.log.bind(console) ),
  writable
);
```

Returns **ReadableWritable** 

## throttle

This function takes an int `n` and returns a `transform stream`
that throttles the incoming values by `n` ms, only producing
values every `n` ms and dropping the rest.

**Parameters**

-   `interval` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)**  (optional, default `0`)
-   `head` **[boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)**  (optional, default `true`)

Returns **TransformStream** 

## uniq

This function returns a `transform stream` that keeps only unique
values from the input stream and enqueues it on the output stream.

**Examples**

```javascript
let readable, writable,
  res = [];

// Create test streams
readable = createTestReadable( [1,1,2,2,3,3] );
writable = createTestWritable( c => res.push(c) );

// Connect the streams
connect(
  readable,
  uniq(),
  writable
); // res == [1,2,3]
```

Returns **ReadableWritable**