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# Introduction
This is an idiomatic, minimally-opinionated `Channel` type for JavaScript that's inspired by [Go](https://golang.org/)'s concurrency model. It works in browsers and in Node.js. If you know how to use `Array` then you already know most of how to use `Channel`.
## Why
Go's concurrency model is amazing and with JavaScript's async/await feature we have the basis for it as well. All that's missing is a `Channel` type. There are existing libraries that implement one but they're generally ports from other languages and don't feel like they were made for JavaScript. I wanted an idiomatic `Channel` type that's simple and minimally-opinionated.
This document assumes you're familiar with Go's concurrency model and why you'd want to use it. For explanatory background, read my [blog article](https://www.NodeGuy.com/concurrency-in-javascript) on the subject.
## Requirements
ES 2017
## Installation
```shell
$ npm install @nodeguy/channel
```
## Basic Use
Create a channel with `Channel()`.
To send an value to a channel, use `push`. To receive an value from a channel, use `shift`. Always precede the method calls with `await`:
```JavaScript
const assert = require('assert')
const Channel = require('@nodeguy/channel')
const channel = Channel()
;(async () => {
await channel.push(42)
})()
;(async () => {
assert.equal(await channel.shift(), 42)
})()
```
The `push` and `shift` methods are usually called in different async functions. They represent the two different ends of the channel and act to coordinate the behavior of the async functions.
# API
## New Properties
The following properties don't have equivalents in `Array`.
### close() -> async
Closes the channel so that no more values can be pushed to it. Returns a promise that resolves when any remaining pushes in flight complete.
Attempting to push to a closed channel will throw an exception and shifting from a closed channel will immediately return `undefined`.
### readOnly() -> Channel
Returns a version of the channel that provides only read methods.
### value
Set to the most recently `shift`ed value. This is useful when used in combination with `select`.
### writeOnly() -> Channel
Returns a version of the channel that provides only write methods.
### Channel.select(methods) -> async channel
`Channel.select` attempts to call multiple channel `methods` in parallel and returns the channel of the first one that succeeds. Only the winning method is executed to completion—the other methods have no effect.
#### Examples
Imagine you're at a party and your next conversation depends on whom you run
into first: Alice, Bob, or Charlie.
```JavaScript
switch (await Channel.select(alice.shift(), bob.shift(), charlie.push(`Hi!`)) {
case alice:
console.log(`Alice said ${alice.value}.`)
break
case bob:
console.log(`Bob said ${bob.value}.`)
break
case charlie:
console.log(`I said "hi" to Charlie.`)
break
}
```
Be careful of unintended side effects, however. Even though only one value is pushed in the following example, the counter is incremented twice.
```
let counter = 0
const increment = () => {
counter++
return counter
}
await Channel.select(alice.push(increment()), bob.push(increment()))
assert.equal(counter, 2)
```
Sometimes you don't want to wait until a method completes. You can use this pattern to return immediately even if no methods are ready:
```JavaScript
const closed = Channel()
closed.close()
switch (await Channel.select(alice.shift(), bob.shift(), closed.shift()) {
case alice:
console.log(`Alice said ${alice.value}.`)
break
case bob:
console.log(`Bob said ${bob.value}.`)
break
default:
console.log(`No one has anything to say yet.`)
}
```
You can also arrange it so that the select completes within a timeout:
```JavaScript
const timeout = Channel()
setTimeout(timeout.close, 1000)
switch (await Channel.select(alice.shift(), bob.shift(), timeout.shift()) {
case alice:
console.log(`Alice said ${alice.value}.`)
break
case bob:
console.log(`Bob said ${bob.value}.`)
break
default:
console.log(`I stopped listening after one second.`)
}
```
## Array-like Methods
These methods are similar to the equivalently named methods of `Array`.
### Channel
#### Channel([bufferLength = 0]) -> Channel
Create a new `Channel` with an optional buffer.
#### Channel.of(...values) -> Channel
#### Channel.from(iterable | stream.Readable) -> Channel
Create a new `Channel` from an iterable or a [Node.js readable stream](https://nodejs.org/api/stream.html#stream_readable_streams).
### Channel Object
#### filter(callbackfn[, thisArg]) -> Channel
#### forEach(callbackfn[, thisArg]) -> async
The promise returned by `forEach` resolves when the channel is closed:
```JavaScript
const toArray = async (channel) => {
const array = []
await channel.forEach((value) => {
array.push(value)
})
return array
}
```
If `callbackfn` is async then `forEach` will wait for it before iterating to the next value:
```JavaScript
const pipe = async (source, sink) => {
await source.forEach(sink.push)
sink.close()
}
```
#### join(separator) -> async String
#### map(callbackfn[, thisArg]) -> Channel
#### push(value) -> async bufferLength
Unlike `Array`'s method, `push` accepts only one `value` at a time.
Sends the value into the channel and returns a promise that resolves when the value has been shifted out or placed in the buffer.
* Throws a `TypeError` when attempting to push to a closed channel.
* Throws a `TypeError` when attempting to push `undefined` because it's a reserved value used to indicate a closed channel.
#### reduce(callbackfn[, initialValue])
#### shift() -> Promise
Returns a promise that resolves when an value is received from the channel. Closed channels always return `undefined` immediately.
#### slice(start, end) -> Channel
# Contributing
Please [submit an issue](https://github.com/NodeGuy/channel/issues/new) if you would like me to make `Channel` more `Array`-like (e.g., by adding another `Array` method).
# Similar Projects
* [Channel](https://github.com/gozala/channel)
* [cochan](https://github.com/skozin/cochan)
* [js-csp](https://github.com/ubolonton/js-csp)
* [node-csp](https://github.com/olahol/node-csp)
# Copyright
Copyright 2017 [David Braun](https://www.NodeGuy.com/)
Licensed under the Apache License, Version 2.0 (the "License"); you may not use
these files except in compliance with the License. You may obtain a copy of the
License at `http://www.apache.org/licenses/LICENSE-2.0`. Unless required by
applicable law or agreed to in writing, software distributed under the License
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the specific language
governing permissions and limitations under the License.

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'use strict'
// An order represents a pending push or shift.
const Order = (channel) => {
let order
const preonFulfilleds = []
const promise = new Promise((resolve, reject) => {
order = {
resolve: (value) => {
preonFulfilleds.forEach((preonFulfilled) => {
preonFulfilled(value)
})
resolve(value)
},
reject
}
})
Object.assign(promise, {
cancel: () => {
order.cancelled = true
},
channel,
prethen: (onFulfilled) => {
preonFulfilleds.push(onFulfilled)
}
})
return {order, promise}
}
const Channel = function (bufferLength = 0) {
let buffered = 0
let closed = false
let lastValue
let resolvedIndex = 0
const pushes = []
const shifts = []
// Process the push and shift queues like an order book, looking for matches.
const processOrders = () => {
const index = {push: 0, shift: 0}
// Match pushes and shifts.
while ((index.push < pushes.length) && (index.shift < shifts.length)) {
const push = pushes[index.push]
const shift = shifts[index.shift]
if (push.cancelled) {
index.push++
} else if (shift.cancelled) {
index.shift++
} else {
lastValue = push.value
shift.resolve(lastValue)
buffered = Math.max(0, buffered - 1)
index.push++
index.shift++
}
}
// Resolve push promises up to the end of the buffer.
for (
;
(resolvedIndex < index.push) ||
((resolvedIndex < pushes.length) && (buffered < bufferLength));
resolvedIndex++
) {
const {cancelled, resolve} = pushes[resolvedIndex]
if (!cancelled) {
if (resolvedIndex > index.push) {
buffered++
}
resolve(bufferLength)
}
}
// If the channel is closed then resolve 'undefined' to remaining shifts.
if (closed) {
for (;
index.shift < shifts.length;
index.shift++
) {
const {cancelled, resolve} = shifts[index.shift]
if (!cancelled) {
lastValue = undefined
resolve(lastValue)
}
}
}
pushes.splice(0, index.push)
shifts.splice(0, index.shift)
resolvedIndex -= index.push
}
const readOnly = Object.freeze({
filter: (callbackfn, thisArg) => {
const output = Channel()
;(async () => {
for (;;) {
const value = await readOnly.shift()
if (value === undefined) {
await output.close()
break
} else if (callbackfn.call(thisArg, value)) {
await output.push(value)
}
}
})()
return output
},
forEach: async (callbackfn, thisArg) => {
for (;;) {
const value = await readOnly.shift()
if (value === undefined) {
break
} else {
await callbackfn.call(thisArg, value)
}
}
},
join: async (separator) => {
const elements = []
await readOnly.forEach((element) => {
elements.push(element)
})
return elements.join(separator)
},
map: (callbackfn, thisArg) => {
const output = Channel()
;(async () => {
await readOnly.forEach((value) =>
output.push(callbackfn.call(thisArg, value))
)
output.close()
})()
return output
},
readOnly: () => readOnly,
reduce: async (callbackfn, ...initialValue) => {
let previousValue = initialValue[0]
let previousValueDefined = initialValue.length > 0
await readOnly.forEach((currentValue) => {
if (previousValueDefined) {
previousValue = callbackfn(previousValue, currentValue, 0, readOnly)
} else {
previousValue = currentValue
previousValueDefined = true
}
})
return previousValue
},
shift: function () {
const {order, promise} = Order(this)
shifts.push(order)
setImmediate(processOrders)
return Object.freeze(promise)
},
slice: (start, end = Infinity) => {
const output = Channel()
;(async () => {
for (let index = 0; index < end; index++) {
const value = await readOnly.shift()
if (value === undefined) {
break
} else if (index >= start) {
await output.push(value)
}
}
await output.close()
})()
return output
},
get value () {
return lastValue
}
})
const writeOnly = Object.freeze({
close: () =>
new Promise((resolve, reject) => {
if (closed) {
reject(new Error(`Can't close an already-closed channel.`))
} else {
closed = true
processOrders()
// Give remaining orders in flight time to resolve before returning.
setImmediate(resolve)
}
}),
push: function (value) {
const {order, promise} = Order(this)
order.value = value
if (closed) {
order.reject(new Error(`Can't push to closed channel.`))
} else if (value === undefined) {
order.reject(new TypeError(
`Can't push 'undefined' to channel, use close instead.`
))
} else {
pushes.push(order)
setImmediate(processOrders)
}
return Object.freeze(promise)
},
writeOnly: () => writeOnly
})
// Use Object.create because readOnly has a getter.
return Object.freeze(Object.assign(Object.create(readOnly), writeOnly))
}
Channel.from = (values) => {
const channel = Channel()
;(async () => {
try {
// iterable
for (let value of values) {
await channel.push(value)
}
await channel.close()
} catch (exception) {
// Assume it's a Node.js stream.readable.
values.on('readable', async () => {
while (true) {
const data = values.read()
if (data === null) {
break
} else {
await channel.push(data)
}
}
})
values.once('end', channel.close)
}
})()
return channel
}
Channel.of = (...values) =>
Channel.from(values)
Channel.select = (...methodPromises) => {
const promise = new Promise((resolve, reject) => {
methodPromises.forEach(async (promise) => {
promise.prethen(() => {
// We've been given a heads-up that this method will complete first so
// cancel the other method calls.
methodPromises.forEach((other) => {
if (other !== promise) {
other.cancel()
}
})
})
try {
await promise
} catch (exception) {
reject(exception)
}
resolve(promise.channel)
})
})
return promise
}
module.exports = Object.freeze(Channel)

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{
"name": "@nodeguy/channel",
"description": "a Channel type for Go-like concurrency",
"version": "0.0.0",
"author": {
"name": "David Braun",
"email": "David@NodeGuy.com",
"url": "https://www.NodeGuy.com/"
},
"bugs": {
"url": "https://github.com/NodeGuy/channel/issues"
},
"dependencies": {},
"devDependencies": {
"@nodeguy/assert": "0.1.1",
"husky": "0.14.3",
"mocha": "3.5.3",
"standard": "10.0.3"
},
"homepage": "https://github.com/NodeGuy/channel",
"keywords": [
"CSP",
"channel",
"concurrency"
],
"license": "Apache-2.0",
"main": "lib/index.js",
"repository": {
"type": "git",
"url": "git+https://github.com/NodeGuy/channel.git"
},
"scripts": {
"precommit": "standard",
"pretest": "standard",
"test": "mocha --recursive"
},
"standard": {
"globals": [
"after",
"afterEach",
"before",
"beforeEach",
"describe",
"it",
"run"
]
}
}

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The tests in this directory are translated from [Go's tests](https://github.com/golang/go/tree/bad5abf64d76f9c302c084c5f62e6f70920d3c81/test/chan).

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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Test the situation in which two cases of a select can
// both end up running. See http://codereview.appspot.com/180068.
'use strict'
const Channel = require('../../lib')
it(`doubleselect`, async function () {
this.timeout(10 * 1000)
const iterations = 100000 // number of iterations
// sender sends a counter to one of four different channels. If two cases both
// end up running in the same iteration, the same value will be sent to two
// different channels.
const sender = async (n, c1, c2, c3, c4) => {
for (let i = 0; i < n; i++) {
await Channel.select(c1.push(i), c2.push(i), c3.push(i), c4.push(i))
}
c1.close()
c2.close()
c3.close()
c4.close()
}
// mux receives the values from sender and forwards them onto another channel.
// It would be simpler to just have sender's four cases all be the same
// channel, but this doesn't actually trigger the bug.
const mux = async (output, input, done) => {
await input.forEach(async (value) => {
await output.push(value)
})
await done.push(true)
}
// recver gets a steam of values from the four mux's and checks for
// duplicates.
const recver = (input) => {
const seen = new Map()
input.forEach((v) => {
if (seen.has(v)) {
throw new Error(`got duplicate value: ${v}`)
}
seen.set(v, true)
})
}
const c1 = Channel()
const c2 = Channel()
const c3 = Channel()
const c4 = Channel()
const done = Channel()
const cmux = Channel()
sender(iterations, c1, c2, c3, c4)
mux(cmux, c1, done)
mux(cmux, c2, done)
mux(cmux, c3, done)
mux(cmux, c4, done)
// We keep the recver because it might catch more bugs in the future.
// However, the result of the bug linked to at the top is that we'll
// end up panicking with: "throw: bad g->status in ready".
recver(cmux)
await done.shift()
await done.shift()
await done.shift()
await done.shift()
cmux.close()
})

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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Test that unbuffered channels act as pure fifos.
'use strict'
const Channel = require('../../lib')
it(`fifo`, function () {
const N = 10
const AsynchFifo = async () => {
const ch = Channel(10)
for (let i = 0; i < N; i++) {
await ch.push(i)
}
for (let i = 0; i < N; i++) {
if (await ch.shift() !== i) {
throw new Error(`bad receive`)
}
}
}
const Chain = async (ch, val, input, output) => {
await input.shift()
if (await ch.shift() !== val) {
throw new Error(val)
}
await output.push(1)
}
// thread together a daisy chain to read the elements in sequence
const SynchFifo = async () => {
const ch = Channel()
let input = Channel()
let start = input
for (let i = 0; i < N; i++) {
const output = Channel()
Chain(ch, i, input, output)
input = output
}
await start.push(0)
for (let i = 0; i < N; i++) {
await ch.push(i)
}
await input.shift()
}
AsynchFifo()
SynchFifo()
})

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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Torture test for goroutines.
// Make a lot of goroutines, threaded together, and tear them down cleanly.
'use strict'
const Channel = require('../../lib')
it(`goroutines`, async function () {
const f = async (left, right) => {
await left.push(await right.shift())
}
const n = 10000
const leftmost = Channel()
let right = leftmost
let left = leftmost
for (let i = 0; i < n; i++) {
right = Channel()
f(left, right)
left = right
}
;(async (c) => {
await c.push(1)
})(right)
await leftmost.shift()
})

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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Test simple select.
'use strict'
const Channel = require('../../lib')
it(`select`, async function () {
const closed = Channel()
closed.close()
let counter = 0
let shift = 0
const GetValue = () => {
counter++
return 1 << shift
}
const Send = async (a, b) => {
let done = false
let i = 0
do {
switch (await Channel.select(a.push(GetValue()), b.push(GetValue()),
closed.shift())) {
case a:
i++
a = Channel()
break
case b:
i++
b = Channel()
break
default:
done = true
}
shift++
} while (!done)
return i
}
let a = Channel(1)
let b = Channel(1)
let v = await Send(a, b)
if (v !== 2) {
throw new Error(`Send returned ${v} !== 2`)
}
const av = await a.shift()
const bv = await b.shift()
if ((av | bv) !== 3) {
throw new Error(`bad values ${av} ${bv}`)
}
v = await Send(a, Channel())
if (v !== 1) {
throw new Error(`Send returned ${v} !== 1`)
}
if (counter !== 10) {
throw new Error(`counter is ${counter} !== 10`)
}
})

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'use strict'
const assert = require('@nodeguy/assert')
const Channel = require('../lib')
const stream = require('stream')
const toArray = async (channel) => {
const array = []
await channel.forEach((item) => {
array.push(item)
})
return array
}
describe(`Channel`, function () {
it(`allows the use of new`, function () {
return new Channel()
})
it(`is frozen`, function () {
assert.throws(() => {
Channel.frozen = false
})
})
it(`creates a frozen object`, function () {
assert.throws(() => {
Channel().frozen = false
})
})
it(`creates a buffered channel`, async function () {
const channel = Channel(2)
;(async () => {
assert.equal(await channel.shift(), 0)
})()
await channel.push(0, 1, 2)
})
describe(`from`, function () {
it(`iterable`, async function () {
assert.deepEqual(await toArray(Channel.from([0, 1, 2])), [0, 1, 2])
})
it(`Node.js's stream.readOnly`, async function () {
const readOnly = stream.PassThrough({objectMode: true})
readOnly.write(0)
readOnly.write(1)
readOnly.end(2)
assert.deepEqual(await toArray(Channel.from(readOnly)), [0, 1, 2])
})
})
it(`of`, async function () {
assert.deepEqual(await toArray(Channel.of(0, 1, 2)), [0, 1, 2])
})
describe(`select`, function () {
it(`miscellaneous`, async function () {
const a = Channel()
const b = Channel()
;(async () => {
await b.push(0)
await a.push(1)
await a.shift()
})()
assert.equal(await Channel.select(a.shift(), b.shift()), b)
assert.equal(b.value, 0)
assert.equal(await a.shift(), 1)
assert.equal(await Channel.select(a.push(0), b.shift()), a)
})
})
it(`allows for non-blocking selects`, async function () {
const a = Channel()
const b = Channel()
const nonBlocking = Channel()
nonBlocking.close()
switch (await Channel.select(a.shift(), b.push(0), nonBlocking.shift())) {
case a:
assert(false)
break
case b:
assert(false)
break
default:
assert(true)
break
}
})
})
describe(`Channel object`, function () {
describe(`close`, function () {
it(`can't close an already closed channel`, function (done) {
const channel = Channel()
channel.close()
channel.close()
.then(() => {
done(new Error())
})
.catch((reason) => {
assert.deepEqual(
reason,
new Error(`Can't close an already-closed channel.`)
)
done()
})
})
it(`can't push to a closed channel`, async function () {
const channel = Channel()
channel.close()
return (async () => {
await channel.push(0)
})().then(() => {
assert(false)
}).catch((reason) => {
assert.deepEqual(reason, new Error(`Can't push to closed channel.`))
})
})
it(`returns 'undefined' immediately from shift`, async function () {
const channel = Channel()
channel.close()
assert.strictEqual(await channel.shift(), undefined)
})
})
it(`filter`, async function () {
assert.deepEqual(
await toArray(Channel.of(0, 1, 2, 3, 4, 5).filter(x => x % 2 !== 0)),
[1, 3, 5]
)
})
it(`forEach`, async function () {
const output = Channel()
;(async () => {
await Channel.of(0, 1, 2).forEach(output.push)
output.close()
})()
assert.deepEqual(await toArray(output), [0, 1, 2])
})
it(`join`, async function () {
assert.equal(await Channel.of(`a`, `b`, `c`).join(), `a,b,c`)
})
it(`map`, async function () {
assert.deepEqual(
await toArray(Channel.of(`a`, `b`, `c`).map(x => x.toUpperCase())),
[`A`, `B`, `C`]
)
})
describe(`push`, function () {
it(`with shift`, async function () {
const channel = Channel()
;(async () => {
await channel.push(0)
})()
assert.equal(await channel.shift(), 0)
})
describe(`undefined`, function () {
it(`outside select`, function () {
const channel = Channel()
return (async () => {
await channel.push(undefined)
})().then(() => {
assert(false)
}).catch((reason) => {
assert.deepEqual(
reason,
new TypeError(`Can't push 'undefined' to channel, use close instead.`)
)
})
})
it(`inside select`, function () {
const channel = Channel()
return (async () => {
await Channel.select(channel.push(undefined))
})().then(() => {
assert(false)
}).catch((reason) => {
assert.deepEqual(
reason,
new TypeError(`Can't push 'undefined' to channel, use close instead.`)
)
})
})
})
it(`returns a frozen promise`, function () {
assert.throws(() => {
Channel().push(0).frozen = false
})
})
})
it(`readOnly`, async function () {
const channel = Channel()
const readOnly = channel.readOnly()
assert.throws(() => {
readOnly.close()
})
assert.throws(() => {
readOnly.push(0)
})
assert.throws(() => {
readOnly.writeOnly()
})
;(async () => {
await channel.push(1)
})()
assert.equal(readOnly.readOnly(), readOnly)
assert.equal(await readOnly.shift(), 1)
assert.equal(readOnly.value, 1)
assert.throws(() => {
readOnly.frozen = false
})
})
it(`reduce`, async function () {
assert.equal(await Channel.of(0, 1, 2)
.reduce((previous, current) => previous + current),
3
)
assert.equal(await Channel.of(0, 1, 2)
.reduce((previous, current) => previous + current, 10),
13
)
})
describe(`shift`, function () {
it(`with push`, async function () {
const channel = Channel()
;(async () => {
await channel.push(0)
})()
assert.equal(await channel.shift(), 0)
})
it(`returns a frozen promise`, function () {
assert.throws(() => {
Channel().shift().frozen = false
})
})
})
describe(`slice`, function () {
it(`start`, async function () {
assert.deepEqual(await toArray(Channel.of(0, 1, 2).slice(1)), [1, 2])
})
it(`end`, async function () {
assert.deepEqual(
await toArray(Channel.of(0, 1, 2, 3, 4).slice(1, 4)),
[1, 2, 3]
)
})
})
it(`value`, async function () {
const channel = Channel()
;(async () => {
await channel.push(0)
})()
await channel.shift()
assert.equal(channel.value, 0)
assert.throws(() => {
channel.value = 1
})
channel.close()
await channel.shift()
assert.equal(channel.value, undefined)
})
describe(`writeOnly`, function () {
it(`provides only write methods`, async function () {
const channel = Channel()
const writeOnly = channel.writeOnly()
assert.throws(() => {
writeOnly.readOnly()
})
assert.throws(() => {
writeOnly.shift()
})
assert.equal(writeOnly.value, undefined)
;(async () => {
await channel.shift()
})()
await writeOnly.push(0)
writeOnly.close()
assert.throws(() => {
writeOnly.frozen = false
})
})
})
})

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module.exports = () => ({
files: ['lib/**/*.js'],
tests: ['test/*.js'],
env: {type: 'node'}
})