makepromise

makepromise

makepromise can be used to get a Promise from a function with a callback. It will also make sure that the error stack starts at the line where the makepromise was called.

yarn add -E makepromise

Table Of Contents

• Table Of Contents
• API
  • async makePromise(fn: function(...args, cb), args: (*[]|*))
  • async makePromise(fn: function(...args, cb), args: (*[]|*), resolveValue: *)
  • Binding Methods
• Error Stack
• Copyright

API

The package exports a default makepromise function.

import makePromise from 'makepromise'

async makePromise(
  fn: function(...args, cb),
  args: (*[]|*),
): void

Create a promise from a function which accepts a callback as the last argument, and where the callback will be called with 2 arguments: error and result. The arguments must be passed either as an array, or a single value.

The example below shows how to use makePromise with an array of arguments (promisified truncate) and a single argument (promisified unlink). The context of the example is 2 methods from a lib to create a temp file, and read data from a file.

import { truncate, unlink, existsSync } from 'fs'
import makePromise from 'makepromise'
import { createTempFile, readFile } from './lib'

(async () => {
  try {
    // 0. SETUP: create a temp file.
    const path = await createTempFile('hello-world')
    const data = readFile(path)
    console.log('Created temp file %s', path)
    console.log('Exists: %s', existsSync(path))
    console.log('Content: "%s"', data)

    // 1. TRUNCATE to 5 characters.
    await makePromise(truncate, [path, 5])
    const data2 = await readFile(path)
    console.log('Content: "%s"', data2)

    // 2. ERASE the temp file.
    await makePromise(unlink, path)
    console.log('Exists: %s', existsSync(path))
  } catch (err) {
    console.log(err)
  }
})()

Created temp file example/temp.data
Exists: true
Content: "hello-world"
Content: "hello"
Exists: false

async makePromise(
  fn: function(...args, cb),
  args: (*[]|*),
  resolveValue: *,
): void

When resolveValue is passed as the last argument to the makePromise function, the returned promise will be forced to resolve with it.

import { unlink } from 'fs'
import makePromise from 'makepromise'
import { createTempFile } from './lib'

(async () => {
  try {
    // 0. SETUP: create a temp file.
    const path = await createTempFile()

    // 1. UNLINK and return the path to the temp file.
    const erasedPath = await makePromise(unlink, path, path)
    console.log('Erased: %s', erasedPath)
  } catch (err) {
    console.log(err)
  }
})()

Erased: example/temp.data

Binding Methods

Sometimes, it is important to bind methods of instances to their contexts, otherwise they will loose access to this.

For example. when closing a Writable stream, its close method must be bound to its instance.

import { createWriteStream, unlink } from 'fs'
import makePromise from 'makepromise'
import { createTempFile, readFile } from './lib'

(async () => {
  try {
    // 0. SETUP: create a temp file.
    const path = await createTempFile()

    // 1. CREATE a write stream, and end it with data.
    const ws = createWriteStream(path)
    await makePromise(ws.end.bind(ws), 'example-data')

    // 2. CHECK that data has been written.
    const data = await readFile(path)
    console.log('Read file: "%s"', data)

    // 3. TEAR-DOWN: remove file.
    await makePromise(unlink, path)
  } catch (err) {
    console.log(err)
  }
})()

Read file: "example-data"

Error Stack

This modules will make sure that errors are updated to include the stack trace of when makePromise was called, rather than have the Node's internal error stack or no stack at all.

import { unlink } from 'fs'
import makePromise from 'makepromise'

(async () => {
  try {
    await makePromise(unlink, 'error-test-file')
  } catch ({ stack }) {
    console.log(stack)
  }
})()

Error: ENOENT: no such file or directory, unlink 'error-test-file'
    at /Users/zavr/adc/makepromise/example/error-stack.js:6:11
    at Object.<anonymous> (/Users/zavr/adc/makepromise/example/error-stack.js:10:3)

Without this functionality, the error stack would not appear.

import { unlink } from 'fs'

(async () => {
  try {
    await new Promise((r, j) => {
      unlink('error-test-file', (err) => {
        if (err) return j(err)
        return r()
      })
    })
  } catch ({ stack }) {
    console.log(stack)
  }
})()

Error: ENOENT: no such file or directory, unlink 'error-test-file'

Copyright

(c) Art Deco 2019