Executing a function without a callback within an Async library

Question

Executing a function without a callback within an Async library

I am facing an issue with this particular example mentioned in the Async documentation:

async.map(['file1','file2','file3'], fs.stat, function(err, results) {
   // results is now an array of stats for each file
});

In this example, the fs.stat function is called for each element in the array using (item,callback), but I am confused about which callback function to use and where it is defined.

javascript fs async.js

Answer 1

Answer №1

To streamline your code, consider utilizing node's built-in util.promisify instead of relying on async.map and async.js:

const { promisify } = require('util')

const fs = require('fs')

const files = ['file1', 'file2', 'file3']

Promise.all(files.map(promisify(fs.stat)))
  .then(results => /* results is now an array of stats for each file */)
  .catch(err => /* err is the first error to occur */)

Promises have become the go-to method for handling concurrency in modern JavaScript environments. They can seamlessly replace node-style error-first callbacks, following the format of (err, res) => { ... }, which is commonly seen with async.map.

Promises offer a solution to the issues associated with "callback hell" and provide a cleaner approach to asynchronous programming. If you still prefer using node-style callbacks, the below example showcasing asyncMap and an async function demonstrates the orchestration of callbacks:

// Code example demonstrating asyncMap functionality

// Define a function that doubles a number after a delay
const delayedDouble = (x, callback) =>
  setTimeout
    ( () =>
        callback
          ( null
          , x * 2
          )
    , 1000
    )

// Implement asyncMap to handle async operations and callbacks
const asyncMap = (arr, func, cb) =>
{ const loop = (res, i) =>
    i >= arr.length
      ? cb(null, res)
      : func
          ( arr[i]
          , (err, x) =>
              err
                ? cb(err, null)
                : loop
                    ( [ ...res, x ]
                    , i + 1
                    )
          )
  return loop
     ( []
     , 0
     ) 
}
  
// Demonstrate the usage of asyncMap with a delayedDouble function
asyncMap
  ( [ 1, 2, 3 ]
  , delayedDouble
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  
console.log('please wait 3 seconds...')
// please wait 3 seconds...
// <3 seconds later>
// result [ 2, 4, 6 ]

In the event of a potential error, asyncMap handles error propagation effectively:

// Example showcasing error handling in asyncMap

const tenDividedBy = (x, callback) =>
  setTimeout
    ( () =>
        x === 0
          ? callback(Error('cannot divide 10 by zero'), null)
          : callback(null, 10 / x)
    , 1000
    )

asyncMap
  ( [ 1, 0, 6 ]
  , tenDividedBy
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  // error Error: cannot divide 10 by zero

When executed without errors, asyncMap returns the results as expected:

asyncMap
  ( [ 1, 2, 3, ]
  , tenDividedBy
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  // result [ 10, 5, 3.3333333333333335 ]

Transitioning to Promises offers a more streamlined approach, reducing callback nesting and providing automatic error handling:

// Refactored example using Promises for smoother asynchronous operations

// Implement asyncMap using Promises
const asyncMap = (arr, func) =>
{ const loop = (res, i) =>
    i >= arr.length
      ? Promise.resolve(res)
      : func(arr[i]).then(x => loop([...res, x], i + 1))
  return loop ([], 0)
}

// Function using Promises to handle division
const tenDividedBy = x =>
  x === 0
    ? Promise.reject(Error('cannot divide 10 by zero'))
    : Promise.resolve(10 / x)

// Demonstrate the usage of asyncMap with Promises
asyncMap([1, 2, 0], tenDividedBy)
  .then(res => console.log('result', res))
  .catch(err => console.error('error', err))
// Error: cannot divide 10 by zero

asyncMap([1, 2, 3], tenDividedBy)
  .then(res => console.log('result', res))
  .catch(err => console.error('error', err))
// result [ 10, 5, 3.3333 ]

While manual implementation is great for learning purposes, leveraging Promise.all eliminates the need for custom solutions like asyncMap and offers parallel processing for enhanced efficiency.

Answer 2

To streamline your code, consider utilizing node's built-in util.promisify instead of relying on async.map and async.js:

const { promisify } = require('util')

const fs = require('fs')

const files = ['file1', 'file2', 'file3']

Promise.all(files.map(promisify(fs.stat)))
  .then(results => /* results is now an array of stats for each file */)
  .catch(err => /* err is the first error to occur */)

Promises have become the go-to method for handling concurrency in modern JavaScript environments. They can seamlessly replace node-style error-first callbacks, following the format of (err, res) => { ... }, which is commonly seen with async.map.

Promises offer a solution to the issues associated with "callback hell" and provide a cleaner approach to asynchronous programming. If you still prefer using node-style callbacks, the below example showcasing asyncMap and an async function demonstrates the orchestration of callbacks:

// Code example demonstrating asyncMap functionality

// Define a function that doubles a number after a delay
const delayedDouble = (x, callback) =>
  setTimeout
    ( () =>
        callback
          ( null
          , x * 2
          )
    , 1000
    )

// Implement asyncMap to handle async operations and callbacks
const asyncMap = (arr, func, cb) =>
{ const loop = (res, i) =>
    i >= arr.length
      ? cb(null, res)
      : func
          ( arr[i]
          , (err, x) =>
              err
                ? cb(err, null)
                : loop
                    ( [ ...res, x ]
                    , i + 1
                    )
          )
  return loop
     ( []
     , 0
     ) 
}
  
// Demonstrate the usage of asyncMap with a delayedDouble function
asyncMap
  ( [ 1, 2, 3 ]
  , delayedDouble
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  
console.log('please wait 3 seconds...')
// please wait 3 seconds...
// <3 seconds later>
// result [ 2, 4, 6 ]

In the event of a potential error, asyncMap handles error propagation effectively:

// Example showcasing error handling in asyncMap

const tenDividedBy = (x, callback) =>
  setTimeout
    ( () =>
        x === 0
          ? callback(Error('cannot divide 10 by zero'), null)
          : callback(null, 10 / x)
    , 1000
    )

asyncMap
  ( [ 1, 0, 6 ]
  , tenDividedBy
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  // error Error: cannot divide 10 by zero

When executed without errors, asyncMap returns the results as expected:

asyncMap
  ( [ 1, 2, 3, ]
  , tenDividedBy
  , (err, res) =>
      err
        ? console.error('error', err)
        : console.log('result', res)
  )
  // result [ 10, 5, 3.3333333333333335 ]

Transitioning to Promises offers a more streamlined approach, reducing callback nesting and providing automatic error handling:

// Refactored example using Promises for smoother asynchronous operations

// Implement asyncMap using Promises
const asyncMap = (arr, func) =>
{ const loop = (res, i) =>
    i >= arr.length
      ? Promise.resolve(res)
      : func(arr[i]).then(x => loop([...res, x], i + 1))
  return loop ([], 0)
}

// Function using Promises to handle division
const tenDividedBy = x =>
  x === 0
    ? Promise.reject(Error('cannot divide 10 by zero'))
    : Promise.resolve(10 / x)

// Demonstrate the usage of asyncMap with Promises
asyncMap([1, 2, 0], tenDividedBy)
  .then(res => console.log('result', res))
  .catch(err => console.error('error', err))
// Error: cannot divide 10 by zero

asyncMap([1, 2, 3], tenDividedBy)
  .then(res => console.log('result', res))
  .catch(err => console.error('error', err))
// result [ 10, 5, 3.3333 ]

While manual implementation is great for learning purposes, leveraging Promise.all eliminates the need for custom solutions like asyncMap and offers parallel processing for enhanced efficiency.

Executing a function without a callback within an Async library

Answer №1

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