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Promisify based solution for lab 8.2 .

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Posts: 58
in LFW211 Class Forum

I have tried a promisify based solution to lab 8.2 .
Share with you.
Any additional advice is welcome.

Solution 1:

  1. 'use strict'
  2. const { promisify } = require('util')
  3.  
  4. const print = (err, contents) => {
  5.   if (err) console.error(err)
  6.   else console.log(contents)
  7. }
  8.  
  9. const opA = (cb) => {
  10.   setTimeout(() => {
  11.     cb(null, 'A')
  12.   }, 500)
  13. }
  14.  
  15. const opB = (cb) => {
  16.   setTimeout(() => {
  17.     cb(null, 'B')
  18.   }, 250)
  19. }
  20.  
  21. const opC = (cb) => {
  22.   setTimeout(() => {
  23.     cb(null, 'C')
  24.   }, 125)
  25. }
  26.  
  27. // solution 1
  28. const printHelper = (contents) => print(null, contents)
  29.  
  30. const pOpA = promisify(opA)
  31. const pOpB = promisify(opB)
  32. const pOpC = promisify(opC)
  33. pOpA()
  34.   .then(printHelper)
  35.   .then(() => pOpB())
  36.   .then(printHelper)
  37.   .then(() => pOpC())
  38.   .then(printHelper)

Solution 2:

  1. 'use strict'
  2. const { promisify } = require('util')
  3.  
  4. const print = (err, contents) => {
  5.   if (err) console.error(err)
  6.   else console.log(contents)
  7. }
  8.  
  9. const opA = (cb) => {
  10.   setTimeout(() => {
  11.     cb(null, 'A')
  12.   }, 500)
  13. }
  14.  
  15. const opB = (cb) => {
  16.   setTimeout(() => {
  17.     cb(null, 'B')
  18.   }, 250)
  19. }
  20.  
  21. const opC = (cb) => {
  22.   setTimeout(() => {
  23.     cb(null, 'C')
  24.   }, 125)
  25. }
  26.  
  27. // solution 2
  28.  
  29. const pOpA = promisify(opA)
  30. const pOpB = promisify(opB)
  31. const pOpC = promisify(opC)
  32.  
  33. const operate = async () => {
  34.   print(null, await pOpA())
  35.   print(null, await pOpB())
  36.   print(null, await pOpC())
  37. }
  38.  
  39. operate()

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Comments

  • Posts: 3

    Hi,

    I followed the .then approach according to the training Promises (Cont.) chapter where we want to execute serial each function.

    const contentA = promisify(opA)
    const contentB = promisify(opB)
    const contentC = promisify(opC)

    contentA()
    .then((contents) => {
    print(contents)
    return contentB()
    })
    .then((contents) => {
    print (contents)
    return contentC()
    })
    .then(print)

  • Posts: 58

    @ghortat said:
    Hi,

    I followed the .then approach according to the training Promises (Cont.) chapter where we want to execute serial each function.

    const contentA = promisify(opA)
    const contentB = promisify(opB)
    const contentC = promisify(opC)

    contentA()
    .then((contents) => {
    print(contents)
    return contentB()
    })
    .then((contents) => {
    print (contents)
    return contentC()
    })
    .then(print)

    That looks similar as mine. I abstracted the print function into a printHelper to make the code compact.

  • Posts: 1

    @krave said:

    show previous quotes

    That looks similar as mine. I abstracted the print function into a printHelper to make the code compact.

    You actually don't need to abstract the print function

    1. const { promisify } = require("util");
    2.  
    3. const print = (err, contents) => {
    4.   if (err) console.error(err);
    5.   else console.log(contents);
    6. };
    7.  
    8. const opA = (cb) => {
    9.   setTimeout(() => {
    10.     cb(null, "A");
    11.   }, 500);
    12. };
    13.  
    14. const opB = (cb) => {
    15.   setTimeout(() => {
    16.     cb(null, "B");
    17.   }, 250);
    18. };
    19.  
    20. const opC = (cb) => {
    21.   setTimeout(() => {
    22.     cb(null, "C");
    23.   }, 125);
    24. };
    25.  
    26. const opAProm = promisify(opA);
    27. const opBProm = promisify(opB);
    28. const opCProm = promisify(opC);
    29.  
    30. opAProm()
    31.   .then(print)
    32.   .then(() => opBProm())
    33.   .then(print)
    34.   .then(() => opCProm())
    35.   .then(print);
  • Posts: 58

    @nosvalds said:

    show previous quotes

    You actually don't need to abstract the print function

    1. const { promisify } = require("util");
    2.  
    3. const print = (err, contents) => {
    4.   if (err) console.error(err);
    5.   else console.log(contents);
    6. };
    7.  
    8. const opA = (cb) => {
    9.   setTimeout(() => {
    10.     cb(null, "A");
    11.   }, 500);
    12. };
    13.  
    14. const opB = (cb) => {
    15.   setTimeout(() => {
    16.     cb(null, "B");
    17.   }, 250);
    18. };
    19.  
    20. const opC = (cb) => {
    21.   setTimeout(() => {
    22.     cb(null, "C");
    23.   }, 125);
    24. };
    25.  
    26. const opAProm = promisify(opA);
    27. const opBProm = promisify(opB);
    28. const opCProm = promisify(opC);
    29.  
    30. opAProm()
    31.   .then(print)
    32.   .then(() => opBProm())
    33.   .then(print)
    34.   .then(() => opCProm())
    35.   .then(print);

    Hi, nosvalds

    If you look close at the code and set a debugger in the print function. Run the code in debug mode and you will notice that the actual method which log out A, B, C is console.error.

    I believe the intention of this function is for whatever error occurs. Not the normal workflow. So I adjusted the handler.

  • Posts: 270

    Prefer async/await to .then and .catch because

    • it allows for async stack traces (stack traces over multiple event loop ticks that trace the awaiting of promises in async functions)
    • if you forget the .catch handler you end up with a unhandled rejection handler
    • less code noise with async await + less chance of deep nesting
  • Posts: 58

    @davidmarkclements Thank you for your advices.

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