Promisify based solution for lab 8.2 .

krave

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

Solution 1:

'use strict'
const { promisify } = require('util')

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents)
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

const opC = (cb) => {
  setTimeout(() => {
    cb(null, 'C')
  }, 125)
}

// solution 1
const printHelper = (contents) => print(null, contents)

const pOpA = promisify(opA)
const pOpB = promisify(opB)
const pOpC = promisify(opC)
pOpA()
  .then(printHelper)
  .then(() => pOpB())
  .then(printHelper)
  .then(() => pOpC())
  .then(printHelper)

Solution 2:

'use strict'
const { promisify } = require('util')

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents)
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

const opC = (cb) => {
  setTimeout(() => {
    cb(null, 'C')
  }, 125)
}

// solution 2

const pOpA = promisify(opA)
const pOpB = promisify(opB)
const pOpC = promisify(opC)

const operate = async () => {
  print(null, await pOpA())
  print(null, await pOpB())
  print(null, await pOpC())
}

operate()

ghortat

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)

krave

@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.

nosvalds

@krave said:

@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.

You actually don't need to abstract the print function

const { promisify } = require("util");

const print = (err, contents) => {
  if (err) console.error(err);
  else console.log(contents);
};

const opA = (cb) => {
  setTimeout(() => {
    cb(null, "A");
  }, 500);
};

const opB = (cb) => {
  setTimeout(() => {
    cb(null, "B");
  }, 250);
};

const opC = (cb) => {
  setTimeout(() => {
    cb(null, "C");
  }, 125);
};

const opAProm = promisify(opA);
const opBProm = promisify(opB);
const opCProm = promisify(opC);

opAProm()
  .then(print)
  .then(() => opBProm())
  .then(print)
  .then(() => opCProm())
  .then(print);

krave

@nosvalds said:

@krave said:

@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.

You actually don't need to abstract the print function

const { promisify } = require("util");

const print = (err, contents) => {
  if (err) console.error(err);
  else console.log(contents);
};

const opA = (cb) => {
  setTimeout(() => {
    cb(null, "A");
  }, 500);
};

const opB = (cb) => {
  setTimeout(() => {
    cb(null, "B");
  }, 250);
};

const opC = (cb) => {
  setTimeout(() => {
    cb(null, "C");
  }, 125);
};

const opAProm = promisify(opA);
const opBProm = promisify(opB);
const opCProm = promisify(opC);

opAProm()
  .then(print)
  .then(() => opBProm())
  .then(print)
  .then(() => opCProm())
  .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.

davidmarkclements

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

krave

@davidmarkclements Thank you for your advices.