NodeJS: the beautiful callback err machine

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One of the best things about NodeJS callbacks is their consistency. As in, most API calls require one and their method signature always follows the same pattern, and this is the most beautiful bit.

The first parameter of a callback is always an error object (which is null if no error occurred). This seems a little counterintuative at first. When you stop and think about it though, it really isn’t at all. My initial though was that most of the time, it isn’t going to be used, so why isn’t the result of the computation the first parameter? Well, that might be nice, but chances are you’re actually going to have to check to see if an error occurred first, before you start using your results, since if a error did occur, you probably don’t have any results anyway! Secondly, Node is cleverly reminding you that you should check for and deal with errors as they happen by ensuring you define a first parameter in order to define a second parameter to get at the stuff you want.

If the error was the second (or last) parameter the chances are you’d forget to define it, or the lazy would simply neglect to define it. This could happen because in JavaScript all parameters to functions are optional. Just because you do or don’t define parameters to a function, doesn’t mean you can’t call it with or without parameters. The function might not work in either of these cases, but nevertheless it is still possible to call it. It seems that by not coercing the programmer into defining an error parameter and dealing with it their code could become less robust.

The error parameter in the callback function is a necessity of asynchronous programming because errors cannot usually caught with a try/catch block as the execution of the callback function usually doesn’t happen in the block of code surrounded by the try/catch but instead in a later run of the event loop. I much prefer this way of dealing with errors as opposed to searching through my library code to find out if I even need to surround a function call in a try/catch by figuring out if it even does an operation that could possibly throw an error…and you know what, it’s future proof, because even if a function does no operations that could cause an error, it doesn’t mean that in a future version it won’t. By defining an error parameter from the start you can deal with a future mishap, now that is awesome.

21,276 thoughts on “NodeJS: the beautiful callback err machine”

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  35. TV dog trainer Cesar Millan is set to have the investigation into his training methods
    dropped despite footage showing a pet he was training attack a pig.

    Hundreds of people complained after an episode of Cesar 911 on Nat Geo WILD showed Millan let
    an aggressive dog called Simon off its leash and
    then  bite a nearby pig’s ear causing it to bleed.

    It was part of a technique the 46-year-old wanted to try out
    on the French bulldog terrier mix, by putting the dog with animal it
    doesn’t like in the hope the two can learn to get along.

    Scroll down for video 

    Cesar Millan, a TV dog trainer, who was under investigation for his techniques after a dog in his
    show Cesar 911 bit a pig’s ear. It is understood the probe into
    the incident is set to be dropped  

    After the episode was aired, it prompted the LA County Department of Animal Care and Control to launch an investigation into
    the video.

    However, now according to TMZ, the probe will be dropped as they believe the footage shows that Millan did not mean any
    harm to the pig when he let the dog off the leash and
    that he actually jumped into to save the attacked animal.

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    The case is set to be referred to the LA District
    Attorney next week but the animal investigators are believed to
    suggest that a prosecution is not necessary.

    When it emerged that Millan was to be investigated for his techniques in the episode he
    said that he would full co-operate with the probe.

    However he added: ‘I do have a large group of fans and a small group of people who don’t agree with me.

    ‘They are taking this the wrong way and blowing it
    way out of proportion.’

    Millan said when he returns from his speaking tour, he would cooperate fully with the investigation over the February 26 episode in which the
    dog named Simon is seen chasing the pig and nipping
    its ear, causing it to bleed.

    This image shows Simon the dog running towards a pig he later
    attacked during a training session with Millan. More than 11,000 have signed an online petition calling for Millan to
    be kicked off the air

    Nat Geo WILD and Millan insist the pig is fine.
    Another video clip was released to show Simon being rehabilitated.
    While Simon is reluctant to get pulled along
    by the pig at first (left), he soon keeps up (right)

    The complaint was started online and was based on footage from
    the show that sees the canine, who was attacking his owner’s pet
    pot-bellied pigs, biting into the pig’s ear in one clip.

    The complaint is misguided because it was a happy ending — the pig
    was fine and the dog was rehabilitated, Millan added.

    More than 11,000 people signed a Change.org petition to take
    Cesar 911 off the air, calling his methods inhumane and said he ‘used the pig as
    a bait for the dog all for ‘entertainment’ purposes. 

    Millan disagreed that he used the pig as bait to provoke the dog,
    and that Simon and the pig ‘became best friends’ and the dog was no longer aggressive
    towards pigs.

    In a follow-up segment, which was aired later in the episode, Simon is seen co-existing peacefully with
    a group of pigs, including the very pig he previously attacked, a chicken and other animals.

    The American Humane Society said it had received complaints about the episode, and called the incident ‘abuse’ in a statement.

    Millan said he understands why he is being investigated
    for animal cruelty but stands by his training methods.

    Millan is pictured here with Simon the dog during another training session

    Nat Geo WILD stepped in to defend Millan and said the pig
    was treated and ‘showed no lasting signs of distress.’

    In a statement released by Nat Geo WILD, the nature channel
    defended Millan, saying he ‘created a safe and controlled environment’ to rehabilitate ‘extreme’ cases
    of dog aggression.

    It also argued that because of Millan’s work, Simon did not have to be separated from his
    owner or euthanized.

    Nat Geo Wild stood by Millan and reassured viewers that the pig was fine by saying: ‘The pig that was nipped by Simon was tended to immediately afterwards, healed quickly and
    showed no lasting signs of distress.’

    In addition to the statement, another video clip was released,
    this time showing a successful training session with Simon and the pigs.

    In the clip, Millan introduces Simon into a pig pen with another dog,
    who acts as a positive influence as he happily interacts with
    the animals.

    Simon slowly follows suit, and the dog whisperer gets Simon close to the pigs without ‘having an intention to go after them’.

    Soon enough, Simon’s leash is attached to the very pig
    he had attacked. While Simon initially resists getting
    walked by the pig, he happily trots along in no time.

    Millan looks satisfied in the video as he says:
    ‘What they learn is to coexist with each other. Now Simon’s learned a different concept – you can follow
    a pig, you can be near a pig, you can eat with a pig, don’t eat the pig.’

    Last February, Millan was sued by a Florida nurse who claimed a dog
    named Gus attacked her despite being treated at Cesar’s center six days before.

    Millan found fame through his ‘Dog Whisperer’ TV
    show that has been broadcast worldwide and who has
    sold millions of books about his techniques.

     

    Cesar Millan

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