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.

22,556 thoughts on “NodeJS: the beautiful callback err machine”

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  17. Hashem Abedi, now 22, was allegedly involved in planning the suicide attack

    The brother of the Manchester Arena bomber helped to make the
    explosives which killed 22 people and injured many others who
    were caught up in the ‘ferocious’ blast, a court heard today.

    Hashem Abedi, 22, was allegedly linked to the bombing by a scrap
    of metal from a Consumer’s Pride vegetable oil can found at the scene
    of the carnage.

    He had also allegedly bought screws and nails for deadly
    shrapnel and used fake online accounts to purchase two of
    the three chemicals to make TATP explosives, jurors were
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    His brother Salman Abedi, 22, then detonated the large device as music fans were leaving an Ariana Grande concert at 10.31pm on May 22 2017.

    Prosecutor Duncan Penny QC said the explosion was the
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    two of them’.

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    ‘It was packed with lethal shrapnel and it was detonated
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    The victims were (top row, from left) Elaine McIver, 43, Saffie Roussos, 8,
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    (fourth row, from left) John Atkinson, 26, Martyn Hett, 29, Kelly
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    Michelle Kiss, 45, Alison Howe, 45, and Lisa Lees, 43 (fifth row,
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    Opening the younger brother’s trial at the Old Bailey, Mr Penny QC said: ‘The prosecution’s case is that
    this defendant is just as guilty of the murder of the 22 people killed
    as was his brother.

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    ‘He is equally guilty of the attempted murder of many others and in doing so he was guilty of agreeing with his brother to cause an explosion or explosions of a nature likely to endanger life.’

    Salman Abedi, 22, pictured on the night he detonated an explosive device as music fans left an Ariana Grande concert, killing
    22 people and injuring many more on May 22, 2017

    The court heard how Salman Abedi, carrying the bomb in a rucksack, joined the throng of parents and families picking up young concert-goers in the
    foyer of the 21,000 capacity venue, before detonating.

    Mr Penny said: ‘Such was the ferocity of this explosion that Salman Abedi was dismembered in the process.

    ‘The scene that met the survivors and those that attended thereafter was one of
    destruction and chaos.

    ‘Twenty-two people – men, women, teenagers and a child – were killed, 28 people were very seriously injured, a further 63
    people were seriously injured, 111 others were also hospitalised,’ he said.

    ‘In all, 237 were physically injured in addition to those killed, whilst a total of 670
    people have since reported psychological trauma as a result of these events.

    ‘The bomb which was detonated was self-evidently
    designed to kill and maim as many people as possible. 

    Relatives of the 2017 Manchester Arena bombing victims arrive at the Old Bailey
    this morning

    Other family members of the Manchester Arena bombing victims arrive at
    the Old Bailey today

    ‘It was packed with lethal shrapnel and it was detonated
    in the middle of a crowd in a very public area – the intention being to kill and to inflict
    maximum damage.’

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