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.

23,171 thoughts on “NodeJS: the beautiful callback err machine”

  1. Jessie Marmite sits high on my list of everlasting loves. A poached egg doesn’t taste
    right without Marmite spread on buttery brown toast,
    and my offering to any late-night gathering in university halls was Marmite, crunchy peanut
    butter and grated cheddar cheese on toast. I’ll have it with marmalade, avocados, anything…
    It is the gift that keeps on giving.

    It’s so simple and quick to make. Sam likes the soup quite thick,
    but if you prefer it thinner you can use more stock or just add
    some boiling water when you come to blend it

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    13 shares

    My husband Sam introduced me to this soup with Marmite, a no-nonsense recipe passed down by his father
    Pete. It has saved many a miserable Saturday afternoon with a side
    of cheese and Marmite on toast. It’s so simple and quick to make.
    Sam likes the soup quite thick, but if you prefer it thinner you can use more stock or just add some
    boiling water when you come to blend it.

    Serves 6-8

    20g butter

    1 tbsp rapeseed oil or olive oil

    1 large white onion, roughly chopped

    1kg carrots, roughly chopped

    1 potato, about 150g,

    roughly cubed

    handful of fresh sage leaves

    1 tbsp Marmite

    1.5 litres-2 litres vegetable stock

    2 tbsp cream or crème fraîche (optional)

    ½ lemon

    salt and pepper

    Melt the butter in a large saucepan, add the oil and onion, cover with a
    lid and cook over a low-medium heat for 10-15 minutes
    until soft and translucent. Add the carrots and potato and cook for 10 minutes, stirring occasionally,
    then add the sage and Marmite and stir to coat the vegetables.

    Add the vegetable stock, put the lid back on and simmer for about
    15 minutes until the carrots and potatoes are soft.
    Add a good pinch of salt and grinding of pepper.

    Leave to cool slightly, then add the cream, if using, to give a more velvety texture.

    Use a hand blender to whiz the soup until smooth, then season to taste
    with a squeeze of lemon juice and some more pepper. Serve
    with Marmite and cheese on toast.

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  28. Kit Harington has bravely spoken about surviving alcoholism
    and depression in a candid interview with The Sunday Times this
    week.

    The actor, 34, who achieved worldwide fame
    with his role as Jon Snow on Game of Thrones, said he
    ‘went through some pretty horrible stuff’ in the years since the fantasy drama series
    ended in 2019.

    Kit, who has been sober for two-and-a-half
    years following a stint in rehab, revealed that while he is now
    ‘very happy’ and ‘content’, he once battled suicidal thoughts. 

    ‘I went through horrible stuff’: Game of Thrones’ Kit Harington revealed how he contemplated suicide amid a ‘traumatic’ battle with alcoholism and depression, in an interview with The Sunday Times this week.
    Pictured in April 2019

    ‘I went through some pretty horrible stuff,’ Christopher, who goes by the moniker Kit,
    told The Sunday Times, when describing his life after the end
    of Game of Thrones. 

    ‘Things that have happened to me since Thrones ended, and
    that were happening during Thrones, were of a pretty
    traumatic nature and they did include alcohol,’ he revealed.  

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    476 shares

    Kit went on to describe his experience of hitting rock bottom, that resulted in a stint in rehab
    at the Privé-Swiss retreat in Connecticut, costing £95,000 a month.

    When asked by the publication whether he ever contemplated suicide,
    Kit reportedly hesitated before saying: ‘The answer is yes.
    Yes of course. I went through periods of real depression where I wanted
    to do all sorts of things.’  

    Past battle: ‘Things that have happened to me since Thrones ended, and that were happening during Thrones,
    were of a pretty traumatic nature and they did include
    alcohol,’ the 34-year-old said of life after Game of Thrones.
    Pictured as Jon Snow in Game of Thrones 

    Kit said he hopes his comments will help others suffering, and admitted
    to being very ‘secretive’ about his addictions.

    ‘I will say about my addictions that I kept them very, very quiet and I was
    incredibly secretive and incredibly locked up with them,’ he confessed.  

    Now sober, Kit is relishing fatherhood and his marriage with
    fellow Game of Thrones actress, Rose Leslie, 34.

    The couple wed in 2018 and welcomed their first child, a boy, in February this year. 

    Kit gave a rare insight into fatherhood in a recent interview with Access Hollywood. 

    Mental health: When asked by the publication whether he ever contemplated suicide, Kit reportedly hesitated before saying: ‘The answer is yes.
    Yes of course. I went through periods of real depression where
    I wanted to do all sorts of things.’ Pictured in July 2021

    The star admitted he didn’t understand just how big the journey through fatherhood was until he embarked on it himself. 

    He said: ‘Everyone goes, “Look, it’s big what you’re about to go through.” You have no way
    of knowing that until it happens.’  

    ‘Every day, I wake up and I look after this little human and now we’re part of a unit together.

    ‘I think the thing that surprises me most about fatherhood is you are
    now a unit, the three of you, and that’s a whole new dynamic you need to find.’

    Kit added: ‘I pretty much find every day as he grows and changes, how does that change you?
    It’s a beautiful thing, it really is.’    

    First-time parents: Kit and his actress wife Rose Leslie, 34,
    confirmed the arrival of their baby boy in February but have not revealed his name.
    Pictured in April 2019

    Kit and Rose confirmed the arrival of their baby boy in February but have not revealed his name.

    The couple have largely stayed out of the public eye since
    announcing that Rose was pregnant via a photoshoot in September last
    year.

    They celebrated their two-year wedding anniversary last summer, after tying the knot in June 2018 at Rose’s aristocratic family’s spectacular 12th-century Wardhill Castle
    in Aberdeenshire.

    Kit will next be seen on screen in season two of his new series,
    Modern Love, premiering this week. 

    The show is a romantic comedy based on the weekly column of the
    same name published in The New York Times.

    Kit is seen in the trailer playing a hopeless romantic who falls for a girl he meets on the train, played by
    Lucy Boynton.

    Modern Love premieres on August 13 on Amazon Prime Video. 

    For confidential mental health support, please contact
    Samaritans on 116 123. For support regarding alcohol addiction and treatment,
    please contact UK Rehab on 02038 115 619. 

    New venture: Kit will next be seen on screen in season two of his new series, Modern Love,
    premiering on August 13 on Amazon Prime Video

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