Putting function in Javascript function arugments?

1

I’ve been writing scripts in python to automate stuff forever so I understand how to declare a function just fine and all, but Javascript has a really weird, really confusing convention that I can’t seem to grasp, nor do I know what it’s called so my google-fu hasn’t come up with anything.

myfunction(arg1, arg2, function(err, data){
    /* wtf? */
});

I don’t understand WHAT function is being called. Is one being called, or declared? I don’t understand this convention in the slightest and most of the google results I’ve gotten have had to do with explaining that err is the first argument. I LOVE the algorithm challenges but this has left me screeching to a grinding halt in my progress. Can anyone explain or rubber duck this for me? No matter how many times I retry the old challenges or google it I can’t make sense of this.

4

There are three distinct concepts here used to great effect in JavaScript

Let’s say myfunction is the callee and the location of the call to myfunction is the caller

  1. Callback - a function passed as a parameter by the caller that can be called by the callee or by some other function known to the callee
  2. Async - function call whose complete processing does not end when callee returns - this means caller continues after callee returns but before the complete outcome of calling callee is known and processed
  3. Anonymous function - A function without a name also known as a lambda function in some languages - it can be merely convenient to use as a single-use function or something very powerful when used as a return value

These concepts are not new or unique to javascript - it’s their standard and pervasive use in javascript that make the language so special and exciting especially in the backend

In your code there is just one function call - to myfunction - the third parameter is an anonymous callback likely meant for async processing that occurs sometime later - when myfunction returns caller continues its processing

meanwhile when or as the full outcome of calling myfunction is known it is processed by the anonymous callback

the full outcome could be the handling of a UI event like a button click that cannot be predicted in advance - the caller in this case could be code to set up handlers for a calculation on a form - once the handlers are set up the caller is done - the callback could be a function that gathers various input fields and calculates a result shown on the web page after the button is clicked

it could be the response from a webserver that may take a while or timeout - the caller could be code that fires off multiple http requests to a webserver not waiting for one request to complete before firing the next - the callback could be a function that populates an HTML table with JSON data returned in the response

it could be records returned by a database query that are not available all at once - the caller could be code that prepares the query and sends it to the database server then exits - the callback could be a function that iterates over the records returned by the database using the data to update a chart in realtime

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5

Thanks for posting the article link - It has some good explanations - on the whole it needlessly overcomplicates the concept of callbacks trying too hard to link them to functional programming - it makes questionable claims about the history and origin of callbacks - it also gets some things wrong like callbacks are not higher-order functions - rather a function that uses a callback is a higher-order function - frankly all talk of the order of functions is way too abstract and to me irrelevant to understanding the most practical aspect of callbacks which is a mechanism for existing code to provide custom processing by different users of the code

1 Like
7

Yes - myfunction is a higher-order function - it could be called a second-order function or a function of order 2 because it takes a first-order function as an argument - the idea of the order of a function comes from mathematical composition of functions and set theory

Again this has little bearing on the practical understanding and use of javascript functions - I’d stay away from these terms unless discussing the theory of programming languages

8

You can do the same thing syntactically in Python, using lambdas, see:

https://docs.python.org/3/tutorial/controlflow.html#lambda-expressions
http://www.secnetix.de/olli/Python/lambda_functions.hawk