pass a callback function with some arguments already set

Question

I am trying to preload arguments into a callback function. Here's a stripped down example of what I'm attempting.

function doSomething(callback) {
  // code to obtain a 3rd argument, eg API call or access a file.
  // let's say we are getting the number 5.
  const c = 5;
  // invoke the callback with its final argument.
  callback(c);
}

function toBeCalled(arg1, arg2, arg3) {
  // do a calculation with these arguments.
  console.log((arg1 * arg3) + arg2);
}

// invoke doSomething with the first two arguments already defined.
// eg user input.
doSomething(toBeCalled(a, b));

what I want to happen:

doSomething(toBeCalled(4, 2));

console: 22

I've given the callback its first two arguments when I invoke doSomething. doSomething grabs the value of the third argument from somewhere and invokes the callback with the 3rd value added.

what actually happens:

As far as I'm aware the code above would invoke toBeCalled too early which is causing the error:

TypeError: callback is not a function

Thank you for any help!

Place a closure over the callback that contains the first arguments. That's the typical way of "partially applying" a function. I'm on my phone, so it would be difficult for me to write up an answer. — Carcigenicate, Apr 28 '17 at 11:40
You can work with promises - many libraries do support it (jQuery Ajax e.g): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise — Larce, Apr 28 '17 at 11:42
[What is 'Currying'?](http://stackoverflow.com/questions/36314/what-is-currying) — Andreas, Apr 28 '17 at 11:44
@Carcigenicate If I run `doSomething({toBeCalled(4,2)})` I get `Unexpected number`. Is that where you said to put the closure? — Nick Ramsbottom, Apr 28 '17 at 11:48
@Nick Closure was actually the wrong word to use. See my answer. — Carcigenicate, Apr 28 '17 at 11:48

Carcigenicate · Accepted Answer · 2017-04-28T17:41:59.953

1

Just wrap it in another function:

f = function (c) {
        toBeCalled(4, 2, c)
    }

Then pass f as the callback to receive the final argument:

doSomething(f)

edited Apr 28 '17 at 17:41

answered Apr 28 '17 at 11:45

Carcigenicate

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this did the trick, thank you for your help. I don't have enough rep to up vote you but will come back and do so when I do! – Nick Ramsbottom Apr 28 '17 at 11:51

pass a callback function with some arguments already set

1 Answers1