JavaScript: get to arguments object of callback

Question

Fiddle here

I'm looking at making some very high level logic into helper functions for curiosity's sake. I would like to be able to execute a function with its parameters in the _if function without having to define something like _callback ahead of time? I feel like I'm missing something here.

var _if = function(predicate,callback){
    if(predicate){
        callback(); //callback(arguments) is not arguments for callback
    }
};
var text = 'some text';
_if(1 > 0,function(){
    console.log('hello world'); //hello world
});
_if(1 > 0,function(text){
    console.log(text); //undefined
});
//define callback for this situation
var _callback = function(x){
    console.log(x);
}
_if(1 > 0,function(){
    _callback(text); //some text
});

Community · Accepted Answer · 2017-05-23 11:56:56Z

1

Not sure what you want, but maybe this helps:

You can call your function like this too:

_if(1 > 0,_callback.bind(null,text));  //null is the value of this

_if(1,function(text){
    console.log(text); //this way not undefined
}.bind(null,text));

1) This works because of the logic of JavaScript. In further versions you can use your version too if you use let text = 'some text'; - ref

2) null is the this value for the function, (more info here) but I think you can pass this too if you need to use it in the function (null will be automatically replaced in non-strict mode to the global object. That's why you can use window.colsole.log inside the function. - ref)

edited May 23, 2017 at 11:56

CommunityBot

11 silver badge

answered Nov 14, 2014 at 15:31

user669677

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2 Comments

wootscootinboogie Over a year ago

i'm looking for a way to get at the arguments object of the callback function in the _if function, without having to define _callback beforehand. In the second call to _if you can really see what i'm trying to get at. that function prints undefined when i pass in parameters to the callback.

wootscootinboogie Over a year ago

thanks, that's what i was looking for. two questions: 1.) why does this work, 2.) why pass null instead of this? as the context?

Platinum Azure · Accepted Answer · 2014-11-14 15:54:43Z

1

Why not just take the arguments to the callback function as extra parameters on _if?

var _if = function (predicate, callback, context) {
    if (predicate) {
        var callbackArgs = [].slice.call(arguments, 3);
        callback.apply(context || null, callbackArgs);
    }
};

// Usage:
_if(true, console.log, console, "text");

edited Nov 14, 2014 at 15:54

answered Nov 14, 2014 at 15:47

Platinum Azure

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5 Comments

wootscootinboogie Over a year ago

because i knew that way would work and i was looking for reasons why i hadn't seen what i asked about :)

wootscootinboogie Over a year ago

why pass null at the context to apply?

Platinum Azure Over a year ago

You didn't specify you wanted a calling context as well. You could easily have a 3rd named argument for the context and change the slice argument to 3, and that would work. With some clever thinking about your argument list, you could probably even find a way to require only one function (which can deduce if a calling context was passed in or not).

Platinum Azure Over a year ago

Oops, never mind, now I see. It wasn't clear that your question was specifically about this. I'll modify my answer.

wootscootinboogie Over a year ago

i think i actually like this better than what i was going after. it reads nicely to have the parameters meant for the callback passed last.

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