Function with parameter called from object

Question

I am new to javascript and I'm trying some different things. Take this following example:

function obj(){
    this.execute = function(codeToExecute){
        codeToExecute();
    }
}

// Object init and function call

var obj = obj();
obj.execute(function(){
    alert("G'Day!");
}

This will execute the alert message. All good until now but now I'm trying to alert a message transmitted through a parameter:

var obj = obj();
obj.execute(function(message){
    alert(message);
}

What should be the structure of the function obj() now that I have to insert that parameter somewhere?

I couldn't find anything useful on google because honestly I don't know exactly what I should be looking for. Thank you!

Alnitak · Accepted Answer · 2012-05-17 09:24:17Z

4

You can extend execute so that any additional parameters are passed to the supplied function:

function Obj() {
    this.execute = function(f) {
        var args = [].slice.call(arguments, 1);
        f.apply(this, args);
    }
}

var obj = new Obj();
obj.execute(function(message){
    alert(message);
}, "boo!");

This line is the "magic" one:

var args = [].slice.call(arguments, 1);

It uses the Array.prototype.slice function which is used to copy arrays, but (kind of) tricks the function into using the arguments pseudo-array as the source array (instead of the supplied []), copying all of the elements apart from the first.

You can't just use arguments.slice(1) because arguments isn't a real JS array. It has a .length property, and you can access arguments[n], but it doesn't have all of the extra functions in its prototype that a real array has. It's close enough though that the implementation of .slice() doesn't know any better.

NB: you should use new to create an object instance - in your original code you're just calling obj() immediately and then reassigning the (undefined) result back to obj - that code could never have worked at all.

edited May 17, 2012 at 9:24

answered May 17, 2012 at 8:20

Alnitak

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3 Comments

Ja͢ck Over a year ago

+1 though the OP might need an example of how to call it from his code :)

Nemoden Over a year ago

Deleted my answer. This is the answer.

TGM Over a year ago

@Alnitak Thank you! It works. Could you please explain [].slice.call(arguments, 1)? I found some details about call() and apply() but nothing about []. Thanks again!.

Steve Rowe · Accepted Answer · 2012-05-17 08:28:55Z

2

You can do it like this:

function obj(){
 this.execute = function(codeToExecute, arg){
     codeToExecute(arg);
   }
} 

var obj = new Obj(); 
obj.execute(function(message){
               alert(message);},
           "yourmessage");

Essentially, you pass 2 arguments to your obj.execute: the function, and the argument to that function.

answered May 17, 2012 at 8:28

Steve Rowe

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2 Comments

Steve Rowe Over a year ago

For someone new to JavaScript, this might be a little easier to understand. Using apply and using [] to get the argument array is a little more advanced, even though it is more general purpose.

Decrypter Over a year ago

Someone new to js should not get into these bad habits. Make maintenance difficult

Decrypter · Accepted Answer · 2012-05-17 08:38:52Z

1

You could use the good old set/get methods. Below I'm passing in the message into the object constructor and it can be accessed with the getMessage function. You can include a set function if you don't want to pass into the constructor

function Obj(message){
this.getMessage = function(){
return message;
}
    this.execute = function(codeToExecute){
        codeToExecute();
    }
}

var obj = new Obj("hello");
obj.execute(function(){
    alert(obj.getMessage());
});

answered May 17, 2012 at 8:38

Decrypter

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1 Comment

Alnitak Over a year ago

that's really horrible. It makes the function being passed unnecessarily dependent on Obj.

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