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!

Answer 1

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.