foreach function in javascript

Question

Ok my questions comes from an example from a book that i'm trying to understand.Keep in mind i just got into javascript.

So we have the object set and we define the function foreach. It takes another function as a parameter and invokes it for every item of an array "values" that belongs to set.

set.foreach = function(f,c) {
    for(var i = 0; i < this.values.length; i++) 
        f.call(c,this.values[i]);
};

So far so good..

But i can't understand the usage of the foreach function in the second snipet.In particular i don't understand the role of the variable v. It is not defined enywhere else in the book and i m having a really hard time to understand how this work.We define another function in set to take the values as an array

set.toArray = function() {
    var a = [];
    this.foreach(function(v) { a.push(v); }); //where did v came from???
    return a;
}

Answer 1

set.foreach = function(f,c) {
    for(var i = 0; i < this.values.length; i++) 
        f.call(c,this.values[i]);
}; //                 ^-------------- being passed right here

The function you passed in is f, and f is invoked having the this value of its calling context set to c, and this.values[i] passed as the first argument.

       // ------v---------your function "f" in the forEach
this.foreach(function(v) { a.push(v); });
       // ------------^------references the first argument (after the "this" arg)
       //                      that was passed to "f"

---

Here's a simpler example:

This function accepts a function as a parameter. The only thing it does is call the function:

function my_func( fn ) {
    fn();
}

   // call my_func, which will call the function you give it
my_func( function() { alert( "hi" ); } );

Live Example: http://jsfiddle.net/6a54b/1/

...so passing the function to my_func will alert the string "hi". No surprise.

---

But what if my_func provided the value to be alerted?

function my_func( fn ) {
    fn( "message from my_func" );  // call the fn passed, giving it an argument
}  //            ^------------------------------------------------|
   //                                                             |
   //               v------references the arg passed by my_func---|
my_func( function( arg ) { alert( arg ); } );

Live Example: http://jsfiddle.net/6a54b/

Now you can see that an argument is being passed to the function we're sending over, and we reference that argument with the arg parameter.

It alerts whatever my_func gave it.

---

We can even take it one step further, by passing a second argument to my_func that my_func will simply take and pass it to the function we pass in.

function my_func( fn, str ) {
    fn( str );  // call the fn passed, giving it
}                                  //    the string we passed in

   //               v------the arg we passed here-----v
my_func( function( arg ) { alert( arg ); }, "I'm getting dizzy!" );

Live Example: http://jsfiddle.net/6a54b/2/

And you can see that both arguments are given to my_func, and my_func calls the function we passed in, passing it the string argument we gave it.

Answer 2

The variable vis an argument being passed to the function. It allows you to work with whatever the function receives. It is no different than namein the following example:

function sayHello(name) {
  console.log('Hello '+name);
}