javascript arguments anonymous function

Question

Could someone please explain function(tx) in the code snippet below, from this page: http://www.webkit.org/demos/sticky-notes/. Where and how is tx assigned? I have looked here for information but am still in the dark.

What I think I understand is that that the saveAsNew method of the object is being defined as an anonymous function that first creates a timestamp and creates a local reference to itself (note=this), and then invokes the transaction method of the db object, providing to that method a parameter which is yet another anonymous function that has an argument tx. But I don't understand where tx is coming from.

   .
   .
   .
      saveAsNew: function()
         {
             this.timestamp = new Date().getTime();        
             var note = this;
             db.transaction(function (tx)   
                 {
                 tx.executeSql("INSERT INTO WebKitStickyNotes (id, note, timestamp, left, top, zindex) VALUES (?, ?, ?, ?, ?,                   ?)", [note.id, note.text, note.timestamp, note.left, note.top, note.zIndex]);
                 }); 
    },
    .
    .
    .

FULL CODE PASTE

Answer 1

You could have used any variable, so long as in the your anonymous method definition you use the same variable. The transaction method will pass a value as the first parameter when calling your anonymous method and it will be assigned to tx.

Answer 2

If I knew what transaction() was I might be able to help you more.

But I believe it is something that function returns.

Like click has a event that is returned

So somewhere in the transaction code a variable is sent to the function which you can then name and use in your code.

---

From earlier on the code:

db = openDatabase("NoteTest", "1.0", "HTML5 Database API example", 200000);

You have to look into what openDatabase does that has the transaction function.

You can read up on openDatabase() here

Answer 3

The function saveAsNew returns a function that takes tx as an argument:

var new = saveAsNew();
new(tx);

as to where tx comes from, it's impossible to say from this snippet.

Answer 4

The db.transaction() function must take an argument that is a function that takes one parameter (function(tx) in your example).

It is probably documented as db.transaction(callback).

The parameter tx is sent to the callback function (anonymous function(tx) in your case) by the db.transaction() function.

Answer 5

First, tx is a parameter definition. You can choose any name, it could also be bar. It is not different from defining a function as

function foo(bar) {

}

If you wonder "who" is passing this argument, then it is probably db.transaction. You are passing the function as a callback ^[Wikipedia]. Somewhere db.transaction will call:

callback(transaction);

Maybe this example helps it to understand:

function hello(callback) {
    // do some very important world changing computations... then:
    callback('Hello ');
}

hello(function(foobar) {
    alert(foobar + 'Tim!');
});
// alerts 'Hello Tim!'

Here, the hello function passes one parameter to the callback function.

Answer 6

Your code is equivalent to this:

 saveAsNew: function()
     {
         this.timestamp = new Date().getTime();        
         var note = this;
         function booga(tx)
         {
             tx.executeSql("INSERT INTO WebKitStickyNotes (id, note, timestamp, left, top, zindex) VALUES (?, ?, ?, ?, ?,                   ?)", [note.id, note.text, note.timestamp, note.left, note.top, note.zIndex]);
         }
         db.transaction(booga);
},

Where does the "tx" come from in "booga(tx)"? Answer: Whoever calls booga will pass a parameter, and tx is the name we gave to that parameter.

Answer 7

The function call db.transaction() itself requires one parameter. That parameter is itself a function that db.transaction() will call and when it calls that function, it will pass it one parameter. The name tx can be anything, it's just a name for the first parameter to this function.

Your code could have been written with that function that you are passing to db.transaction() as an anonymous function like you did here:

db.transaction(function (tx)   
{
    tx.executeSql("INSERT INTO WebKitStickyNotes (id, note, timestamp, left, top, zindex) VALUES (?, ?, ?, ?, ?,                   ?)", [note.id, note.text, note.timestamp, note.left, note.top, note.zIndex]);
}); 

or it could have been written this way where it's a little more obvious what is happening, but not as compact:

function writeSql(tx) {
    tx.executeSql("INSERT INTO WebKitStickyNotes (id, note, timestamp, left, top, zindex) VALUES (?, ?, ?, ?, ?,                   ?)", [note.id, note.text, note.timestamp, note.left, note.top, note.zIndex]);
}

db.transaction(writeSql);