Why am I able to define same function twice in javascript? - javascript

I have a javascript file in which I write a bunch of jquery functions. I have a function to return the angular scope. I found out that if I were to write the same function twice, the code still executes.
function getngScope()
{
alert(2);
return angular.element($('#data-area')).scope();
}
function getngScope()
{
alert(1);
return angular.element($('#data-area')).scope();
}
When I call getngScope() I get "1" alerted and the scope returned. Why does it have this behavior?

The second definition of an Object overwrites the first one. In general the last definition of an object overwrites all previous definitions.
Functions in JavaScript are Objects:
(function () {}) instanceof Object === true
When you create a new global function f it's equivalent to creating a property in the window object and assigning the function definition to that variable, if you create a function:
function myFun() { console.log('my function') };
and then check the value of window.myFun you'll notice it is the same function as myFun:
window.myFun === myFun // true
You'll also notice that modifying window.myFun changes/overwrites myFun.
E.g.
function myFun() { console.log('myFun') };
myFun(); // Prints: 'myFun'
// Overwrite myFun
window.myFun = function () { console.log('NoFun') };
myFun(); // Prints: 'NoFun'
The second definition of the function takes precedence.
I recommend you read the chapter on Functions from JavaScript: the good parts by Crockford.

functions are data in memory stack, so when you define another function with the same name, it overrides the previous one.

Well obviously you’re not meant to define the same function twice. However, when you do, the latter definition is the only 1 that applies. Try not to do that. Find another way other than giving two functions the same name.

The second function replaced the first function,you could always change this by modifying the name of the function ,if not you can add multiple arguments ..if that is ever needed...and for the explaination to this behaviour,unlike other programming languages javascript doesnt return any errors while being executed..so u can assume that it just corrects itself during the execution by overwriting the function.

Related

About function in javascript

Sorry for the rather beginner question. What's the differences of function usage between
$(document).keypress(function() {
//some code
});
and
var somethingElse = function() {
//some code
};
I know the latter is to create a function similar to Java's
public void somethingElse() {
//some code
}
but I always assume the former as anonymous function that act inside a method. Can someone shed me some light regarding this? Thanks.
The first one is a jQuery shortcut to create an event listener.
It's equivalent to:
document.addEventListener('keypress', function() {
// some code
});
More info: http://www.w3schools.com/jsref/met_document_addeventlistener.asp
Now, about named or anonymous functions, what's the difference between
var doSomething = function() {
// some code
};
and this?
function doSomething() {
// some code
}
There's no difference for the developer. Of course there's a difference on memory, but in javascript developers don't have to take care of it.
Actually for the case of an event handler or other techniques that use callback functions, you can pass an anonymous function or a previously declared one, it's exactly the same:
$(document).keypress(doSomething);
or
$(document).keypress(function() {
// some code
});
creates an anon function and passes it to the handler
creates an anonymous function and a variable that references it.
creates a named function - that is hoisted
the hoisted function becomes available to you at any line within your function scope, while the non-hoisted function will be undefined until the line where it is declared runs.
there is no difference between #2 and #3 (other than the hoisting) - some people think that the first one creates an object and the 2nd one is some magical thing, or a global function, but nope - they are both function objects within your scope.
The former is a callback, meaning some instructions that will be executed ONLY as soon as the keypress event in your example is triggered.
Thus, a function's name is not required.
Function expressions, the latter, is mostly used when adding an object's property acting as a method like:
var myObject = {};
myObject.sayHello = function() {
alert("Hello");
}

JavaScript: invocation as a function/method

I am trying to sharpen my JavaScript skills and I am aware that there are four basic ways to invoke a function - which alter the way this is defined. The two I am interested in are the basic two:
Invocation as a function
Invocation as a method
Which is fine. The first, this will refer to the window object:
function doSomething() {
console.log(this);
}
doSomething(); // window is logged
And the second this will refer to the object it is being executed from within:
var t = {
doSomething: function () {
console.log(this);
}
};
t.doSomething(); // t is logged
Which is all fine. However, is it correct to say, that in these two invoking methods, this is always going to return the object the method is contained within (if that makes sense)?
In the first example, doSomething() is, in reality, defined within the window object - so is a property of the window object, even if we do not define it (or reference it).
Therefore, can it not be said that, in reality, invocation as a function is invocation as a method? Or not?
Imagine defining your function, not in the scope of the global object like you did, but in the scope of another function:
function outer() {
var inner = function(){console.log(this)};
inner();
};
outer();
What do you think the this will be bound to ? What will be printed ?
You might believe it will be the outer function object but its turns out that it will always be the global object.
It is just as if you had written:
function outer() {
var inner = function(){console.log(this)};
inner.call(window);
};
outer();
This is a quite hazardous and far from intuitive JS "feature".
So, to get back to your first example, what you are doing is basically this:
doSomething = function () {
console.log(this);
}
doSomething().call(window);
And, like Yoshi pointed out, if you were using the ES5's strict mode, you would end up doing something like that instead, which is, in my opinion, far less dangerous:
doSomething().call(undefined);
It also gives you a strong hint about the fact that you should never use this in a function ;)

Adding multiple event listeners in javascript to element

I've got a small drag and drop set up up and running, but it's using inline javascript and I'd prefer to move it all to an external file. Theoretically it's an easy swap, but I'm getting referenceErrors in my inspector and my minifcation is failing.
From what I can tell, the issue is coming from the return.
Original HTML
<section id="titles" ondrop="dropClip(this, event)" ondragenter="return false" ondragover="return false"></section>
Desired HTML
<section id="titles"></section>
JavaScript
var titles = document.getElementById('titles');
titles
.addEventListener('drop', dropClip(this, event))
.addEventListener('dragenter', return false)
.addEventListener('dragover', return false);
Javascript is a language where functions are "first-class objects". This means, possibly contrary to other programming languages that you may have experience in, that you can treat a function like any other object: you can pass it around, you can return it, you can have a function with a function inside it with a function inside it, and so on.
The consequence of this may be a very unexpected programming style required to be successful in Javascript. In other languages, UI event binding occurs in a variety of ways (such as C#'s Button1.Click += new System.EventHandler(this.myEventHandler);, or Classic VB's Button_Click()). In C#, you can pass around delegates, which are special objects with a specifically defined set of parameters and return values, to which a function can be bound. But all that disappears in javascript because you can simply attach a function to a property directly. For browser DOM event handling, the property is assumed to be a function reference, and during the native event handling code for that event, it calls the function attached to the property with the same name as the event.
Okay, you say, we have functions. And we can pass them around and treat them just like variables. Now what?
First, please take special note that the following two functions declarations are identical. They yield the exact same result, of declaring a function named myfun in the current scope (in a browser, the window object or the current function that is running).
function myfun(param1, param2) {
//do some stuff
};
var myfun = function (param1, param2) {
//do some stuff
};
(Actually, the first one will end up with a name property that the second one won't, and possibly some other minor differences, but for all practical intents and purposes they are identical).
The first one is just a shortcut for the second one. Basically, you create a function (that may or may not have a name) and assign it to a variable. Programmers use the convenient shortcut of calling the result "the myfun function", but in reality the case is "the myfun variable--which contains a particular function right now".
You can get many references to the same function--which is a true object--just by assigning it to other variables:
function myfun(param1, param2) {
//do some stuff
};
var a = myfun, b = myfun, c = myfun;
a(); // runs `myfun`
b(); // runs `myfun`
c(); // runs `myfun`
The next thing to notice is that to invoke a function, you must use parentheses after its name, and any parameters go inside the parentheses.
var result = myfun('a', 1); // invoke the `myfun` function
// and store its return value in variable `result`
But take a look back at our assignment statement making a, b, and c all be aliases of the myfun function: in those cases we didn't use parentheses, because--and here is where it gets really important, so pay attention:
To invoke a function (and get the function's return value), use parentheses after its name.
To pass a function reference, do not use parentheses.
What if we had done this instead:
var a = myfun(), b = myfun(), c = myfun();
a, b, and c would no longer be pointers to the myfun function. They would all be the result of myfun and would contain whatever myfun returned--or undefined if it didn't return anything. If you tried to invoke one of these, say a() you would get some error similar to:
> TypeError: a is not a function
Now that I've painted all that background, there is one simple thing to know that will get you on track to being successful with addEventListener: it expects a function as the second parameter. In your example code, you've put the contents of functions you'd like to run when addEventListener calls them, but no actual functions.
You can solve that by actually declaring the functions first, such as:
function doNothing() {
return false;
}
titles.addEventListener('dragenter', doNothing);
// Note: this is not invocation like `doNothing()`, but passing a reference
Or, you can simply wrap your function statements into an anonymous function. Remember, functions in javascript don't actually have names, we just have variables that contain functions. An anonymous function is one that has no name at all, and it is invoked either by an implicit name assignment (such as being passed as a parameter--where it will have the parameter's name) or by being directly invoked by doing the magic invocation action of putting parentheses after it.
That would look something like this:
titles.addEventListener('dragenter', function () { // anonymous function
return false;
});
If you want to invoke an anonymous function, you do have to let javascript know you want to treat it like a value as opposed to the normal shortcut-method of creating a named function in the current scope (where function myfun is treated like var myfun = function). That is done by wrapping the entire thing in one more set of parentheses, like this:
(function () { // begin a function value containing an anonymous function
// do something
}()); //invoke it, and close the function value
I hope this helps you understand more about javascript, why your code was not working, and what you need to do to make it work.
For sure you have to wrap the function contents you give to addEventListener into a function.
For instance instead of .addEventListener('drop', dropClip(this, event)) you will write .addEventListener('drop', function(event) { dropClip(this, event); })

How do I read Javascript Closure Syntax?

Based on some code in a lecture by Doug Crockford, I've created this.
var isAlphaUser = (function() {
alert("Forming Alpha User List");
let AlphaUsers = {
1234: true,
5678: true
};
return function(id){
alert("Checking Alpha Users:",id);
return AlphaUsers[id];};
}());
alert("starting");
alert(isAlphaUser(1234));
alert(isAlphaUser(5678));
alert(isAlphaUser(3456));
which gives me this:
Forming Alpha User List
starting
Checking Alpha Users: 1234
true
Checking Alpha Users: 5678
true
Checking Alpha Users: 3456
undefined
Which is quite cool, as it does the expensive setup once only, and every further call is a cheap check.
However, I can't decipher the code that does this. Specifically, I can't understand why I need the "()" at the end of the function declaration.
Can somebody explain how this syntax is working?
() calls a function. function() { } defines a function. Appending () right after immediately calls it1, and the result (also an anonymous function) is assigned to isAlphaUser.
The function() { ... }() pattern is frequently used to isolate variables to an inner scope, so those variables don't become part of the global scope.
In this case, this is what happens:
An anonymous function is run, defining a variable AlphaUsers inside that scope.
That function returns another function that takes 1 parameter. This function is a closure to which the AlphaUsers variable becomes bound (in other words, available). This function checks if the parameter passed in is contained in AlphaUsers (actually, it returns the item at that index, which is just a boolean).
The return value is assigned to a variable isAlphaUser.
Since isAlphaUser is now a function, it can be called to see if the parameter is contained in the AlphaUsers variable, but no direct access to AlphaUsers is available in the global scope (it become a sort of private variable).
1 — Note: As cwolves mentioned in the comments, beware that while () appended directly after the } works in this case, it is only because in this case the function definition is a function expression. If function is the first word on the line, the line becomes a function declaration, and that is all that line can do, the function is not anonymous (it will require a name, otherwise it's a syntax error) and cannot be called immediately inline. See Function Declarations vs. Function Expressions for more info.
The () at the end of the code is separate from the closure issue. By wrapping your function in parens and adding the () at the end you are creating an anonymous function that is run immediately with whatever arguments you pass into ().
Specifically, I can't understand why I need the "()" at the end of the
function declaration.
It creates self-invoking function, in other words, the function is executed as soon as it is parsed.
It is basically same thing when you call a function by suffixing it with () like:
myfunc(); // call this func
The top-level anonymous function returns the function that the isAlphasUser varaible refers to.
You need to call the top-level function, to get the inner-function reference.
Think of it like this, the outer anonymous function is a function factory, i.e., it returns a function.
In order to use any function (even one that returns a function) you must call it.

Javascript closure

The following program returns "local" and, according to the tutorial Im reading, it is designed to demonstrate the phenomenon ofclosure`
What I don`t understand is why, at the end, in order to call parentfunction, it assigns it to the variable "child" and then calls "child."
Why doesn`t it work by just writing parentFunction(); at the end?
var variable = "top-level";
function parentFunction() {
var variable = "local";
function childFunction() {
print(variable);
}
return childFunction;
}
var child = parentFunction();
child();
parentFunction() returns another function which you assign to var child. Then, you call child() to invoke the function returned by the call to parentFunction().
Running just parentFunction(); at the end wouldn't do anything useful because you would just discard its return value which is a function. But this would work:
parentFunction()();
See this fiddle: http://jsfiddle.net/USCjn/
Update: A simpler example:
function outer() { // outer function returns a function
return function() {
alert('inner function called');
}
}
x = outer(); // what is now in x? the inner function
// this is the same as saying:
// x = function() {
// alert('inner function called');
// }
x(); // now the inner function is called
See this fiddle: http://jsfiddle.net/bBqPY/
Functions in JavaScript can return functions (that can return functions (that can return functions ...)). If you have a function that returns another function then it means that when you call the outer function what you get is the inner function but it is not called yet. You have to call the value that you got as a function to actually run the body of the inner function. So:
x = f();
means - run a function f and store what it returns (which may be a string, a number, an object, an array, or a function) in x. But this:
x = f()();
means - run a function f, expect it to return a function and run that returned function as well (the second parentheses) and store in x what the returned function returned.
The function f here is a higher order function because it returns another function. Functions can also take another functions as arguments. One of the most powerful ideas of functional programming languages in general and JavaScript in particular is that functions are just normal values like arrays or numbers that can be returned and passed around.
You have to first grasp the idea of higher order functions to understand closures and the event system in JavaScript.
2016 Update
Note that currently this:
function outer() {
return function() {
alert('inner function called');
}
}
can be written as:
let outer = () => () => alert('inner function called');
using the ES6 arrow function syntax.
The amazing part about closures is that an inner function (in this case, childFunction) can refer to variables outside of its scope (in this case, variable). parentFunction doesn't return the result of childFunction, but an actual reference to the function!
This means that when you do the following...
var child = parentFunction();
...now, child has a reference to childFunction, and childFunction still has access to any variables it had when the function was created, even if they no longer exist.
In order to have parentFunction call childFunction, you'd need to change your code as follows:
From...
return childFunction;
To:
return childFunction();
Douglas Crockford (Pioneer of JSON, among other things) has a whole article devoted to closures, and scoping in javascript, and it would be well worth it to check out his other articles on javascript.
The point that is being demonstrated is that the function that was returned and assigned to child is still referencing the variable that was declared inside parentFunction instead of the one that was declared outside where child() is being invoked.
The only way to create a variable scope in javascript is in a function body. Normally the variable inside the parentFunction would have been discarded after the function returned.
But because you declared a function inside parentFunction that referenced variable in that scope and passed it out of parentFunction, the variable in the parentFunction is retained via the reference made in the new function.
This protects variable from outside manipulation except by functions that closed around it inside parentFunction.

Categories