I have done a fair amount of research but have not been able to find any answers, to what seems like a simple question:
I want to associate a property with a function (to use as a static variable in this function), like so:
function foo() {
if (!foo.counter) {
foo.counter = 1;
}
else {
foo.counter++
}
// rest of the function code goes here...
}
If I change the name of the function later, I don't want to have to change references to it inside function definition.
So, is there a way to refer to the currently executing function? (other than arguments.callee which is now deprecated). A keyword like thisFunction?
If not, what is the reason for not having something like this?
I do not know of an identifier or keyword like thisFunction (besides arguments.callee) that JavaScript exposes, but one way to achieve the same effect is to name the function foo in a closure, and return foo from that closure. That way you can always use the name foo within foo to refer to itself, regardless of what variable it's assigned to by your program:
var fn = (function() {
function foo() {
if (!foo.counter) {
foo.counter = 1;
}
else {
foo.counter++
}
// rest of the function code goes here...
}
return foo;
})();
function foo() {
if (typeof foo.counter=="undefined") {
foo.counter = 1;
}
else {
foo.counter++
}
return foo;
}
var x = new foo();
alert(x.counter); //1
var y = new foo();
alert(x.counter); //2
If you don't need to refer to the counter outside of the function, this will work. Instead of setting a variable of the function itself, you define a variable outside of the function and let the function modify that instead. Every time you call makeFoo, it makes a new variable x and returns a function using x. Every time you call the returned function, it still refers to the same x created by makeFoo. Additionally, this entirely encapsulates the variable, ensuring that almost nothing outside of the returned function can change it (it definitely won't be changed by most things outside of the returned function, but there is enough funky javascript reflection that I can't guarantee that nothing will change it). Certainly, moving the function between variables won't affect the counter (ie x=makeFoo();y=x;x=null;).
function makeFoo() {
var x=0;
return function () {
x++;
return x;
}
}
//different counters
x=makeFoo();
y=makeFoo();
x();//returns 1
y();//returns 1
x();//returns 2
y();//returns 2
//same counter
x=makeFoo();
y=x;
x();//returns 1
y();//returns 2
x();//returns 3
y();//returns 4
Related
I'm trying to get a solid understanding of closures, and I'm struggling with the mechanics of it. I've looked on w3schools (https://www.w3schools.com/js/js_function_closures.asp) and MDN (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures) and a few other places. I understand what a closure is and how to make it work but I don't understand why subsequent calls after the first call to the outer function just seem to go straight to the inner one instead.
Here is my code - a simple, working closure:
var add = (function () {
var a = 0;
alert('hi');
function addInner(){
a += 1;
return a;
}
return addInner;
})();
function getAdd() {
document.getElementById("test").innerHTML = add();
}
I have two questions that probably both have the same answer: Why does a not get reset every time I call add()? And why does the alert not pop up except for on the first time?
I attached getAdd() to a button, and it works great, but it but doesn't pop up with the alert past the first time. Why does it do this?
Edit: I also found the first answer to this question (Why Don't Variables Reset in a Closure (Javascript)) really helpful.
add is a reference to addInner, not the anonymous "outer" function, since the "outer" function returns addInner. Then, you call that anonymous function once - that last set of ()- and store the resulting function, with its own private 'a', in add.
Imagine that the outer function was a named function called constructAdder, and you called
var add = constructAdder();
var add2 = constructAdder();
That's basically what you've done, but with an anonymous function, if that makes it any clearer. You have an outer function that constructs an inner function that can count, due to the magic of closures.
Those questions do both have the same answer. And it's fairly simple. This statement:
var val = (function() {
return 42
}())
sets val to 42. Not a function. This is called an IIFE, or immediately-invoked function expression. We are declaring a function and calling it immediately. We never save that function. We just use it once to get the value it returns:
val // 42
Contrast that to:
var val = function() {
return 42
}
In this case we set val to a function that, when called, returns 42.
val() // 42
val() // 42
We can get pretty crazy with IIFEs:
var val = (function() {
return (function() {
return (function() {
return 42
}())
}())
}())
The value returned by that mess is still 42.
val // 42
All those functions are declared once, used once, and thrown away. But IIFEs are capable of so much more:
var add = (function() {
var counter = 0
return function() {
return counter++
}
}())
We're now using the IIFE to create a private variable! counter cannot be accessed outside the scope of the IIFE. It's safe. But the inner function can access and modify it. It's a fully encapsulated, stateful function!
add() // 0
add() // 1
counter // Uncaught ReferenceError: counter is not defined
This is what your code is doing:
var add = (function () {
var a = 0;
alert('hi');
function addInner(){
a += 1;
return a;
}
return addInner;
})();
It is encapsulating the a variable so that it cannot be accessed outside the scope of the IIFE. The addInner function, however, does have access to a and can modify it at will.
The alert() is only called once because that IIFE is only called once, then thrown away. Note that the function itself is the only thing thrown away. Since addInner maintains a reference to the scope (closure) created by the IIFE, that scope is safe from garbage collection.
In fact, that's a helpful way to think about IIFEs:
var val = function() {}
creates a function and
val()
creates a closure (the context that the function's body runs in). IIFEs are used when we don't care about the function, we just want the closure it creates.
Hope this helps. Have fun out there!
var add = (function () {
var a = 0;
alert('hi');
function addInner(){
a += 1;
return a;
}
return addInner;
})(); <-- This
The () at the end means that add is added forever, and if you look at the line above, then it says return addInner. This means that add() is actually addInner(), the good thing with this is that addInner has access to a. Actually writing the function like this is more proper.
var add = (function () {
this.a = 0;
alert('hi');
var addInner = function(){
this.a += 1;
return this.a;
}.bind(this);
return addInner;
})();
Because now you would think that add.a would make a available because it is written as this.a, but since addInner is returned, then it is not available.
You should focus on the
(function(){
// impl
})()
block.
It is called an Immediately Invoked Function Expression or IIFE.MDN reference
So when the code executes to line 1, it first evaluate the right side and the IIFE returns a closure which was then assigned to the variable add. Meanwhile, an alert was invoked in the IIFE. Understanding the IIFE can help you solve the problems.
In my opinion , the key is IIFE, after the code below is executed at the first time,
var add = (function () {
var a = 0;
alert('hi');
function addInner(){
a += 1;
return a;
}
return addInner;
})();
(Look out ! There is no add() in the code above because IIFE will be Invoked Immediately and the return addInner finished the initialization of variable add )
the function add has been changed to :
add = function addInner() {
a += 1;
return a;
}
Of course, the alert() only executed one time because the add has been changed at the begining.
Why does a not get reset every time I call add()?
And why does the alert not pop up except for on the first time?
that is the answer of your question.
variable a didn't destroy because the function add still have a reference, and this is about the closure.
function emergency() {
var ambulance = 100;
var callAmbulance = function() { alert(ambulance); }
ambulance++;
return callAmbulance;
}
var accident = emergency();
accident(); // alerts 101
I am referring to the variable 'ambulance'.
When I call accident(); it should call emergency() which should use the declared variable 'ambulance' [considering the global scope thing in javascript, still it could set the value to global] but its using old value 101 instead of setting again back to 100 - behaving more like static var.
What's the Explanation?
What you have there is called a closure. It means a function can access variables declared in outer functions (or in the global scope). It retains access even after the 'parent' function has returned. What you need to understand is that you don't get a copy. The changes performed on that variable are visible to your inner function, which in theory means you could have multiple functions sharing access to the same variable.
function f () {
var x = 0;
function a() { x += 1; }
function b() { x += 2; }
function show() { console.log(x); }
return {a:a, b:b, show:show};
}
var funcs = f();
funcs.a();
funcs.b();
funcs.show(); // 3
One thing to be aware of is that a subsequent call to f will create a new scope. This means a new x variable will be created (new a, b, show functions will be created as well).
var newFuncs = f();
newFuncs.a();
newFuncs.show(); // 1
funcs.a();
funcs.show(); // 4
So, how do you get a copy? Create a new scope.
function g () {
var x = 0;
var a;
(function (myLocal) {
a = function () { myLocal += 1; }
}(x));
x += 200;
return a;
}
JS only has pass-by-value so when you call the anonymous function, the xvariable's value will be copied into the myLocal parameter. Since a will always use the myLocal variable, not x, you can be certain that changes performed on the x variable will not affect your a function.
If, by any chance, you're coming from a PHP background you are probably used to do something like
use (&$message)
to allow modifications to be reflected in your function. In JS, this is happening by default.
You are creating a function definition which is not compiled at this time:
var callAmbulance = function() { alert(ambulance); }
And before you are sending it to the called function, you are incrementing the num:
ambulance++;
And then you are sending it to the called function:
return callAmbulance;
But whether you are sending it there or not, it doesn't matter. The below statement executes or compiles the function:
var accident = emergency();
And this takes in the current ambulance value which is 101 after the increment. This is an expected behaviour in creating function but not executing it. Please let me know, if you didn't understand this behaviour. I will explain it more clearly.
En example can be found in Twitter'a typeahead.js here:
function () {
// ...
return this.each(initialize);
function initialize() {
// ...
}
}
Questions:
What are the scopes and what function sees what?
What is the reason for using such a construct (usage scenarios and advantages)?
Javascript has function based scope, which means that every thing defined inside a function is available from the first line, since the definition is "hoisted" by the complier.
That goes for both variable and function definitions - variable values however, are not available until after assignment.
You can read all about javascript scoping and hoisting here
This means that the function initialize is available from the first line of the wrapping anonymous function.
There is no real reason, and no advantages, for doing it that way, unless you count the code structure as an advantage.
Personally I don't see any reason to do this. For me even it looks a little bit weird. Martin is right. You should be careful, because the defined variables are not accessible like functions. For example this doesn't work:
var getValue = function(func) {
return func();
}
var f = function() {
return getValue(now);
var now = function() {
return 10;
}
}
alert(f());
However, this works:
var getValue = function(func) {
return func();
}
var f = function() {
return getValue(now);
function now() {
return 10;
}
}
alert(f());
Is there any way to refer to the function object that you're currently executing in? If it's not a method of any object or not called with .call() or .apply(), the this pointer is likely just window, not the function object.
I often use a design pattern like this for global variables that I want scoped to a particular function as this keeps them out of the top level namespace:
function generateRandom() {
if (!generateRandom.prevNums) {
generateRandom.prevNums = {}; // generateRandom.prevNums is a global variable
}
var random;
do {
random = Math.floor((Math.random() * (99999999 - 10000000 + 1)) + 10000000);
} while (generateRandom.prevNums[random])
generateRandom.prevNums[random] = true;
return(random.toString());
}
But, I'd rather not have to spell out the function name every time I want to use a variable scoped to that object. If the name of the function ever changes, there are then a lot of places to change the name.
Is there any way to get the currently executing function object?
Well, you could use arguments.callee()...
https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope/arguments/callee
From MDN:
Description
callee is a property of the arguments object. It can be used to refer
to the currently executing function inside the function body of that
function. This is for example useful when you don't know the name of
this function, which is for example the case with anonymous functions.
Note: You should avoid using arguments.callee() and just give every
function (expression) a name.
BUT...
What you really want are Javascript Prototypes.
function RandomSomethingGenerator()
{
this.prevNums = {};
}
RandomSomethingGenerator.prototype.generate = function() {
var random;
do {
random = Math.floor((Math.random() * (99999999 - 10000000 + 1)) + 10000000);
} while (this.prevNums[random])
this.prevNums[random] = true;
return(random.toString());
};
Why do I say this?
1.) You're dirtying the global space with all those functions.
2.) Even if you like Jani's suggestion, and you want a "static" function like you have now, my suggestion would be the same, but with a twist: Create your global function, and wrap an instance of an object (built from a prototype) inside the closure and make the call to it (so, basically, make yourself a singleton).
As in this (adapted from Jani's answer):
var randomSomething = (function() {
var randomSomethingGenerator = new RandomSomethingGenerator();
return function() {
randomSomethingGenerator.generate();
};
})();
I don't think there's any way to do exactly what you ask, but you could use a closure for your function-local static variables instead.
You can easily achieve this using an IIFE:
var generateRandom = (function() {
//any function's static variables go here
var prevNums = {};
return function() {
//function code goes here
var random;
do {
random = Math....
}
prevNums[random] = true;
return random.toString();
};
})();
You want arguments.callee. From MDN - callee:
callee is a property of the arguments object. It can be used to refer to the currently executing function inside the function body of that function. This is for example useful when you don't know the name of this function, which is for example the case with anonymous functions.
For example:
> foo = function() { console.log(arguments.callee); };
> bar = function() { foo() };
> bar();
function () { console.log(arguments.callee) }
However, I think this is being deprecated. The above link says, "The 5th edition of ECMAScript forbids use of arguments.callee() in strict mode."
Is there any way to break a closure easily in JavaScript? The closest I have gotten is this:
var src = 3;
function foo () {
return function () {
return src; }
}
function bar (func) {
var src = 9;
return eval('('+func.toString()+')')(); // This line
}
alert(bar(foo()));
This prints '9', instead of '3', as a closure would dictate. However, this approach seems kind of ugly to me, are there any better ways?
Your code is not breaking the closure, you're just taking the code the makes up a function and evaluating it in a different context (where the identifier src has a different value). It has nothing at all to do with the closure that you've created over the original src.
It is impossible to inspect data that has been captured in a closure. In a sense, such data are even more "private" than private members in Java, C++, C# etc where you can always use reflection or pointer magic to access them anyway.
This could be useful if you are trying to create multiple similar methods in a loop. For example, if you're creating a click handler in a loop that relies on a loop variable to do something a little different in each handler. (I've removed the "eval" because it is unnecessary, and should generally never be used).
// Assign initial value
var src = 3;
// This is the regular js closure. Variables are saved by reference. So, changing the later will
// change the internal value.
var byref = function() {
return src;
}
// To "break" the closure or freeze the external value the external function is create and executed
// immidiatly. It is used like a constructor function which freezes the value of "src".
var byval = function(s) {
return function() { return s };
}(src);
src = 9;
alert("byref: " + byref()); // output: 9
alert("byval: " + byval()); // output: 3
As others said this doesn't seem to be the right thing to do. You should explain why you want this and what you want to achieve.
Anyway, one possible approach could be to access properties of an object inside your function. Example:
var src = 3;
function foo (context) {
context = context || window; // Fall back to the global namespace as default context
return function () {
return context.src;
}
}
function bar (func) {
var context = {src: 9};
return func(context);
}
alert(bar(foo));
If you want to access a variable in a wider scope, just don't reuse the variable name in a narrower scope.
That's how it is supposed to work. Work with it instead of trying to fight it.
Here is the code see if you can understand , closures defined within a loop .
var clicked = false;
for(var i=0;i<temp.length;i++){
(function(index){
if(clicked) return false;
$(temp[index]).on('click',function(){
if($(temp[index]).text()=="" && !$(".cell1").val()){
$(this).text(player1Val);
$(".cell1").val(true);
console.log("first player clicked ");
clicked = true;
$(this).off();
for(var j=0;j<temp.length;j++){
$(temp[j]).off('click');
}
return false;
}
else return false;
});
})(i);
}