Javascript closure and IIFE (immediately invoked function expressions) - javascript

Closures store their outer variables by reference (and not by value). In the below code, however, I want to store by value. Can anyone show me how to do it using IIFE?
var i = -1;
var f = function () {
return i; // I want to capture i = -1 here!
};
i = 1;
f(); // => 1, but I want -1

What you've posted is in fact not an IIFE: that stands for immediately invoked function expression; you have a function but you're not immediately invoking it!
That aside, the idea here is to store the interesting bit of state in a function argument, so that it's a distinct reference. You do that by creating another function (the Function Expression part), and then invoking it with the globals whose state you want to capture (the Immediately Invoked part). Here's what it looks like:
var i = -1;
var f = (function(state) { // this will hold a snapshot of i
return function() {
return state; // this returns what was in the snapshot
};
})(i); // here we invoke the outermost function, passing it i (which is -1).
// it returns the inner function, with state as -1
i = 1; // has no impact on the state variable
f(); // now we invoke the inner function, and it looks up state, not i

As IIFE - Immediately invoke the function.
var i = -1;
var f = function () {
return i; // I want to capture i = -1 here!
}();// invoked here
i = 1;
console.log(f);

Related

In what scenario would you use a function that returns another function (Javascript)?

Thanks to this question I understand how a function may take two sets of parentheses and how a function may return another function which fires immediately after being returned.
What I do not understand is why this is good to know? How could this be applied to solve a problem?
The example of:
function add(x){
return function(y){
return x + y;
};
}
add(3)(4) === 7; // true
Works fine - sure. But why not just write it like this instead?
function add(a, b) {
return a + b;
}
add(3, 4) === 7; // true
Lets take the same code which you have mentioned.
function add(x) {
return function(y) {
return x + y;
};
}
var adder3 = add(3); //Forming adder3
var op1 = adder3(4) // 7
var op1 = adder3(5) // 9
// Now adder 10:
var adder10 = add(10); //Forming adder3
var op1 = adder10(4) // 14
var op1 = adder10(5) // 15;
Hope you understand!!
Revert me if you need more info on closure.
Your example called as closures
Closures’ Rules and Side Effects
Closures have access to the outer function’s variable even after the outer function returns:
One of the most important and ticklish features with closures is that the inner function still has access to the outer function’s variables even after the outer function has returned. Yep, you read that correctly. When functions in JavaScript execute, they use the same scope chain that was in effect when they were created. This means that even after the outer function has returned, the inner function still has access to the outer function’s variables. Therefore, you can call the inner function later in your program. This example demonstrates:
function celebrityName(firstName) {
var nameIntro = "This celebrity is ";
// this inner function has access to the outer function's variables, including the parameter​
function lastName(theLastName) {
return nameIntro + firstName + " " + theLastName;
}
return lastName;
}​​
var mjName = celebrityName("Michael"); // At this juncture, the celebrityName outer function has returned.​
​​ // The closure (lastName) is called here after the outer function has returned above​
​ // Yet, the closure still has access to the outer function's variables and parameter​
mjName("Jackson"); // This celebrity is Michael Jackson

Closures store references to the outer function’s variables; they do not store the actual value. 
Closures get more interesting when the value of the outer function’s variable changes before the closure is called. And this powerful feature can be harnessed in creative ways, such as this private variables example first demonstrated by Douglas Crockford:
function celebrityID() {
var celebrityID = 999;
// We are returning an object with some inner functions​
// All the inner functions have access to the outer function's variables​
return {
getID: function() {
// This inner function will return the UPDATED celebrityID variable​
// It will return the current value of celebrityID, even after the changeTheID function changes it​
return celebrityID;
},
setID: function(theNewID) {
// This inner function will change the outer function's variable anytime​
celebrityID = theNewID;
}
}​
}​​
var mjID = celebrityID(); // At this juncture, the celebrityID outer function has returned.​
mjID.getID(); // 999​
mjID.setID(567); // Changes the outer function's variable​
mjID.getID(); // 567: It returns the updated celebrityId variable


Reference site : http://javascriptissexy.com/understand-javascript-closures-with-ease/
Functions which return functions are useful when you want similar functions which depend on some parameters.
A real life example: [].sort can be called with a custom comparator function, but it can make sense to define a comparator function to allow more customizations:
function comparator(options) { // Function which returns a function
return function(a, b, tmp) {
if(options.reverse) tmp = a, a = b, b = tmp;
if(options.map) a = options.map(a), b = options.map(b);
if(options.func) return options.func(a, b);
return a < b ? -1 : (b < a ? 1 : 0);
}
}
Then you can use
[1,11,10,2].sort(comparator({map: String})); // [1, 10, 11, 2]
[1,11,10,2].sort(comparator({reverse: true})); // [11, 10, 2, 1]
[1,11,10,2].sort(comparator({func: Function.prototype})); // [1, 11, 10, 2]
if we require a function in certain state with certain value then we can use it inside another function and return that, so that the return function with certain state can be directly used in different scenario.
you can check out various example on closure.
http://javascriptissexy.com/understand-javascript-closures-with-ease/
If you know the first param is always going to be the same, then it will convenient to have it closure, instead of passing it again and again. For simple programs, it may not make sense. But, for programs which handles repetitive params more often, this technique definitely comes handy.
Other than closures you can also use it for pre-processing as a one time job, consider if you had to do something intensive, e.g. generate a million things;
function generateSessionSecrets(lock) {
var secrets = [], i = 1000000;
while (i-- > 0) {
secrets[i] = Math.random();
}
return function(key, i) {
if (key === lock) return secrets[i];
}
}
var chest = generateSessionSecrets('fizz');
chest('fizz', 0); // e.g. 0.2096199430525303
chest('fizz', 1); // e.g. 0.30329699837602675
// ...
chest('fizz', 0); // still 0.2096199430525303
(This is an example of concept, not an example of real security)
there isn't a point to using it immediately. you would use it to create a function to attach it to an event or use as a callback for an asynchronous function. an example might be such:
function factory(param){
return function(result) {
if (result==param) dosomething();
}
}
$('#domobject').click({
param = $('#domvalue').value;
asynch_function(factory(param));
});
Here I've attached a click event to presumably a button. When it is clicked it will retrieve the value of an input and create a function based on it and call an asynchronous function with the newly created function as it's callback. The asynchronous function might be an ajax request. When the asynchronous function completes the function that factory created, the callback, will be called. It will check the return value the asynchronous function passed to the callback against the param specified when the event was attached.
If we move the dom lookup to inside the callback function, then we wouldn't need factory or param, but then it would use the value that is in the input at the time when the asynch function has returned, rather than when the button was clicked which is later and the value might have changed.
Sometimes, you won't be able to obtain a value you need in the context of the callback for other reasons. Or it might just be that you want to abstract out a class of functions so you don't have to retype a slightly different version of it in all the places you use it.

Why is non-static variable behaving like static?

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.

Execution order of simple function

I am a bit new to javascript, i was just trying the below snippet:
_getUniqueID = (function () {
var i = 1;
return function () {
return i++;
};
}());
s = _getUniqueID();
console.log(s); // 1
console.log(_getUniqueID()); // 2
I was under the impression that i would have to do s() to get 1 as the result and i was thinking that _getUniqueID() returns a function rather than execute the funtion inside it. Can somebody explain the exact execution of this function please ?
What you're seeing here is a combination of Javascript's notion of closure combined with the pattern of an immediately invoked function expression.
I'll try to illustrate what's happening as briefly as possible:
_getUniqueID = (function () {
var i = 1;
return function () {
return i++;
};
}()); <-- The () after the closing } invokes this function immediately.
_getUniqueID is assigned the return value of this immediately invoked function expression. What gets returned from the IIFE is a function with a closure that includes that variable i. i becomes something like a private field owned by the function that returns i++ whenever it's invoked.
s = _getUniqueID();
Here the returned function (the one with the body return i++;) gets invoked and s is assigned the return value of 1.
Hope that helps. If you're new to Javascript, you should read the book "Javascript, the Good Parts". It will explain all of this in more detail.
_getUniqueID = (function () {
var i = 1;
return function () {
return i++;
};
}());
s = _getUniqueID();
console.log(s); // 1
console.log(_getUniqueID()); // 1
when you do () it calls the function,
a- makes function recognize i as global for this function.
b- assigns function to _getUniqueID
you do s = _getUniqueID();,
a - it assigns s with return value of function in _getUniqueID that is 1 and makes i as 2
when you do _getUniqueID() again it will call the return function again
a- return 2 as the value and
b makes value of i as 3.
This is a pattern used in Javascript to encapsulate variables. The following functions equivalently:
var i = 1;
function increment() {
return i ++;
}
function getUniqueId() {
return increment();
}
But to avoid polluting the global scope with 3 names (i, increment and getUniqueId), you need to understand the following steps to refactor the above. What happens first is that the increment() function is declared locally, so it can make use of the local scope of the getUniqueId() function:
function getUniqueId() {
var i = 0;
var increment = function() {
return i ++;
};
return increment();
}
Now the increment function can be anonymized:
function getUniqueId() {
var i = 0;
return function() {
return i ++;
}();
}
Now the outer function declaration is rewritten as a local variable declaration, which, again, avoids polluting the global scope:
var getUniqueId = function() {
var i = 0;
return (function() {
return i ++;
})();
}
You need the parentheses to have the function declaration act as an inline expression the call operator (() can operate on.
As the execution order of the inner and the outer function now no longer make a difference (i.e. getting the inner generator function and calling it, or generate the number and returning that) you can rewrite the above as
var getUniqueId = (function() {
var i = 0;
return function() {
return i ++;
};
})();
The pattern is more or less modeled after Crockford's private pattern
_getUniqueID = (function () {
var i = 1;
return function () {
return i++;
};
}());
console.log(_getUniqueID()); // 1 , this surprised me initially , I was expecting a function definition to be printed or rather _getUniqueID()() to be called in this fashion for 1 to be printed
So the above snippet of code was really confusing me because I was't understanding that the above script works in the following manner, by the time the IFFE executes _getUniqueID is essentially just the following:
_getUniqueID = function () {
i = 1
return i++;
};
and hence,
_getUniqueID() // prints 1.
prints 1.
Note: please note that I understand how closures and IFFE's work.

Expression contains parentheses only. Function argument after } [duplicate]

This question already has answers here:
What is the (function() { } )() construct in JavaScript?
(28 answers)
Closed 9 years ago.
I got so confused here. Can someone help me peel the layers off the function definition here? What are the out-most parentheses for? Why argument (p) can be after }, and which function is it for? Where is q required at all? Why can p directly call publish() later?
var p = {};
(function(q) {
q.publish = function(topic, data){
...
};
}(p));
...
p.publish("inbox", "hello");
It simply creates the function and executes it immediately.
For example:
var foo = function() {
return 5;
}();
console.log(foo);
will print 5.
If you want to learn more about it, please read this excellent article http://benalman.com/news/2010/11/immediately-invoked-function-expression/
I would like to quote module pattern example from the link I shared.
var counter = (function(){
var i = 0;
return {
get: function(){
return i;
},
set: function( val ){
i = val;
},
increment: function() {
return ++i;
}
};
}());
console.log(counter.get());
counter.set(3);
console.log(counter.increment());
console.log(counter.i);
Output
0
4
undefined
Here we are dynamically creating an Object at the same time we are making i private. This is called closure property.
Putting the function in parentheses means the JS interpreter takes it as a function expression, so it can be anonymous (i.e., not have a declared name). Adding () at the end means the function is called immediately. So, e.g.:
(function() {
alert('Hi');
}());
...declares a really simple function with no arguments, and calls it immediately. A slightly more complicated one with an argument might look like this:
(function(someArgument) {
alert(someArgument);
}('Hi'));
That would alert 'Hi', because the string 'Hi' is passed to the anonymous function when it is called.
So with that as background, here's what your code is doing line by line:
var p = {}; // declare variable p, referencing an empty object
(function(q) { // make an anonymous function expression that takes one argument, q
q.publish = function(topic, data){ // add a property 'publish' to whatever q is,
... // where 'publish' is a function that takes
}; // two arguments
}(p)); // call the anonymous function, passing p in
...
p.publish("inbox", "hello"); // call the method 'publish' that was added to p
The q argument that you asked about takes the value from p, so it refers to the same empty object, but then it adds the .publish() method to that object.
This general concept is called an "Immediated invoked function expression", or IIFE.
Usually if an IIFE is just sitting there by itself like that (i.e., another variable is not assigned equal to it) it is done so that working variables/functions can be created temporarily without adding them to the global scope, because in JavaScript the scope options are basically global or function. The code you show doesn't do that, but here's a simple example:
(function() {
var x = 0;
for (var i = 0; i < 100; i++)
x += i;
alert (x);
}());
// here x and i are not accessible because they're local to the function above.
It's a self executing function. Using q is useless here since q === p.
var p = 'hello';
function f(q){ return q; }; f(p); // "hello"
(function(q){ return q; }(p)); // "hello"
(function(){ return p; }()); // "hello"
/*(*/function(){ return p; }()/*)*/; // SyntaxError: Unexpected token (
This pattern is usually used to create a private context for variables. Here is a well known example :
var counter = function(){
var i = 0;
return function(){
return ++i;
};
}();
counter(); // 1
counter(); // 2
i; // undefined
Have a look at this great article : http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html.

What is the difference in the following JS syntax?

Following are two ways to define BW.Timer. Can someone tell me what the difference is? I am not certain the first is even valid, but if it is valid, what is different about using the myfunc=(function(){}()) syntax?
BW.Timer = (function () {
return {
Add: function (o) {
alert(o);
},
Remove: function (o) {
alert(o);
}
};
} ());
And...
BW.Timer = function () {
return {
Add: function (o) {
alert(o);
},
Remove: function (o) {
alert(o);
}
};
};
The first is the return-value of the immediately-invoked function. The second is a function. It essentially comes down to what the difference is between these:
var f = (function() { return 0; })();
var f = function() { return 0; };
Since the first function is called immediately, the value of 0 is given to the variable f. The first f is not a function. However, the second f we must call in order to get the value:
f(); // 0
It follows that in your example, the first BW.Timer is the object literal itself and the second is a function returning an object literal. You must call the function in order to get to the object:
BW.Timer().Add(x);
Why use the first then?
You might ask yourself why one would use a syntax like a = (function() { return {}; })() instead of a = {}, but there's a good reason. An IIFE (Immeditately-Invoked Function Expression), unlike a regular function allows the emulation of static variables (variables that maintain their value through a single instance). For example:
var module = (function() {
var x = 0;
return { get: function() { return x },
set: function(n) { x = n }
};
})();
The above a text-book example of the Module Pattern. Since the function is called right away, the variable x is instantiated and the return value (the object) is given to module. There's no way we can get to x other than by using the get and set methods provided for us. Therefore, x is static, meaning its variable won't be overridden each time you use module.
module.set(5);
module.get(); // 5
On the other hand, let's see an example where module is declared as a function instead:
// assume module was made as a function
module().set(5);
module().get(); // 0
When we call module() the x variable is overridden each time. So we're effectively using different instances of module and x each time we call module.
The difference is rather large.
In the first case, BW.Timer is executed when it is first encountered, and that is the static version assigned to BW.Timer. In that instance, BW.Timer.Add(1) may be used. Each call to BW.Timer will be the same object.
In the second case, BW.Timer is not executed when first encountered, and instead is a function referece which must be invoked BW.Timer(). For Add to be used, this must be the case BW.Timer().Add(1). Also, you can issue var timer = new BM.Timer();. Each instance of BW.Timer() will be unique here.
In the first example BW.Timer references an object that the self-executing function returns, while in the second example it references a function object, in other words it's a function that can be executed BW.Timer().

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