While learning about javascript closures, I came across this answer https://stackoverflow.com/a/111111/3886155 on stackoverflow.
It's a very good explanation of closures.
But I have some confusion in Example 4 and Example 5.
I just copy entire snippet here:
Example 4:
var gLogNumber, gIncreaseNumber, gSetNumber;
function setupSomeGlobals() {
// Local variable that ends up within closure
var num = 666;
// Store some references to functions as global variables
gLogNumber = function() { console.log(num); }
gIncreaseNumber = function() { num++; }
gSetNumber = function(x) { num = x; }
}
setupSomeGlobals();
gIncreaseNumber();
gLogNumber(); // 667
gSetNumber(5);
gLogNumber(); // 5
var oldLog = gLogNumber;
setupSomeGlobals();
gLogNumber(); // 666
oldLog() // 5
After reading some examples I can say that whenever function inside the function executes it can always remember the variables declared inside outer function.
I agree that if these closure variable updated anyway it still refers to the new variable value.
My problem in this example is specially related to var oldLog=gLogNumber;
How it can return old number after call to the setupSomeGlobals();?
because now the var num has reset.So why it is not using this new num value 666?
Now Example 5:
function buildList(list) {
var result = [];
for (var i = 0; i < list.length; i++) {
var item = 'item' + i;
result.push( function() {console.log(item + ' ' + list[i])} );
}
return result;
}
function testList() {
var fnlist = buildList([1,2,3]);
// Using j only to help prevent confusion -- could use i.
for (var j = 0; j < fnlist.length; j++) {
fnlist[j]();
}
}
Here they have pushed functions into array and executed them after loop finishes.But now reference to the closure variables is the latest one after loop finishes.Why not old one?
In both examples you are just assigning function definition to variable or array index.But first one points to old and second one points to latest.Why?
How it can return old number after call to the setupSomeGlobals()
num is not globally scoped, so the value num is in the context of which reference of gLogNumber is invoked.
After invocation of setupSomeGlobals method again, reference to gLogNumber got changed. try this
console.log(Object.is( gLogNumber, oldLog )); //true
setupSomeGlobals();
console.log(Object.is( gLogNumber, oldLog )); //false
So, oldLog retained old reference and hence old value of num, but gLogNumber got new num.
But now reference to the closure variables is the latest one after
loop finishes.Why not old one?
For this problem, have a look at
JavaScript closure inside loops – simple practical example.
Related
This question already has answers here:
Explanation of `let` and block scoping with for loops
(5 answers)
Closed 5 years ago.
function first(){
var items = document.getElementsByTagName("li");
for(var x = 0; x < items.length; x++){
items[x].onclick = function() {
console.log(x);
}
}
}
function second(){
var items = document.getElementsByTagName("li");
for(var x = 0; x < items.length; x++){
(function(val) {
items[val].onclick = function() {
console.log(val);
}
})(x);
}
}
function third(){
var items = document.getElementsByTagName("li");
for(let x = 0; x < items.length; x++){
items[x].onclick = function() {
console.log(x);
}
}
}
There are 4 elements in the list. Outputs of the 3 functions:
first: 4 4 4 4
second: 0 1 2 3
third: 0 1 2 3
I am not able to understand the output from the third function. In the second function, each call to the IIFE creates a new function object and hence, a new val variable. But in the third function, there is a single copy of the variable x, then how is the output: 0 1 2 3
Please correct me if I am wrong.
In the documentation for let from MDN they have an example covering this exact case in the Cleaner Code in inner functions section:
for (let i = 1; i <= 5; i++) {
let item = document.createElement('li');
item.appendChild(document.createTextNode('Item ' + i));
item.onclick = function(ev) {
console.log('Item ' + i + ' is clicked.');
};
list.appendChild(item);
}
The example above works as intended because the five instances of the (anonymous) inner function refer to five different instances of the variable i. Note that it does not work as intended if you replace let with var, since all of the inner functions would then return the same final value of i: 6. Also, we can keep the scope around the loop cleaner by moving the code that creates the new elements into the scope of each loop.
The same thing applies to your case, because you use let each anonymous function refers to a different instance of x. There is a different instance on each iteration of the loop. This happens because let has a block-level scope instead of the global function scope that var has.
From the docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let
let allows you to declare variables that are limited in scope to the
block, statement, or expression on which it is used. This is unlike
the var keyword, which defines a variable globally, or locally to an
entire function regardless of block scope.
When it is var it gets hoisted like all variables.
When it is let the scope is the block it is defined in.
This is one of the trickiest examples of let keyword.
The fact that Let binds variables to the block(& in this case for-loop) means that it binds the variable to every iteration of the loop. So, when loop is finished, you have 4 items (items from item[0] to item[3]) listening to click event.
In fact the for-loop in the third function produces the following:
items[0].onclick = function() {
console.log(0);
}
items[1].onclick = function() {
console.log(1);
}
items[2].onclick = function() {
console.log(2);
}
items[3].onclick = function() {
console.log(3);
}
Make sure to read more about Let here in MDN Some other exciting cases could be found there.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
Say I have this function:
function printFruits(fruits) {
for (var i = 0; i < fruits.length; i++) {
setTimeout( function() {
console.log( fruits[i]);
}, i * 1000);
}
}
printFruits(["Lemon", "Orange", "Mango"])
So this returns undefined 3 times.
I can see on a high level that since variables are stored not by value but by reference inside the closure... the loop is finishing first and by the time the functions are dequeued from maybe the Event Loop... the variable is already at undefined (fruits.length evaluates to 3 which is too high for this array size). But why does this perform strangely... it prints "apple" 3 times.
function printFruits(fruits) {
for (var i = 0; i < fruits.length; i++) {
var someConstant = i;
setTimeout( function() {
console.log( fruits[someConstant]);
}, someConstant * 100);
}
}
printFruits(["mango", "banana", "apple"])
Shouldn't someConstant be changing as well with i? Why does it seem to be 2 always?
Also this works:
function printFruits(fruits) {
for (var i = 0; i < fruits.length; i++) {
(function() {
var current = i;
setTimeout( function() {
console.log( fruits[current]);
}, current * 1000);
})();
}
}
Why is the IIFE necessary to fix this problem?
2nd Example
function printFruits(fruits) {
for (var i = 0; i < fruits.length; i++) {
var someConstant = i;
setTimeout(function() {
console.log(fruits[someConstant]);
}, someConstant * 1000);
}
}
printFruits(["Lemon", "Orange", "Mango"])
This logs thrice Mango. Because everytime the someConstant variable is created and re-initialised to i. Recollect how for loop works. i-value is increasing here till 4, checks the condition 4<3, and terminates. So the matter inside the loop executes only thrice. So the last value of someConstant defined in the printFruits functional scope is 2. So when the inner function executes someConstant, its equal to 2. So we get each time Mango.
3rd example
function printFruits(fruits) {
for (var i = 0; i < fruits.length; i++) {
(function() {
var current = i;
setTimeout(function() {
console.log(fruits[current]);
}, current * 1000);
})();
}
}
printFruits(["Lemon", "Orange", "Mango"])
Here the beauty of closures happening. Here its a self executing function being executed, immediately. So when i = 1, it invokes immediately. Every function has a different scope now. There is a seperate current value defined for each. So later when it executes, it recollects whats the value of 'current' when it was defined inside its scope.
The only difference between these samples is that the for loop increments i to 3 before stopping, while the last value which is assigned to someConstant is 2. Your "working" code is outputting Mango three times (index 2 of the array) instead of undefined (index 3 of the array). The general behaviour is the same.
Yes, you do need an IIFE, or ES6's let keyword instead of var.
The difference is that someConstant never gets incremented after the last iteration. The for() loop sets i = 3, the test i < fruits.length fails, so the loop stops. As a result, someConstant is still set to 2 from the last iteration of the loop. Then all the callbacks run, so they all log fruits[2], which is Mango.
You need the IIFE to get each iteration to save its value of i in the closure.
This uses var
var a = [];
for (var i = 0; i < 10; i++) {
a[i] = function() {
console.log(i);
};
}
a[6](); // 10
This uses let
var a = [];
for (let i = 0; i < 10; i++) {
a[i] = function() {
console.log(i);
};
}
a[6](); // 6
I don't understand why the result is different. Can somebody guide me?
The resulting array consists of functions, each function body looks like this:
console.log(i);
The value of i depends on whether we used var or let to declare the variable.
var (ECMAScript 5 and 6)
Here i is a global variable whose value is 10 after exiting the loop. This is the value that is logged.
let (ECMAScript 6)
Here i is a local variable whose scope is restricted to the for statement. Moreover, this variable gets a fresh binding on each iteration. This is best explained by your code transpiled to ECMAScript 5:
"use strict";
var a = [];
var _loop = function(i) {
a[i] = function() {
console.log(i);
};
};
for (var i = 0; i < 10; i++) {
_loop(i);
}
a[6](); // 6
So, on seventh iteration for example, the value of i will be 6 (counting from zero). The function created inside the iteration will refer to this value.
I think it would be much better to not define functions in a loop, you could easily accomplish this with one function definition that returns a closure:
function logNumber(num) {
return function() {
console.log(num);
}
}
var a = [];
for (let i = 0; i < 10; i++) {
a[i] = logNumber(i);
}
a[6]();
Regarding the difference between the two examples, one is using let for block scoping. A better example that shows the difference would be:
ECMA5:
for (var i = 0; i < 10; i++) { }
console.log(i); // 10
ECMA6:
for (let i = 0; i < 10; i++) { }
console.log(i); // i is not defined
Edit: as I stated in my comment to your question, this is more likely a side-effect of the transpiler you are using. Firefox supports block scoping and both versions of your loop produce 10 as output (which they should).
This is correct behavior according to the spec. The behavior with var and let is defined to be different.
See the spec, at https://people.mozilla.org/~jorendorff/es6-draft.html#sec-forbodyevaluation. According to this, the relevant concepts, which make the function declared inside the loop close over the current value of the block-scoped loop index, are things called "per-iteration bindings" and "per-iteration environment".
Babel handles it correctly, producing the following code:
var a = [];
var _loop = function (i) {
a[i] = function () {
console.log(i);
};
};
for (var i = 0; i < 10; i++) {
_loop(i);
}
This implements the semantics of for (let by isolating the contents of the for loop into a separate function parameterized by the index. By virtue of doing that, the function no longer closes over the for loop index, and i is treated separately in each function created. Thus the answer is 6.
Traceur does not produce the correct result. It yields 10.
So the famous question that has been asked 100 times on SO, about why my function declared in a loop and closing over the index index is using the "wrong" value of the loop index, shall be asked no more?
The issue is a bit more nuanced that merely proclaiming that "of course, let is block-scoped". We know that. We get how it works in an if block, for example. But what's going on here is a bit of an twist on block scoping in the context of a for, hitherto unknown to many people including me. It's a variable actually declared outside the "block" (if you think of the block as the body of the for statement) but has a separate existence inside each iteration of the loop.
For more, see https://github.com/babel/babel/issues/1078.
Why is result different in ES6 and ES5?
Because let and var are different. let is block-scoped while var is function-scoped.
In your first example there is only a single variable i. Every function you create has a reference to the same variable i. At the moment you call a[6](), i has the value 10, because that was the termination condition for the loop.
In the second example, every iteration of the loop has it's own variable i. It works exactly like in other languages with block scope.
The W3Schools example for a JS for-each loop (http://www.w3schools.com/js/js_loop_for.asp) is
var person = {fname:"John", lname:"Doe", age:25};
var text = "";
var x;
for (x in person) {
text += person[x];
}
and I'm wondering if there's any particular reason why x is defined outside of the loop if it's only used inside the loop in that particular example, or if that's necessary condition for defining a for-each loop, and if so, why?
This is(/was) an old convention, meant largely to indicate that x does not exist only inside the loop.
Declaring variables at the top of a function is not necessary and is variously encouraged/discouraged by different style tools. It can be convenient to group variable declarations in one place, but may increase the distance between declaration and use (generally considered a bad thing).
In JS, the var declaration works at function-scope, so variables are hoisted to the top of the nearest function and live for the duration of the function. See:
function foo() {
for (var i = 19; i > 0; --i) {
console.log(typeof i); // number
}
console.log(typeof i); // number
}
foo();
Because the variable is accessible throughout the function, it is declared at the start of the function (C used to require this).
ES6 has changed this with the let statement, which introduces the more common block-level scoping. With let, the variable is only available inside of the loop:
function foo() {
for (let i = 19; i > 0; --i) {
console.log(typeof i); // number
}
console.log(typeof i); // undefined
}
Can someone please explain how this gets 50, and how does the algorithm works. Thanks.
var multipliers = function makeMultipliers (max) {
var result = [];
for (var i = 0; i < max; i++)
result.push (function (x) {return x * i;});
return result;
}
multipliers(10) [2] (5)
???
what’s the value?
50, not 5
can you fix it?
Source: http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-170-software-studio-spring-2013/lecture-notes/MIT6_170S13_35-java-fn-cls.pdf
page 30
It's a scope issue. By the time the last function is called, i is where it was last seen at that scope last which would be 10. You can prevent this with a closure.
function multipliers(max){
var a = [];
for(var i=0; i<max; i++){
(function(i){
a.push(function(x){return x*i});
})(i);
}
return a;
}
In this case, the closure is a self-executing function that scopes off i at each step of the loop. If this did not happen the scope would go to the multipliers function itself. Since the loop has already completed by the time you are calling the function which you pass 5 to, which happens to be at step 2 of the Array that multipliers returns, i has already reached 10. Obviously, 10*5 is 50. When the closure is used is last position is 2 within the scope it is in, giving you the result that is sought. 5*2 = 10.
console.log(multipliers(10)[2](5));
This code creates an array of functions which will incorrectly all end up referring to the final value of i, but which are intended to freeze the value of i during each iteration.
To fix it you simply could add a level of locality to the iteration variable by embedding a new local variable that freezes the value of i within another function. For example, below the existing local variable i is passed into the dummy function as an argument, and the value of i at that instant is frozen into the new local variable i which is independent of the original i and not changed by the subsequent iterations of the original i.
result.push (function (x) {return x * i;})
result.push((function(i) {
return function (x) {return x * i;};
})(i));