This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
My code is:
for (i = 0; i < 3; i++) {
elem[i].click(function(event) { alert(i); });
}
Unfortunately that does not work. Any idea how to make it alert the same i as the i in elem[i].
Thank you!
You should use let for defining variable i. Unlike var, variables defined by let are block scoped. As a result, they won't be overwritten after each iteration.
for (let i = 0; i < 3; i++) {
elem[i].click(function(event) { alert(i); });
}
By the way, when ES6 is available, it's generally a bad idea to use closures in this case. It's kind of overkill.
Related
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Explanation of `let` and block scoping with for loops
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JavaScript closure inside loops – simple practical example
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(3 answers)
JavaScript: Understanding let scope inside for loop [duplicate]
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This question is inspired by a very famous trick interview question for javascript closures, however I am more interested into another aspect of it. The code goes as follows:
let i
for (i = 0; i < 3; i++) {
setTimeout(function log() {
console.log(i); // What is logged?
}, 1000);
}
The answer is 3 3 3, and (I think) I know the explanation, each callback function can access the same i variable, by the time they are executed its value is 3.
When we write let inside for loop like this:
for (let i = 0; i < 3; i++) {
setTimeout(function log() {
console.log(i); // What is logged?
}, 1000);
}
Now we get 0 1 2. Ok, here the explanation is that when declared this way, i is scoped to this loop, not available outside, and each iteration has its copy of i.
The part that bugs me: it does not seem to be a different variable in each step. If we change i in a first step, it will be changed for other steps.
for (let i = 0; i < 3; i++) {
setTimeout(function log() {
console.log(i); // What is logged?
}, 1000);
if (i === 0) i ++
}
The result now is 1 2, skipped one step.
So I see two possible explanations for this, but I cannot confirm this anywhere.
Closures work differently for block-scoped and function or global scoped variable
After each step of the loop, the current state of the variable declared in a loop is copied to a new variable for the next step of the loop
???
Note: Please let's not focus on why values are 3 3 3 instead of 2 2 2, let's not go into event loop with setTimeout etc. unless relevant to the answer. I feel like closures and variable scopes will be most relevant here.
This question already has answers here:
How do JavaScript closures work?
(86 answers)
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 1 year ago.
I don't understand why the behavior of the function is funny.
let printNumTwo;
for (let i = 0; i < 3; i++) {
if (i === 2) {
printNumTwo = function() {
return i;
};
}
}
let i =5
console.log(printNumTwo());
I thought the function is created globally and it will have to return 5. Why the function has small scope? Where could i read about what more.
PS Sorry for my english and my probably stuped question. I need more understandable info for this TY
This question already has answers here:
What is the difference between "let" and "var"?
(39 answers)
Closed 4 years ago.
The question pop up in my head when I read example 6 in this post https://stackoverflow.com/a/111111/6359753
Will there ever be a difference between
// Example 1
let i;
var arr = [1,2,3]
for (i=0; i<arr.length; i++){
// do stuff
}
and
// Example 2
var arr = [1,2,3]
for (let i=0; i<arr.length; i++){
// do stuff
}
If they are the same, why are they the same?
In other words, I don't really understand let's scope. If declared outside the loop, are they scoped outside the loop? example 2's let clearly are scoped inside the for loop, but example 1 I'm not so sure.
If it is declared in the for loop, it is visible only in the loop's body. Outside of loop i is not visible.
var arr = [1,2,3];
for (let i=0; i<arr.length; i++) {
}
console.log(i);
If it is declared outside the for loop, the scope of the variable is the closest surrounded block {}.
let i;
var arr = [1,2,3];
for (i = 0; i < arr.length; i++) {
}
console.log(i);
No, scope wise they are not same.
In the first example, variable i is the global variable and can be accessed throughout the program. But in the second example the scope of i is local to the for loop only, thus i can not be accessed from outside of for loop.
Here is an article that explains the difference between var, let and const.
I gives a great overview of how JS handles variables behind the scenes.
It explains hoisting of variables, scope(which was your question) and how to avoid some pitfalls when using let and const.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
How do JavaScript closures work?
(86 answers)
Closed 6 years ago.
I read this article, but I have some issues understanding it.
var funcs = [];
for (var i = 0; i < 3; i++) {
(function IIFE(arg) {
funcs[arg] = function output() {
console.log("My value: " + arg);
};
})(i);
}
for (var j = 0; j < 3; j++) {
funcs[j]();
}
How can I explain the mechanism of this snippet?
Start: i has value of 0. IIFE gets the value of i and stores it in arg.
funcs[0] just get assigned to a function named output.
second time, when i is 1, funcs[1] gets a totally new function? Is this false?
.....
The problem is here, How does funcs[0](); know that arg was 0? How things interact?
Function output() looks up and ask IIFE function to give the required value? right? Can someone say which illustration is correct?
By Providing this picture, for example, number 1, I mean, when funcs[0]() gets executed, output looks for arg, it doesn't find it, then goes to higher scope and finds it. each function has it's own IIFE, for example. I'n not sure how far are this from Javascript or even programming world, but help me get it!
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
I have been trying to do this
function test()
{
$.getJSON("http://myurl.com",function (data){
for( i=0; i<data.length; i++){
test = createElement('img');
test.onclick = function (){ myotherfun(data[i]); }
}
}
}
function myotherfun(data)
{
alert(data);
}
An error message states that data isn't covered by the scope.
Can some one explain why this happens?
Now that you've edited your question I can see the problem XD
The problem is that the onclick function will use the current values of data and (more importantly) i. So essentially it's looking for data[data.length], which by the definition of length doesn't exist.
Instead, you need to "lock" the value of the iterator. Here's how you can do it:
for(i=0; i<data.length; i++) {
(function(i) {
// do stuff that relies on i
})(i);
}