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JavaScript closure inside loops – simple practical example
(44 answers)
How do JavaScript closures work?
(86 answers)
Closed 3 years ago.
I have previously posted on SO:
How would you set the following loop to log like below?
The answer was to change var to let.
I am currently learning javascript after having studied python for a few months. I just wanted to see where my analysis is going wrong as I am struggling to apply the concept of block scope and function scope here.
Here is the original code snippet:
for (var i = 0; i < 5; i++) {
setTimeout(function() { console.log(i); }, i * 1000 );
}
What I do not understand is, if I delete the setTimeout function bit and end up with this:
for (var i = 0; i < 5; i++) {
console.log(i);
}
The output changes from
5
5
5
5
5
to
0
1
2
3
4
which was what I thought it should initially do. I don't understand why in the first case the need to use let i was necessary but in the second case it works fine with var.
Another point of confusion is that, if you take the first code snippet, with the setTimeout function, the value of the final i printed is 5. This does not make sense considering our initial for loop was only supposed to run till i<5.
The last point I am struggling with is that even if i was allowed to take on the value of 5, it seems the action (printing i) is done after the loop? I am saying this because i starts off at 0, and increases in value iteratively, here we are printing the final value of i 5 times. This must mean the computer has gone through the iteration before and then decides to do the action using the final value of i but 5 times?
Essentially I thought it goes as follows, we create a variable i using var, we say that as long as i is less than 5, do something, after doing something increase i by 1 and repeat.
Instead what it looks like from the first codes output is, create a variable i using var, i increases by 1 till i equals 5, do the action as many times as the number of iterations.
Sorry if this question is not coherent but I can't seem to apply the concepts of function scope and block scope here, which I feel is the key issue. Can someone please elaborate on this particular example?
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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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:
setTimeout in for-loop does not print consecutive values [duplicate]
(10 answers)
JavaScript closure inside loops – simple practical example
(44 answers)
What is the difference between "let" and "var"?
(39 answers)
Closed 1 year ago.
I am reading about closures, and I found these small exercises that I solved by chance, but I don't understand why they work.
The exercise is :
for (var i = 0; i < 3; i++) {
setTimeout(function log() {
console.log(i); // What is logged?
}, 1000);
}
I get that 3 will be printed 3 times because the value captured by log() when the for() ends.
Additionally, it was asked how to fix it for the console to print 0,1,2, and I found (by chance, to be honest), that it works by doing :
for ( let i =0 ...
But I guess because Let is block scoped, but i don't really know how to explain it in simple words.
The variables defined with let are blocked scope which means once you go out of the bracket it loses its value and if you try to access it will throw you an error. So, when the for is running the, i present in the console.log() will store the value for it like 0,1,etc.
whereas in the var part the var will be stored in memory and when the timeout is about to expire it will access the variable i in the memory and print whatever is there for that variable at that time.
This question already has answers here:
What is the difference between "let" and "var"?
(39 answers)
JavaScript closure inside loops – simple practical example
(44 answers)
How do JavaScript closures work?
(86 answers)
Closed 4 years ago.
for below code output will print "3" for 3 times, i.e updates same variable for 3 times
for(var k=0;k<3;k++){
setTimeout(()=>{
console.log(k)
},1000)
}
similarly if i use let instead of var 1,2,3
for(var k=0;k<3;k++){
setTimeout(()=>{
console.log(k)
},1000)
}
i know why it prints 0,1,2 as i is different every time. i'm just wondering why its printing 3 in case of var as condition is to checking for less then 3?
The above result is because, var defines variable globally, or locally to an entire function regardless of block scope.
let creates a variable declaration for each loop which is block level declaration. So basically it creates a scope within { }.
If you want to print using var keyword use self invoked functions :
for(var k=0;k<3;k++){
(function(k){
setTimeout(()=>{
console.log(k)
},1000)
})(k);
}
the function setTimeout will execute after the delay provided. The code execution doesn't stop since the javascript is based on concurrency model and event loop, so the execution continues till the delay provided. which in case, increment the k. Also, the post-increment operator will use the value before iteration. So, for last iteration the value of k is 3, but since we have given a delay, it would print 3 every time.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
for (var a=0; a<5; a++)
{
setTimeout(function(){
alert(a);
}, 0);
}
The above code in javascript is prompting me with the output of 5, 5 times infact my expectation was of the output something like 0,1,2,3,4 but rather it shows me the output of 5,5,5,5,5. Why is it so?
Besides that please explain me the concept of context and scope in javascript i always fail to understand it through many examples reading from the web.
Inside setTimeout(), your alert() is called asynchronously so it is very likely that for loop is already finish before alert() get executed.
This question already has an answer here:
Javascript: When writing a for loop, why does it print the last index number?
(1 answer)
Closed 7 years ago.
Though I'm aware of closures and scopes. I need some detailed explanation on the following piece of code.
for (var i = 0; i < 10; i++) {
i // no console.log
}
Output
9
Why would it not display from 0 to 9 ?
Ran on chrome developer console.
Your question is a bit vague, but it seems you are asking why the output would end at 9, instead of 10 (even though, in your example, you provide no output method).
Your loop uses the < operator to compare i and the 10 constant, which translates to "less than 10". Since you are incrementing by whole numbers, 9 will be the last integer before your loop falls outside of the < 10 range, thereby breaking out of the loop.
If you are expecting 10 as your final output, you can use the "less than or equal to" operator (<=) to compare i and 10.
Edit
You've edited your original question, and the answer to it is best summed up in another Stack Overflow answer:
All statements in javascript have a value, including the block
executed in looping constructs. Once the loop block is executed, the
final value is returned (or undefined if no operations take place).
The statement that is implicitly providing the return value "100" is
numbers[i] = i+1;, as the final iteration of i+1 produces 100 and
assignment operations return the value being assigned.
If you're running that in the console, it's because the console shows the final result of the last statement in the program it runs.
In your case, the for loop gives a final value of 9.
While it may seem odd that I'm talking about a for statement giving a result (since statements don't give results), they actually do give a result with respect to the execution of the entire program. This result is obtainable by whatever is executing your code.
Here's an example. You can do the same with eval(), which will give you the result of the final statement.
var n = eval('for(var i = 0; i < 3; i++) {i}');
console.log(n);
This will give you 2 in the console as the result of the program eval'd even though the only result could come from the for loop, which returns the last statement result of its last loop.