So i want to make a changing series of images. They are saved in an array. So i can find each one by using image[i].
I want to make a for loop, or something similar, that will start at index 0, use a setInterval to make the index++. Lets say it goes up to five images. So i need a loop that will reset to 0 when it reaches index of 4.
I cant make this work. I thought it would be easy. Can anyone recommend a loop type?
Various for loops and useEffect
You don't need a for loop. If you have index and setInterval, then every function call of interval function can be treated like a loop cycle.
const images = [
"a.png",
"b.jpeg",
"c.gif"]
let i = -1
let update = () => {
i++
i = i % images.length
console.log(images[i])
}
setInterval(update,1000)
As the function will i++ we need to set initial i=-1 so it'll get 0 when added one. Otherwise, 0++ would be 1, and your images would start showing from second element.
Related
A friend of mine brought to my attention a piece of JavaScript code that gives you unrealistic time scores on the flashcard website Quizlet's match game. It somehow stops the game's timer at the user specified time.
document.getElementsByClassName("UIButton UIButton--hero")[0].click();
setTimeout(function(){for(var F = setTimeout(";"), i = 0; i < F; i++) clearTimeout(i)}, 5100); //Change 5100
Using it is simple, you get on the match game (for example https://quizlet.com/187478162/match) and you simply enter this code in the console in the inspect menu. You then need to complete the game but it doesn't matter how long you take.
I don't know JavaScript (I am very knowledgeable with Python) but I have figured out so far that the first line clicks the start button and the first setTimeout function waits until the specified time to execute the function inside of it. It is the function inside that confuses me. It seems to just create and clear a bunch of Timeouts. I have no Idea how this stops the game timer though.
If someone could explain how it works that would be very much appreciated
Ah, that's a very interesting piece of code, very clever. Here's what's happening.
The inner function basically has the following (expanded for readability):
var F = setTimeout(";")
for(i = 0; i < F; i++) clearTimeout(i)
setTimeout can accept functions or strings that get evaluated after a timeout. They're just passing in the no-op ';', and they're not even passing in a delay. What they care about is the timer-id "F". If you go to the dev-tools (on a non-busy page), and put setTimeout(';') in the console, you'll see that it'll first return the id 0, then 1, then 2, and so on. The ids counts up.
So, you can imagine this timeline:
Random background stuff happens
The webpage starts a timeout with setTimeout, that has, lets say, id 7.
More background stuff
This script executes. It creates a timeout, and gets back an id that's going to be bigger than any timeouts made in the past. Lets say that id is 9.
Now the script goes through all ids from 0 to 9, stopping any active timeouts by passing in the id to clearTimeout. This includes clearing the timeout for id 7.
Before I get to the answer part, you should know a couple of things about setTimeout( ):
The setTimeout( ) function, can accept a callback function as it's first parameter, or, it can also accept code in a 'string'.
Whenever the setTimeout( ) function is called, it returns a timeout id which can be used in order to pass into the clearTimeout( ) function when you want to clear a particular timeout.
Now, please run the below code snippet and see what happens:
a) In the very first setTimeout call, I'm passing a callback function, storing the returned id in the const variable 'id1'.
b) In the second setTimeout call, I'm passing a code string, storing the returned id in a const variable 'id2'.
const id1 = setTimeout(() => console.log('setTimeout with callback fn'), 1000, '_');
const id2 = setTimeout('console.log("setTimeout with code string")', 2000);
console.log('id from 1st setTimeout( ) is :', id1);
console.log('id from 2nd setTimeout( ) is :', id2);
Both the setTimeout calls work without an issue, but the more interesting thing is when you look at the 'id' values for both function, for the first call the id is '1', for the second call the id is '2'.
This means two things: One these IDs are unique and secondly, these IDs are in the same ID pool, think of it like an ID list or an array where for each setTimeout call, a new ID is created by incrementing over the last existing setTimeout ID from the ID Pool.
Now, let's look at the code in question:
In the first line of code, the getElementsByClassName returns a nodelist of all the elements with the class passed in, and with the click( ), you are just simulating a buttonclick for the 0th element from the nodelist that is returned. You can use other functions like querySelector('UIButton UIButton--hero') or querySelecorAll('UIButton UIButton--hero') in this case and it will not make a difference in the way this works.
document.getElementsByClassName("UIButton UIButton--hero")[0].click();
Now, when we look at the below setTimeout( ) function calls with the knowledge we have about how the setTimeout function and setTimeout IDs work, it will be way easier to understand how it stops the actual timer:
setTimeout(function () {
for (var F = setTimeout("console.log(';')"), i = 0; i < F; i++)
clearTimeout(i);
}, 5100); //Change 5100
We are making an outer setTimeout call which takes in a callback function, and the timer for this setTimeout is set to '5100' milliseconds, which is basically a 5 second timer.
Looking at the callback function itself:
function () {
for (var F = setTimeout("console.log(';')"), i = 0; i < F; i++)
clearTimeout(i);
}
This callback function, runs a for loop, in the initialization block of this for loop, two variables are initialized, 'F' and 'i', value of 'F' is the ID returned by the current setTimeout( ) function, 'i' is set to 0.
Now from what we know from the code snippet I posted above is that the ID pool for all the IDs is the same, which means the ID 'F' is definitely going to be greater the setTimeout ID of the original timer that is present on the webpage.
Answer & Conclusion:
Now the loop itself iterates basing on the value of 'i', as long as it is less than 'F', and it clears the interval using clearInterval(i) for each value of i from 0 to F, which means it stops all timers on the page from ID = 0 to ID = 'F'.
And that is the reason this code can be used in order to exploit the website and stop it's timer so users can cheat and complete the quiz. Please let me know if you have any further queries.
Problem is that setTimeout function within for loop is not getting the correct values despite the fact they were passed correctly. Second problem is that the sleep variable is not working, it simply ignores it. Other variables like i are acting strange, they are not going though the loop, they are going in a random order.
Inside function set_delay
console.log(i);
// 3,5,0,2,4,1 should be 0,1,2,3,4,5
console.log(sleep);
// 6000,6000,7000,9000,9000,10000, those ones are displayied in ASC order but shuold be randomly
console.log(share_screen_rate[i]);
//4,1,6,10,6,2,8 - this is not ok it's random
console.log(top);
// 749.5,2998,299.8,499.667,149,374.75 => this should be in order from smallest (149) to biggest (2998)
setTimeout(function() {
}, sleep);
if I change sleep to a numeric value like 2000 it is respected only for first iteration after nothing, it just goes at 0ms.
FULL CODE
https://jsfiddle.net/ojpv2nxu/
EXPECTED OUTPUT
This should be a simple script to scroll down the page and make pause based on the sleep variable and also read the page chunk by chunk based on the share_screen_rate variable
I think it is a logic issue.... You are expecting the setTimeout to happen x seconds after the last one. But they are firing x seconds after you set it. So it is working as expected.
If you want them to fire x seconds after the last, than you need to change the logic to keep track of the seconds and adjust the timers.
so top of the file add
var timeTrack = 0
and than you need to add to that value
timeTrack += sleep
setTimeout(function() {
}, timeTrack);
My file works just fine in the first round of loop when i try to rerun the function again. It shows the previous value of the previous loop when i try to use the value to match and after which it shows the correct value. If i run the function again and again, it keeps holding on to the value of the previous generated random value.
for (var i=0; i<9; i++)
{
var ranD = Math.floor(Math.random()*33);
if (mathStar.indexOf(ranD)== -1) {
mathStar.push(ranD);
item[i].innerHTML = mathStar[i];
}
else {
i--;
}
itemVal[i].value = mathStar[i];
}
Substitute using const and let for var within for loop to avoid creating global variables and --i could have unexpected results within the code where i++ is also used in the foor loop.
Is this the first occurrence of "mathStar"?
If this is the first place you're using mathStar, it means it gets created globally and that usually leads to confusion. In this case, take a look at this.
Looking at just this, it seems that you are not resetting your "mathStar" value. This way, any time you run this loop for the nth time, the values you have added to "mathStar" using mathStar.push(...) also occur in the list of values.
For one of my elements on my page, I want the text to change every ten seconds, and for the class to be changed. Text changing is easy, as is class changing, but I'm having trouble with my for loop, and I feel like I'm missing something.
I want to have the for loop choose a random faction in an array, and then apply that to the element. For my testing, I've been using console.log rather than DOM manipulation.
First, I set up my array:
var factions = ["Enforcers", "Reapers", "Ular Boys", "Roaches"];
Then, I want a variable that is a number chosen at random in reference to this array:
var x = factions[Math.floor(Math.random()*factions.length)];
From that, I want the ability to run the Math.floor and Math.random functions elsewhere.
function reDefine() {
x = factions[Math.floor(Math.random()*factions.length)];
console.log(x);
}
Finally, I want the for loop to run 200 times (I've chosen 200 times because it's far and beyond the time the user will be staying on the site), so I told it to count to 200 (i = 0; i < 200). After that, I wanted each time it iterated, to wait 10s, so I have a Timeout function with a delay of 10000 (milliseconds). Then, the code to reDefine and then, in the case of testing, console.log the new definition of the x variable.
function reChange() {
for (var i = 0; i < 200; i++) {
setTimeout(function() {
reDefine();
console.log("Chosen faction is now: " + x);
}, 10000);
}
}
Instead of counting to 1 (the first iteration), waiting 10000, and then redefining x, it redefines x two hundred times, then logs them all.
Is there something I'm specifically doing wrong here, perhaps with the Timeout function?
Is there something I'm specifically doing wrong here, perhaps with the Timeout function?
Yes! You're scheduling a bunch of deferred callbacks, but not actually waiting until one has finished to schedule the next.
You can fix that with something as simple as:
function reChange(currentIndex) {
setTimeout(function() {
reDefine();
console.log("Chosen faction is now: " + factions[currentIndex]);
// If we haven't gotten to the end of the list, queue up another one
var nextIndex = ++currentIndex;
if (nextIndex < factions.length) {
// Enqueue the next faction
reChange(nextIndex);
}
}, 10000);
}
Make sure to note that the function without the timeout has closure over the value of currentIndex for each call of reChange. That is, the next invocation does not replace currentIndex in any previous timeout, since primitives (including numbers) are passed by value. Closure in JS can be a tricky thing.
The core problem is that your execution right now looks like:
for each item
wait
log
rather than:
for the current item
wait
log
repeat
Because JS is single-threaded (for most intents and purposes), setTimeout adds a callback to be executed later. It doesn't block until the timeout has expired, like a traditional sleep would do.
I'll let you see first what i got:
CODE
I decided to experiment and make a script which order numbers in an array in every possible way.
The first way I'm doing is the "Selection mode", where you find the lower value of the array and you put it at the first position. As you can see I printed it into a table to make it funnier, but here comes my question, i want to put a delay at the end of every loop, where i highlight the values that I'm going to exchange, but for some reason it just organize all the table and then render it instead of going step by step.
Now I'm going to proceed to explain a bit my code ^^u:
If you click at "Restart" you'll get a new bunch of random numbers, and if you click at "Selection" the array will become ordered.
Functions:
generate: generate i numbers with a value from 0 to j.
prinTable: print the table.
renderDots: adds styles at the dots which match with the array values.
seleccion: execute the "Selection" way to organize the array.
highlight: adds styles at the dots which will be exchanged at THIS loop, and remove highlight styles of the others points.
I would like to get an affect like this:
You can use the setInterval function instead of a for loop:
function seleccion(table){
var ordenados = 0;
var timer = setInterval(renderOneStep, 300)
function renderOneStep(){
if (ordenados < table.length){
// do all the calculations
ordenados++;
} else {
clearInterval(timer);
}
}
}
The renderOneStep function updates one column. It is called every 300ms.
Please see the updated jsfiddle.