I want to write a javascript function that takes very close to 5 seconds to run. How can I make that guarantee?
I have tried
function wait(numSeconds) {
var end = new Date().getMilliseconds() + numSeconds * 1000;
while (new Date().getMilliseconds() <= end) {}
}
but this just crashes the page.
You cannot freeze the page without freezing the page.
You're trying to write a function that runs for 5 seconds straight.
Javascript runs on the UI thread, so the page will be frozen whenever any code runs.
In short, you can't do that.
You need to write asynchronous code, probably by using setTimeout().
you can use:
var t = setTimeout(function(){alert('done')},5000);
If you want to wait 5 seconds before doing something, use setTimeout or setInterval. Otherwise, just eating up user time is a bad idea. In fact, the browsers will interrupt your script if it takes too long to do something, which your function most certainly will do.
(Ignoring for the moment why you shouldn't really do this...)
The reason your code is freezing the browser is that your while condition will never become true (for numSeconds of 1 or more).
The .getMilliseconds() method returns just the milliseconds part of the date, i.e., a number between 0 and 999. For numSeconds of 1 of more your end will always be greater than 999.
Try using .getTime() instead of .getMilliseconds(). Or if you don't care about supporting old browsers uses Date.now().
You could just use javascripts setTimeout instead of your own wait function.
setTimeout excecutes a function after a certain amount of time.
For example:
setTimeout(function(){}, 5000);
Would just wait 5 seconds.
See more at w3schools.
Strictly speaking, what you ask cannot be done because, as you've seen, the page will freeze while you wait.
As a practical matter, though, I suspect that what you really want is the something like this:
function waitawhile() {
setTimeout( function() {
alert('5 seconds has passed');
}, 5000);
}
Related
I came across this simple(or what I thought so) question on a Javascript MCQ Test:
After how much time will the following code execute entirely?
setTimeOut(console.log("hi"),1000);
setTimeOut(console.log("hi"),1000);
setTimeOut(console.log("hi"),1000);
setTimeOut(console.log("hi"),1000);
setTimeOut(console.log("hi"),1000);
Options
A) 1 second
B) 2 seconds
C) 4 seconds
D) 5 seconds
I answered as option D) 5 seconds since EACH line above will take 1000 milliseconds to execute i.e a total of 5000 milliseconds = 5 seconds
But in the results, it said that the actual answer is Option A) 1 second.
I executed those five lines in my console (altogether) and the entire code executed after 1 second like the answer said.
I don't understand the logic behind the right answer, and why my reasoning was wrong.
because setTimeout works asynchronously, means all of these 5 statements will be executed simultaneously and all of these will start waiting for 1 second. and after one second all will be executed. hope it clears.
Each call you make it running in it's own background thread (it's own unique operation), when you call setTimeout, you're telling JavaScript that you want to execute your code after 1 second has passed.
If you wanted to make this last 5 seconds you would do something along the lines of:
setTimeout(function() {
console.log("First task")
setTimeout(function() {
console.log("Second task");
},1000);
},1000);
This would execute the first task, once called it will execute the second task
Edit: I saw another post about doing it non-async, you want to avoid doing anything non-async in JavaScript as it will hold up the browser which is general is a bad practice and bad user experience
From the MDN page:
The setTimeout() method ... sets a timer which executes a
function or specified piece of code once the timer expires.
How to input between lines of code some force-timeout (like java Thread.Sleep) in NighWatch.js ?
Have to wait for element to have exact value after page render.
I don't think this is the correct answer. For the original poster it's probably too late but other users might google it:
If you use the Nightwatch testing frame work then you can use these commands:
http://nightwatchjs.org/api#pause
browser.pause(1000); // pauses the test for 1000 milliseconds
or wait for an element to be present:
browser.waitForElementPresent('body', 1000); // waits max. 1000 ms for the element to appear
You won't be able to stop the thread since javascript is single thread non blocking.
What you want to do is this
setInterval(function () {alert("Hello")}, 3000);
The syntax is.
window.setInterval("javascript function", milliseconds);
See : http://www.w3schools.com/js/js_timing.asp
I am trying to get a script from one page and get it again after a certain period of time. But, the time varies each time. On the main page I have
function varcontent() {
$.getScript("custom.php");
}
varcontent();
Then on the custom.php I have my script and
setInterval(varcontent, 20000);
at the end. Each time it may not be 20 seconds. It seems to work at first, but then the old ones are fired again too. I don't know how to get it out of this loop and they keep multiplying.
You probably want setTimeout, not setInterval. setInterval will keep invoking the callback (until you cancel it), whereas setTimeout will only invoke the callback once after the specified delay.
So in your custom.php, you should have instead:
setTimeout(varcontent, 20000);
I've been reading John Resig's "Secrets of a JavaScript Ninja" and it explains that JavaScript is single-threaded. However, I tried testing this and I'm not sure what to take away from here:
// executing this in browser
(function () {
// throw something into event queue
setTimeout(function () {
alert("This will be called back after 1 second.");
}, 1000);
// arbitrary loop to take up some time
for (var i = 0; i < 10000; i += 1) {
console.log(i);
}
})();
Maybe I'm not understanding exactly what being single-threaded means, but I thought that the setTimeout callback wouldn't execute until all of the outer anonymous function is complete. However, running this in the browser shows that the callback function gets called while i's are still being outputted onto the console. To me, this seems like there's 2 threads with anonymous function's invokation occupying 1 thread and then the callback using the 2nd thread.
Can someone help un-confuse me?
console.log() is a weird function in some browsers (like Chrome) and is not synchronous itself so you can't really use it to gauge the single threadedness. What you are probably seeing is that the JS engine executes all the console.log() statements and then the setTimeout() runs to show the alert and, in parallel (in some other process that isn't javascript) all the data is being shown in the console.
Javascript is indeed single threaded. In your example, the setTimeout() callback will not execute until your for loop is done.
You can illustrate it better like this:
(function () {
// throw something into event queue
setTimeout(function () {
alert("This will be called back after 1 second.");
}, 1000);
function now() {
return new Date().getTime();
}
var start = now();
// loop for 1.2 seconds
while (now() - start < 1200) {}
alert("Looping done");
})();
Working jsFiddle demo: http://jsfiddle.net/jfriend00/3sBTb/
This is a bit of a tricky concept to understand. Throwing in things like event listeners also further muddies up the picture.
A simple way to think of it is as if you have a conveyor belt. You have your normal function calls all evenly spaced out, with room in between.
Things that are asynchronous things (timeouts, triggered events, etc.) fill those spots. There isn't an infinite amount of room between each of those normal calls, so it fits what it can from this queue, does a little more of the normal synchronized functions, fills some more space with asynchronous, etc.
The affect appears to be somewhat multi-threaded (and indeed you can cause race conditions of a sort with asynchronous calls). In many cases, the distinction doesn't matter. However, it is important to remember this.
However, when you try to debug things, especially when using tools like Chrome's console.log, it can look like things are scrambled because console.log itself is asynchronous (if it were synchronous, it would freeze your script on a long function).
You can see this yourself if you output something like this:
var input = document.createElement('input');
input.setAttribute('value', 0);
function inc() {
input.setAttribute('value', parseInt(input.getAttribute('value'))+1);
console.log(input);
if (parseInt(input.getAttribute('value')) < 100) {
setTimeout(inc, 10);
}
}
inc();
JSFiddle: http://jsfiddle.net/c2PnP/
What this script does it creates an input element, then every 10 milliseconds, it increments the value of input, then outputs the input element. It repeats 100 times (until value = 100).
If you look at your console, you'll notice that some of the values will be duplicated, it won't be a smooth progression. For example, on a run I just did, I see 5 inputs with a value of "100" and gaps for the missing numbers. This is because console.log is running asynchronously and only outputting when it gets the gap to do so.
(Note: If you have a super fast computer, you may need to decrease the time to something smaller, like 1, and/or increase the number of iterations to a bigger number).
John Resig covered this well. Summarizing:
"JavaScript can only ever execute one piece of code at a time (due to
its single-threaded nature)... when an asynchronous event occurs (like
a mouse click, a timer firing, or an XMLHttpRequest completing) it
gets queued up to be executed later.... After the initial block of
JavaScript finishes executing the browser immediately asks the
question: What is waiting to be executed? The browser then picks one
and executes it immediately. The [rest] will wait until the next
possible time, in order to execute."
I'm having a hard time understanding the logic behind the setTimer method in javascript.
<html><head>
<script>
function Timer () {
var today = new Date();
var h = today.getHours();
var m = today.getMinutes();
var s = today.getSeconds();
document.getElementById('show').innerHTML=h+":"+m+":"+s;
t = setTimeout("Timer()", 1000);
}
</script>
</head>
<body onload="Timer()">
<div id="show"></div>
</body></html>
setTimeout is used to delay a function/method execution. Then why it is being used in a real-time clock?
t = setTimeout("Timer()", 1000);
This part is confusing.
The clock is recursively calling itself, after the elapsed period of time.
Making a real-time clock is impossible in JS.
Because of how JS engines work, if you put Timer in a loop, to run for an infinite period of time, you'd never see the time update on the screen (as changes aren't drawn to the window until a function finishes and there's a gap in the program).
Also, inside that infinite-loop, it would be impossible to do anything else with the page (even closing it), because JS can only do one thing at a time, so it can't listen to any of the user's clicking until it's done with this loop.......
So that's what the setTimeout is for.
Timer is the function which acts as the clock.
Inside of the Timer function, at the end when all of the work is done, it's telling setTimeout to wait 1 second (1000ms) and then to call a function called Timer.
Timer just so happens to be the same function. But setTimeout doesn't know that, and doesn't care.
The t in this case is largely useless. setTimeout will return a number -- like taking a number at the doctor's office.
If, before you go through with it, you decide to back out, you can call clearTimeout(t); and it'll skip over that call (in this case, it would stop calling the clock).
There are a few bad-practices in here, that I figure I should mention, so that you can try not to copy them in your own practice.
First:
Pass setTimeout a reference to a function, and not a string...
var runThisFunction = function () { console.log("It's the future!"); },
time_to_wait = 250;
// DON'T DO THIS
setTimeout( "runThisFunction()", 250 );
// DO THIS
setTimeout( runThisFunction, 250 );
The difference is that setTimeout will run that string through eval, which can be a huge security concern depending on what you're trying to do.
The second problem is setting a random global variable, t... ...and hoping to use that as a solution.
First, in a couple of years, JS engines are going to start yelling at people for doing that stuff. Second, it's a huge hole, because any part of any app on that page could then overwrite t, or you could be relying on t somewhere else in your script, but every 1000ms, it gets written over with a new number.
Instead, they probably should have used a Timer.start(); and Timer.stop(); setup.
Your code:
t = setTimeout("Timer()", 1000);
The first thing you should know is that it's considered bad practice to put the first parameter in a string -- it should be the function name, unquoted, and without brackets, like so:
t = setTimeout(Timer, 1000);
That aside, your question about why it's being used to display a clock:
The use of setTimeout() inside the Timer() function to call itself is a common Javascript pattern to get a function to be called repeatedly. setTimeout() itself only triggers the function to be called a single time, after the given period of time has elapsed, so for a repeating event it needs to be re-triggered every time.
Since the setTimeout call is inside the Timer() function, it won't be set until Timer() is called the first time by some other means. This is where the body onload comes in.
As you suspect, setTimeout() isn't an accurate method for guaranteeing that a function will be called after exactly a given amount of time. Javascript is not multi-threaded, so any event handlers that are triggered must wait for any other code that is running at the same time. If something else is running slowly, this may cause your timer not to be triggered at exactly the moment it wants to be.
However, this isn't really a problem for your clock , because the clock is setting itself to the actual system time rather than relying on the setTimeout loop to keep itself in sync; the setTimeout loop is simply being used to make sure the display is updated (approximately) once a second. If it isn't actually quite exactly once a second, it doesn't really matter.
I hope that helps explain things a bit better.
When the Timer() function is called, it schedules itself to be run again one second later. The end result is once every second, Timer() updates the show element with the current time. (I have no idea why it's assigned to t, unless t is used in some other code on the page.)
The line starts The function again after one second.