set Interval in javascript

Question

I am beginner in javascript. There is strange thing in javascript and I feel stupid

//First statement
var myVar = "Hello";
function hello() {
    document.getElementById("demo").innerHTML = myVar;
}

//second statement
var myVar = setInterval(myTimer, 1000);
function myTimer() {
    document.getElementById("demo").innerHTML = new Date().toLocaleTimeString();
}

Why the second function work without invoked it ? Unlike the first ? This caused a lot of problems to me !

You are passing the function `myTimer` to `setInterval`. `setInterval` calls the function every 1000ms. So while it is not *you* who directly calls the function, it is still called. — Felix Kling, Dec 09 '16 at 14:05
You invoke it here : `var myVar = setInterval(myTimer, 1000);` — jean-max, Dec 09 '16 at 14:05
It's actually the inverse. You're calling the second function but not the first. — Chris, Dec 09 '16 at 14:05
@Jean-maximeBouloc: no, that's merely *passing* the function to `setInterval`. — Felix Kling, Dec 09 '16 at 14:06
@AmrElgendy, I don't see where `hello()` is being called. I do, however, see where `myTimer()` is being called. — Chris, Dec 09 '16 at 14:10
Are you saying `hello` is somehow invoked? Not in your example. Please [edit] your question to clarify it. — Felix Kling, Dec 09 '16 at 14:10
Of course the function "doesn't work". You are not calling it anywhere, nor are you doing anything that gets it invoked. Isn't it obvious that the difference between the first and the second example is that you are passing the second function to `setInterval`, but that you are not doing anything with the first function? — Felix Kling, Dec 09 '16 at 14:14
I'm sorry if I write anything wrong because I speak a little English — AmrElgendy, Dec 09 '16 at 14:16
Ok all things good , but you mean that statement in variable excecute Automatically ??? — AmrElgendy, Dec 09 '16 at 14:18
Not quite, no. The whole point of `setInterval` is to call the function you are passing to it. That's what `setInterval` is implemented to do. — Felix Kling, Dec 09 '16 at 14:20
Maybe also important to know in this case: Functions are *values*, just like any other value. That means you can pass functions to other functions. That's what happens here with `setInterval`: it receives a function as argument and knows that it has to call that function ever x ms (the value of the second argument). — Felix Kling, Dec 09 '16 at 14:31

Felix Kling · Accepted Answer · 2016-12-09T14:26:02.550

Why the second function work without invoked it ? Unlike the first ?

You are passing the function myTimer to setInterval. setInterval calls the function every 1000ms. So while it is not you who directly calls the function, it is still called (by setInterval). setInterval's whole purpose is to call the function that you pass to it as argument.

In contrast, you are not doing anything with hello. You are neither calling it directly, nor are you passing it to any other function that could call it.

score 1 · Answer 2 · answered Dec 09 '16 at 14:22

From what I can gather from the comments, you seem to be confused as to why the myTimer() function works. Here's a brief explanation:

On this line

var myVar = setInterval(myTimer, 1000);

you are calling the setInterval() function. That function takes 2 parameters, which you have defined. The first one is a function; the code to be executed. The second one is the delay between each execution of said function.

On the next line, you have declared the variable myTimer to be a function which is executed with the setInterval.

Have a look at the MDN documentation for details. Specifically, it says:

var intervalID = window.setInterval(func, delay)

The parameters are defined as:

func: A function to be executed every delay milliseconds.

and

delay: The time, in milliseconds (thousandths of a second), the timer should delay in between executions of the specified function or code.

Yes , This is part of my question – AmrElgendy Dec 09 '16 at 14:26 — AmrElgendy, Dec 09 '16 at 14:26
Now I understand all , Thanks – AmrElgendy Dec 09 '16 at 14:26 — AmrElgendy, Dec 09 '16 at 14:26
@AmrElgendy, nice. glad I could help. – Chris Dec 09 '16 at 14:27 — Chris, Dec 09 '16 at 14:27

set Interval in javascript

2 Answers2