Confusion between a general global variable and object as a variable in javaScript

Question

Below is my code. When I'm calling myDate variable outside of myTime() function to create a Date() object, the setInterval() function don't fire but when it's inside myTime() function. As I know myDate variable is a global variable and it should work same inside or outside of the function. But why here the setInterval() method not firing while I'm creating the Date() object outside of the function? Experts explain the things. TIA

var myDate = new Date();
    function myTime(){
        document.getElementById('text').innerHTML = myDate.getHours() + ":" + myDate.getMinutes() + ":" + myDate.getSeconds();
    }
    setInterval(myTime, 1000);

score 1 · Answer 1 · answered Jan 21 '18 at 22:30

1

You are right: myDate variable is accessible from myTime() function, but if it is declared outside of it, its value doesn't change. In the following snippet i've created one more external variable i to show that it is accessible from within the function:

var myDate = new Date();
var i = 0;
function myTime() {
    document.getElementById('text').innerHTML = myDate.getHours() + ":" + myDate.getMinutes() + ":" + myDate.getSeconds() + " (" + i++ + ")";
}
setInterval(myTime, 1000);

<div id="text"></div>

On the other hand, if you declare myDate variable inside the function, it will be created each time function is called - every second:

function myTime() {
    var myDate = new Date();
    document.getElementById('text').innerHTML = myDate.getHours() + ":" + myDate.getMinutes() + ":" + myDate.getSeconds();
}
setInterval(myTime, 1000);

<div id="text"></div>

answered Jan 21 '18 at 22:30

Kirill Simonov

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updated the question. maybe it will help you to better understand. – Ashikur Rahman Jan 21 '18 at 22:35
@AshikurRahman why do you think that `setInterval()` doesn't fire? – Kirill Simonov Jan 21 '18 at 22:37
If you run the first snippet you will se that `i` changes every second – Kirill Simonov Jan 21 '18 at 22:39
yes, but then the seconds not changing as interval time? I'm new in programming – Ashikur Rahman Jan 21 '18 at 22:44
@AshikurRahman _“but if it is declared outside of it, its value doesn't change”_. – Sebastian Simon Jan 21 '18 at 22:51
Right, the seconds not changing because you don't change them - you create this variable only once and it holds the same value – Kirill Simonov Jan 21 '18 at 22:52

Confusion between a general global variable and object as a variable in javaScript

1 Answers1