使用setInterval时，变量不会在全局范围内更新

Question

I want to write a function that will return a variable that I can use inside another function. Another function should use time comparison between current time and some other selected time. I figured I could make time variable accessible in global scope by using return and property and update it by using setInterval() .

Unfortunately when I "import" the variable to another function, the time variable isn't updated.

Why doesn't console.log('time cool outside' + obj.debug ); return the same value as console.log('time cool inside' + timeCool); ?

function checkTime(i) {
    if (i < 10) {i = "0" + i};  // add zero in front of numbers < 10
    return i;
  }

function timeFunction() {
  let today = new Date();
  let hours = today.getHours();
  let minutes = today.getMinutes();
  minutes = checkTime(minutes);
  hours = checkTime(hours);
  let stringHours = hours.toString();
  let stringMinutes = minutes.toString();
  time = stringHours + stringMinutes;
  let timeCool = parseInt(time);
  console.log('time cool inside' + timeCool);
  return {
    debug: timeCool,
  };

}
let obj = timeFunction();
setInterval(timeFunction, 5000);
function a() {console.log('time cool outside' + obj.debug );}
setInterval(a, 5000);

Answer 1

in the lines

let obj = timeFunction();
setInterval(timeFunction, 5000);

you create a global object obj and set it to the output of timeFunction . Then with setInterval you call timeFunction periodically, but never set obj to its output. Thus, when a accesses obj it never changes

Answer 2

Let's see what you are doing here:

let obj = timeFunction();

Here you call the timeFunction and store it's result into obj .

setInterval(timeFunction, 5000);

Here you call timeFunction every 5 seconds. The return value is dismissed.

function a() {console.log('time cool outside' + obj.debug );}

Here you have a new function that references the obj variable defined before. obj is the return value of the first call to timeFunction and has never been changed, so it will always have the same value, no matter how often you call any of those two functions.

If you want to update obj you need to do so inside timeFunction or continue to set it to the return value of timeFunction .

Answer 3

Thank you guys for helping. Indeed I didn't notice that I didn't update "obj" variable.

After implementing this:

let obj;
function objUpdate() {
obj = timeFunction();
}
setInterval(objUpdate, 1000);

Everything looks like I wanted :) Thank you all!