Surprising outcomes when working with JavaScript date and time functionalities

Check out the code snippet below:

let startDate = new Date();
const lastDate = new Date();
lastDate.setMonth(11,31);

const timeArray = [];

while(startDate <lastDate){
  timeArray.push(startDate);
  console.log(startDate)
  startDate =new Date(startDate.setMonth(startDate.getMonth()+1)) 
}
console.log('==================================');
console.log(timeArray)

However, the resulting output displays:

Thu Nov 03 2022 09:12:03 GMT+0530 (India Standard Time)
Sat Dec 03 2022 09:12:03 GMT+0530 (India Standard Time)
==================================
[
Sat Dec 03 2022 09:12:03 GMT+0530 (India Standard Time)
Tue Jan 03 2023 09:12:03 GMT+0530 (India Standard Time)
]

After pushing 'startDate' into the array and checking it immediately, it appears as expected. However, when logging the time array itself, the dates seem to have shifted. Can anyone provide an explanation for this behavior?

javascriptarraysdatetimewhile-loop

Answer №1

It appears that the code snippet below is what you intended to write. In this script, you are cloning the startDate and then making modifications to the cloned date rather than altering the original.

let startDate = new Date();
const lastDate = new Date();
lastDate.setMonth(11,31);

const timeArray = [];

while(startDate <lastDate){
  timeArray.push(startDate);
  console.log(startDate)
  startDate = new Date(startDate); // uncouple it from the other date
  startDate.setMonth(startDate.getMonth()+1)
}
console.log('==================================');
console.log(timeArray)

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