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Odd behaviour when adding Date[] into Date[][] in Typescript/Javascript [duplicate]

So I need to calculate a set of time intervals like this: [8:00 - 8:10] I decided to use a Date type for this and wrote this piece of code to test things:

let todaysDate = new Date();
let startTime: Date = new Date(todaysDate.setHours(5, 0, 0));
let endTime: Date = new Date(todaysDate.setHours(6, 0, 0));
let start : number = startTime.getTime();
let interval: number = 600000;

let firstTime = new Date(startTime.getTime());
let secondTime = new Date(startTime.getTime() + interval);

let timeIntervalsArray: Date[][] = [];

while (secondTime.getTime() < endTime.getTime()) {
    firstTime.setTime(start);
    secondTime.setTime(firstTime.getTime() + interval);

    const times: Date[] = [firstTime, secondTime];
    timeIntervalsArray.push(times);
    start += interval;
}

console.log(timeIntervalsArray);

But when logging the timeIntervalsArray variable I get that all variables in array are the same:

[LOG]: 
[[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"], 
[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"], 
[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"], 
[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"], 
[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"], 
[Date: "2023-04-22T05:50:00.722Z", Date: "2023-04-22T06:00:00.722Z"]]

I managed to fix this behavior by changing this line: const times: Date[] = [firstTime, secondTime]; to this: const times: Date[] = [new Date(firstTime), new Date(secondTime)];

But I don't understand how this works, as if I log as such:

while (secondTime.getTime() < endTime.getTime()) {
    firstTime.setTime(start);
    secondTime.setTime(firstTime.getTime() + interval);

    //Logging firstTime
    console.log(firstTime);
    console.log('As new date', new Date(firstTime));
    //

    const times: Date[] = [firstTime, secondTime];
    timeIntervalsArray.push(times);
    start += interval;
}

I get the same Data object:

[LOG]: Date: "2023-04-22T05:00:00.718Z" 
[LOG]: "As new date",  Date: "2023-04-22T05:00:00.718Z"

What is more odd - when logging the timeIntervalsArray itself:

while (secondTime.getTime() < endTime.getTime()) {
    firstTime.setTime(start);
    secondTime.setTime(firstTime.getTime() + interval);

    //Logging timeInterval
    console.log(timeIntervalsArray);
    //
    
    const times: Date[] = [firstTime, secondTime];
    timeIntervalsArray.push(times);
    start += interval;
}

Fist iteration : [LOG]: [] -> array is empty 
Second iteration : 
[LOG]: [[Date: "2023-04-22T05:10:00.241Z", Date: "2023-04-22T05:20:00.241Z"]] 
-> array has one element - [Date: "2023-04-22T05:10:00.241Z", Date: "2023-04-22T05:20:00.241Z"]

Third iteration : 
[LOG]: [[Date: "2023-04-22T05:20:00.241Z", Date: "2023-04-22T05:30:00.241Z"], 
[Date: "2023-04-22T05:20:00.241Z", Date: "2023-04-22T05:30:00.241Z"]] 
-> the fist element has been changed to 
[Date: "2023-04-22T05:20:00.241Z", Date: "2023-04-22T05:30:00.241Z"]
but it was 
[Date: "2023-04-22T05:10:00.241Z", Date: "2023-04-22T05:20:00.241Z"]

Can somebody explain this behavior because I am out of ideas...

Via Active questions tagged javascript - Stack Overflow https://ift.tt/C9h0mg1
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