Array of Dates

[シモダフランジポータルサイト]

I'm afraid this beginner question will bore the reader, but allow me to ask about the next function, which outputs the array filled with the same dates.

Where is incorrect?

function Test_DateArray(){

  let MyDate = new Date();

  let MyDates = [];

  for(let i= 1; i < 5; i++){

    MyDate.setDate(MyDate.getDate() + 1);

    MyDates.push(MyDate);

    Logger.log(MyDates);

  }

}

[DME]

Good beginner question 👍 — this one actually teaches a very important JavaScript concept.

❌ What’s going wrong?

You are pushing the SAME Date object reference into the array again and again.

👉 Date is an object, not a value type.
So when you do this:

MyDates.push(MyDate);

you’re not saving a new date — you’re saving a reference to the same object.
Later, when MyDate changes, all array entries point to that updated date, so they look identical.

Think of it like this 🧠
📌 You’re putting the same diary into the cupboard 4 times — not 4 different diaries.

---

🔍 Your code flow (simplified)

1. Create one Date object → MyDate

2. Modify it in each loop (setDate)

3. Push same object into array each time

4. Result → array shows same final date everywhere ❌

---

✅ Correct way: push a COPY of the Date

You must create a new Date instance for each entry.

✔️ Fix #1 (Recommended)

function Test_DateArray(){
  let MyDate = new Date();
  let MyDates = [];

  for (let i = 1; i < 5; i++) {
    MyDate.setDate(MyDate.getDate() + 1);
    MyDates.push(new Date(MyDate)); // ✅ clone date
  }

  Logger.log(MyDates);
}

---

✔️ Fix #2 (Cleaner approach)

Avoid mutating the same date at all:

function Test_DateArray(){
  let MyDates = [];

  for (let i = 1; i < 5; i++) {
    let d = new Date();
    d.setDate(d.getDate() + i);
    MyDates.push(d);
  }

  Logger.log(MyDates);
}

---

🧠 Key Concept (Very Important)

Type	Behavior
Number / String	Copied by value
Object / Array / Date	Copied by reference

So always remember:

Push objects by cloning, not reusing 🔁

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Deepak Bansode DME +91-7038193209 produ...@jalaramfeeds.com www.jalaramfeeds.com

[Kildere S Irineu]

This is a very good beginner question, and it points to an essential JavaScript concept that also applies in Google Apps Script.

---

❌ What is wrong in the original function?

function Test_DateArray(){
  let MyDate = new Date();
  let MyDates = [];
  for (let i = 1; i < 5; i++){
    MyDate.setDate(MyDate.getDate() + 1

);
    MyDates.push(MyDate);
    Logger.log(MyDates);
  }
}

The issue is not the loop or setDate.
The issue is object references.

Key problem

Date is an object, not a primitive value.

When this line runs:

MyDates.push(MyDate);

The array stores a reference to the same Date object, not a snapshot of its value.

So:

• There is only one Date object

• It is modified on each iteration

• All array entries point to that same object

• Result → all dates end up identical (the final value)

---

🧠 Important JavaScript rule (very relevant in Apps Script)

Type	Behavior
number, string, boolean	copied by value
Object, Array, Date	copied by reference

---

✅ Correct solution #1 — Clone the Date (recommended)

Create a new Date instance before pushing it into the array.

function Test_DateArray(){
  let MyDate = new Date();
  let MyDates = [];

  for (let i = 1; i < 5; i++){
    MyDate.setDate(MyDate.getDate() + 1);
    MyDates.push(new Date(MyDate)); // clone

  }

  Logger.log(MyDates);
}

✔ Each array element now has its own Date object
✔ Dates remain independent

---

✅ Correct solution #2 — Avoid mutating the same object

Cleaner and often safer:

function Test_DateArray(){
  let MyDates = [];

  for (let i = 1; i < 5; i++){
    let d = new Date();
    d.setDate(d.getDate() + i);
    MyDates.push(d);
  }

  Logger.log(MyDates);
}

✔ No shared references
✔ Easier to reason about
✔ Preferred in most Apps Script projects

---

🔎 Common beginner mistake (very common!)

• Reusing objects inside loops

• Expecting objects to behave like numbers or strings

• Logging inside the loop (can hide the real issue)

💡 Tip: log after the loop when debugging arrays.

---

✅ Best practice takeaway

• Never push mutable objects (Date, Object, Array) into arrays without cloning

• Prefer immutability when possible

• This concept appears everywhere: Sheets data, Calendar events, Drive files, JSON objects

This question is not boring at all — it’s foundational 👍

To view this discussion visit https://groups.google.com/d/msgid/google-apps-script-community/CADK4hK%3Dni%2BeonwRpgn4Srpj8vkuDVAHTx95H4aMtRYSaYqk2dw%40mail.gmail.com.