Great question! Let's break down exactly what's going on here—it all boils down to how JavaScript handles arrays vs. key-value storage.

The Root Cause of Your Issue #

What's Wrong with Approach 1? #

In JavaScript, arrays are specialized objects built for ordered, numerically indexed elements. When you do fileData[data["uniquePropertyName"]] = data, you're not adding an element to the array—you're attaching a custom property to the array object itself.

The critical problem here: adding custom properties doesn't update the array's length property. So even though those key-value pairs exist on the array object, the array still acts like an empty array when you return it, serialize it to JSON, or view it in the console.

Here's a quick example to illustrate:

const arr = [];
arr["user123"] = { id: "user123", name: "Alice" };
console.log(arr); // Logs [] (empty array)
console.log(arr["user123"]); // Logs { id: "user123", name: "Alice" }
console.log(arr.length); // Logs 0

Why Does This Only Show Up in Large Datasets? #

With small datasets, some browsers or Node.js consoles will display the array's custom properties alongside the empty element list. But with large datasets, the console optimizes its output to only show the array's actual numerically indexed elements (which are still empty), so you don't see the hidden properties. The data is still there—it's just not visible in the console output.

Why Approach 2 Works #

In Approach 2, you're using push() to add new objects to the array. This is the correct way to add elements: each push() increases the array's length and adds the object as a numerically indexed element. That's why it works consistently regardless of dataset size—you're actually modifying the array's element collection, not just tacking properties onto the array object.

Fixes for Your Code #

If you need to map data by its uniquePropertyName, use a plain object or Map instead of an array—these tools are designed explicitly for key-value storage.

Option 1: Use a Plain Object #

async getModifiedData() { 
  const dbData = await dbdata.getData(); 
  const fileData = {};
  // Use forEach instead of map (map creates an unused new array)
  dbData.forEach(data => {
    fileData[data.uniquePropertyName] = data; 
  }); 
  return fileData; 
}

Option 2: Use a Map (Ideal for Non-String Keys or Easy Iteration) #

async getModifiedData() { 
  const dbData = await dbdata.getData(); 
  const fileData = new Map();
  dbData.forEach(data => {
    fileData.set(data.uniquePropertyName, data); 
  }); 
  return fileData; 
}

Summary #

Arrays are for ordered, numerically indexed collections. If you need to store data by a unique key, objects or Maps are the right tools—this avoids the confusion of hidden properties on arrays and ensures your data behaves as expected across all dataset sizes.

内容的提问来源于stack exchange，提问作者Naveen Kumar M