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Merge Two Objects But Only Existing Properties

When you're coding, there may come a time when you need to combine two objects in JavaScript. But what if you only want to merge the existing properties between the objects, while ignoring any new properties that might be present? In this article, we'll explore a practical solution to this common scenario.

One way to merge two objects while keeping only the existing properties is by using the `Object.keys()` method along with `reduce()` and `Object.assign()`. Let's break down the process step by step.

First, we need to define the function that will handle the merging of the objects:

Javascript

function mergeObjects(obj1, obj2) {
    return Object.keys(obj1).reduce((acc, key) => {
        if (obj2.hasOwnProperty(key)) {
            acc[key] = obj2[key];
        }
        return acc;
    }, {});
}

In this function, we are utilizing `Object.keys()` to iterate over the keys of `obj1`. For each key, we are checking if `obj2` has the same key using `hasOwnProperty()`. If the common key exists in `obj2`, we assign its value to the accumulator object `acc`.

Let's illustrate this with an example:

Javascript

const obj1 = { a: 1, b: 2, c: 3 };
const obj2 = { b: 20, c: 30, d: 40 };

const mergedObj = mergeObjects(obj1, obj2);

console.log(mergedObj);
// Output: { b: 20, c: 3 }

In this example, only the properties that exist in both `obj1` and `obj2` are merged, meaning 'b' and 'c' in this case.

It's essential to note that this approach only considers properties within the first object (`obj1`) and their counterparts in the second object (`obj2`). Any additional properties present only in `obj2` will not be included in the merged object.

By using this method, you can have better control over the merging process and ensure that only existing properties are retained, providing a cleaner and more predictable outcome for your JavaScript objects.

In conclusion, merging two objects in JavaScript while preserving only the existing properties can be achieved efficiently using the `Object.keys()`, `reduce()`, and `Object.assign()` techniques. This approach allows you to merge objects selectively, maintaining key properties while excluding any additional ones. Consider incorporating this method into your coding arsenal for a streamlined object merging process.

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