When working with arrays in JavaScript, you may often need to add an object to an existing array. There are several ways to achieve this, but the two most common methods are using the push() and splice() methods.

The push() method is the simplest way to add an object to the end of an array. Here's an example of how to use it:

```javascript

let arr = [{name: 'Alice'}, {name: 'Bob'}];

let obj = {name: 'Charlie'};

arr.push(obj);

console.log(arr); // Output: [{name: 'Alice'}, {name: 'Bob'}, {name: 'Charlie'}]

```

As you can see, the push() method adds the object 'obj' to the end of the 'arr' array.

Alternatively, if you want to add an object at a specific index in the array, you can use the splice() method. Here's an example of how to do that:

```javascript

let arr = [{name: 'Alice'}, {name: 'Bob'}, {name: 'Eve'}];

let obj = {name: 'Charlie'};

arr.splice(2, 0, obj);

console.log(arr); // Output: [{name: 'Alice'}, {name: 'Bob'}, {name: 'Charlie'}, {name: 'Eve'}]

```

In this example, the splice() method adds the object 'obj' at index 2 in the 'arr' array, pushing the existing elements to the right.

It's important to note that the push() method modifies the original array and returns the new length of the array, while the splice() method can both add and remove elements from an array.

In addition to these methods, you can also use the spread operator (...) to add an object to an array, like this:

```javascript

let arr = [{name: 'Alice'}, {name: 'Bob'}];

let obj = {name: 'Charlie'};

arr = [...arr, obj];

console.log(arr); // Output: [{name: 'Alice'}, {name: 'Bob'}, {name: 'Charlie'}]

```

This method creates a new array with the existing elements from 'arr' and adds the object 'obj' at the end.

In conclusion, adding an object to an array in JavaScript is a common task, and there are multiple ways to achieve it. Whether you choose the push(), splice(), or spread operator method, it's important to understand the implications of each and use them appropriately in your code.