What is the purpose of the += operator in JavaScript?

The `+=` operator in JavaScript is called the "addition assignment" operator. It is used to add the value on the right-hand side of the operator to the current value of the variable on the left-hand side, and then assign the result back to the variable. In other words, it is a shorthand way of writing `variable = variable + expression`.

Here's an example to illustrate the usage of the `+=` operator:

let num = 5;

// Using the += operator to add 3 to the current value of 'num'

num += 3;

console.log(num);  // Outputs: 8

In this example, `num += 3` is equivalent to `num = num + 3`. The value of `num` is incremented by 3, and the result (8) is assigned back to the variable `num`.

The `+=` operator is commonly used for concise and readable code, especially when you want to update the value of a variable by adding a specific value to it. It is not limited to numbers; it can also be used with strings for concatenation:

let message = "Hello";

// Using the += operator to concatenate a string to the current value of 'message'

message += ", world!";

console.log(message);  // Outputs: Hello, world!

In this case, `message += ", world!"` appends the string ", world!" to the current value of `message`.