The JavaScript expression += is just a shorthand for writing:
value = value + something.

For example:

// let's say you have initial value 5
let someValue = 5;

// you can use this shorthand
someValue += 1;

// and this is equivalent to
someValue = someValue + 1;

// in both cases the end result is 6

However, make sure your code is always immutable because there is almost never a good reason to change the existing values in JavaScript. And it usually leads to bugs.

Hope this helps.

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