What does the '+=' operator do in Javascript?

Question

The operator += is one that I have seen a lot in JavaScript and I have absolutely no idea what it does. Can someone simply define what it is and how I would use it in JavaScript? I found this example and I don't understand what is happening.

var message = "";
if (document.getElementById("x") == "") {
    message += "You must enter your last name."
}

You would get an answer faster if you had just tested it yourself. — Justin Wood, Feb 09 '14 at 22:55
@JustinWood Yes Hello, I know that but I just want to know the theory behind it. I was just slightly confused. — Jack Davis, Feb 09 '14 at 22:56

score 3 · Answer 1 · edited Feb 09 '14 at 22:59

3

It is an assignment shortcut operator

x += y means x = x + y

See the Shorthand Assignment operator table.

In javascript the + operator works on strings as well by concatenating them.. So in your case it will add to the message variable the string on the right side..

edited Feb 09 '14 at 22:59

user2864740

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answered Feb 09 '14 at 22:55

Gabriele Petrioli

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score 2 · Answer 2 · answered Feb 09 '14 at 22:55

2

It does the same thing as it does in C and Java and many other languages:

x += y;

is a shorthand for

x = x + y;

answered Feb 09 '14 at 22:55

Paul

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Not sure why the downvote. This is true (excluding some oddity cases), although a reference would be nice. – user2864740 Feb 09 '14 at 22:56

score 0 · Answer 3 · answered Feb 09 '14 at 22:54

In that case it means message = message + "You must enter your last name.". There might be some edge cases when variables are of different types and are automatically transformed to other types (which might produce unexpected results), but this does not seem to be the case.

What does the '+=' operator do in Javascript?

3 Answers3

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