What is the difference between semicolons in JavaScript and in Python?

JavaScript is designed to "look like C", so semicolons are part of the culture. Python syntax is different enough to not make programmers feel uncomfortable if the semicolons are "missing".

Aside from the syntactical issues, it is partly cultural. In Python culture any extraneous characters are an anathema, and those that are not white-space or alphanumeric, doubly so.

So things like leading $ signs, semi-colons, and curly braces, are not liked. What you do in your code though, is up to you, but to really understand a language it is not enough just to learn the syntax.

Semicolons in Python are totally optional (unless you want to have multiple statements in a single line, of course). I personally think Python code with semicolons at the end of every statement looks very ugly.

Now in Javascript, if you don't write a semicolon, one is automatically inserted¹ at the end of line. And this can cause problems. Consider:

function add(a, b) {
  return
    a + b
}

You'd think this returns a + b, but Javascript just outsmarted you and sees this as:

function add() {
  return;
    a + b;
}

Returning undefined instead.

¹ See page 27, item 7.9 - Automatic Semicolon Insertion on ECMAScript Language Specification for more details and caveats.

This had me confused for the longest time. I thought it was just a cultural difference, and that everyone complaining about semicolon insertion being the worst feature in the language was an idiot. The oft-repeated example from NullUserException's answer didn't sway me because, disregarding indentation, Python behaves the same as JavaScript in that case.

Then one day, I wrote something vaguely like this:

alert(2)
(x = $("#foo")).detach()

I expected it to be interpreted like this:

alert(2);
(x = $("#foo")).detach();

It was actually interpreted like this:

alert(2)(x = $("#foo")).detach();

I now use semicolons.

JavaScript will only¹ treat a newline as a semicolon in these cases:

• It's a syntax error not to.
• The newline is between the throw or return keyword and an expression.
• The newline is between the continue or break keyword and an identifier.
• The newline is between a variable and a postfix ++ or -- operator.

This leaves cases like this where the behaviour is not what you'd expect. Some people² have adopted conventions that only use semicolons where necessary. I prefer to follow the standard convention of always using them, now that I know it's not pointless.

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¹ I've omitted a few minor details, consult ECMA-262 5e Section 7.9 for the exact description.
²Twitter Bootstrap is one high-profile example.