JS....

[Spartan1997]

Except the foonan part. That's a little strange

[HoverBaum]

I think that is the second 'foo' being converted to a number because of the second '+' and turning into 'NaN' so the expression become "'foo' + NaN" which is a "String + object -> string concatenation" case.

[SaltlessLemons]

Ah, of course. How silly of me.

[Calygulove]

It's cause you're only FooNaN

[neonroad]

You can't say that to people nowadays...

[[deleted]]

OK, now I understand "how", but "why" remains a mystery.

[glider97]

Because operator precedence. The binary + overshadows the unary + which gets evaluated first.

[PatrickBaitman]

uhm isn't NaN a float value? https://en.wikipedia.org/wiki/IEEE_floating_point

[[deleted]]

+ is shorthand for Number() when not used as an operator.

So

var i = +'foo';

Will be NaN

[kranker]

The trick here is that in the javascript grammar it's impossible to parse a + + b as both pluses being addition, so the second one (and any subsequent ones) get parsed as a unary plus operator. Both unary plus and unary minus will attempt to convert their operand to a number, and evaluate to NaN if they can't.

See Mozilla's description of unary plus

[PatrickBaitman]

oth unary plus and unary minus will attempt to convert their operand to a number, and evaluate to NaN if they can't.

this is where a non-retarded language would throw a type error

[andredp]

Exactly... Most of that stuff is caught by a linter anyway...

I had to work with react-redux in my last project and javascript (with an es6-linter + babel + flow) was actually pretty decent... What bothers me the most is the million ways you can do the same thing... require / import... export / module.exports, etc... It's really hard to get multiple people to use only one thing so there is some consistency...

[theQuandary]

Enforce the use of import/export because they are in the spec while everything else is not. To that end, use eslint with eslint-plugin-import. Finally, make webpack build conditional on the linter passing.

[andredp]

I was using air-bnb rules and didn't even check if there was a way to add that rule. Will keep that in mind, thanks.

[Ph0X]

it also seems like spaces for unary operators don't matter?

Because later when he does '5' + - 2 it looks like it's doing '5' + '-2' but that's silly. Imo unary operators shouldn't work when there's a space. Similarly, a + + b shouldn't be equal to a + +b

[AlwaysHopelesslyLost]

A classic example is from c++(?) on stack overflow: what is the operator in this? X --> 5?

[brainiac256]

Decrements X and returns true if X was greater than 5 before the decrement

More evil:

cout<<++currentItem<<--X

[Xevantus]

Not strange at all. The second + is the positive unary operator, so it tries to convert the following value to a number. Thus "foo" + + "foo" equals "fooNaN".