Inconsistent behavior of JavaScript's + operator

Question

Why does this

2 + + 3

return 5, but this

'2837363' + + '/'

returns

"2837363NaN"? Even the '/' got lost.

Why would a programming language accept this syntax without throwing a syntax error? When does it assume, the empty place evaluates to 0 (1st example) and when to NaN (2nd example)?

Aaahh... javascript :) You should love it to understand it.

José Cabo
– José Cabo

2015-08-06 10:51:00 +00:00
Commented Aug 6, 2015 at 10:51 — José Cabo
– José Cabo, Commented Aug 6, 2015 at 10:51

AmmarCSE · Accepted Answer · 2015-08-06 11:02:10Z

4

Prepending a variable with a + implies type coercion to a number type.

+ 5 => 5
+ '5' => 5
+'a' => NaN
+'/' => NaN

When you do +'/' the result is NaN

edited Aug 6, 2015 at 11:02

answered Aug 6, 2015 at 10:49

AmmarCSE

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5 Comments

Liglo App Over a year ago

You're right, now I can see: 2 + + 3 evaluates to 2 + (+3), 2nd example similarily. Thanks

hindmost Over a year ago

That's called Unary plus

Liglo App Over a year ago

I know the unary plus, but I didn't recognize it here. I thought, my expression would be something like 2 + SOMETHING + 3 but of course it isn't, which now I can see.

Felix Kling Over a year ago

It converts the value to a number, not an int specifically.

AmmarCSE Over a year ago

+'0.1322' => 0.1322 your right, thanks for pointing out @FelixKling

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