Weird behavior when using + + in JavaScript

Question

Everybody knows the basic concatenation of two strings in JavaScript:

> "Hello " + "World!"
'Hello World!'

But what happens if we use + + instead of +? I just encountered the following weird behavior:

> "Hello " + + "World!"
'Hello NaN'
> "Hello " + + ""
'Hello 0'

From the examples above, I can see that the second string is converted into number. So, passing an object having valueOf property as function, the value returned by that function will be converted.

> "Hello " + + ({valueOf: function () {return 1; }})
'Hello 1'

As expected, it shows "Hello 1".

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Why is the second string converted in Number? Why not throwing a syntax error or so?

Answer 1

The second + is the unary plus operator whose purpose is to convert its operand to a number.

The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

"Hello " + + "World!"

can only be parsed as

"Hello " + (+ "World!")

which is

"Hello " + NaN

hence your result.

This unary operator is very useful in JavaScript and is one of the most common ways to convert from something which may be a number or a string to a number. It also has the advantage over parseFloat that it's not ridiculously tolerant (parseFloat("7up") would return 7) which makes it an easy way to see if a string is the representation of a number (just test s==+s).

Answer 2

Great question. This is because JavaScript has a unary + operator (similar to the unary - as in, ex, -1, which is parsed to - (1)): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Unary_plus_(.2B)

Thus "foo" + + "bar" is parsed to "foo" + (+ "bar") (or if you prefer a lisp-style parse tree: (+ "foo" (+ "bar")).

The unary + converts its operand to a number, or NaN if the operand isn't a sensible number:

> +"42"
42
> +"foo"
NaN

Finally, adding a number (including NaN) to a string results in a string concatenation.

Thus "foo" + +"bar" can be expanded:

"foo " + +"bar"
"foo " + NaN
"foo NaN"

Answer 3

Here + is a unary plus operator Which tries to convert the operator in number & works as follows

+"" //return 0
+"Some" //returns NaN

So here

"Hello " + + "World!" 

will be

"Hello " + (+ "World!")
"Hello " + NaN
"Hello Nan"