What does = +_ mean in JavaScript

Question

I was wondering what the = +_ operator means in JavaScript. It looks like it does assignments.

Example:

hexbin.radius = function(_) {
   if (!arguments.length)
       return r;
   r = +_;
   dx = r * 2 * Math.sin(Math.PI / 3);
   dy = r * 1.5;
   return hexbin;
};

Solution 1

r = +_;

• + tries to cast whatever _ is to a number.
• _ is only a variable name (not an operator), it could be a, foo etc.

Example:

+"1"

cast "1" to pure number 1.

var _ = "1";
var r = +_;

r is now 1, not "1".

Moreover, according to the MDN page on Arithmetic Operators:

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. [...] 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.

It is also noted that

unary plus is the fastest and preferred way of converting something into a number

Solution 2

It is not an assignment operator.

• _ is just a parameter passed to the function.

  hexbin.radius = function(_) {
                  //       ^ It is passed here
      // ...
  };
  

• On the next line r = +_; + infront casts that variable (_) to a number or integer value and assigns it to variable r

DO NOT CONFUSE IT WITH += operator

Solution 3

=+ are actually two operators = is assignment and + and _ is variable name.

like:

i = + 5;
or 
j = + i;
or 
i = + _;

My following codes will help you to show use of =+ to convert a string into int.
example:

y = +'5'
x = y +5
alert(x);

outputs 10

use: So here y is int 5 because of =+
otherwise:

y = '5'
x = y +5
alert(x);

outputs 55

Where as _ is a variable.

_ = + '5'
x = _ + 5
alert(x)

outputs 10

Additionally, It would be interesting to know you could also achieve same thing with ~ (if string is int string (float will be round of to int))

y = ~~'5'  // notice used two time ~
x = y  + 5
alert(x);

also outputs 10

~ is bitwise NOT : Inverts the bits of its operand. I did twice for no change in magnitude.

Solution 4

It's a sneaky one.

The important thing to understand is that the underscore character here is actually a variable name, not an operator.

The plus sign in front of that is getting the positive numerical value of underscore -- ie effectively casting the underscore variable to be an int. You could achieve the same effect with parseInt(), but the plus sign casting is likely used here because it's more concise.

And that just leaves the equals sign as just a standard variable assignment.

It's probably not deliberately written to confuse, as an experienced Javascript programmer will generally recognise underscore as a variable. But if you don't know that it is definitely very confusing. I certainly wouldn't write it like that; I'm not a fan of short meaningless variable names at the best of times -- If you want short variable names in JS code to save space, use a minifier; don't write it with short variables to start with.

Solution 5

It's not =+. In JavaScript, + means change it into number.

+'32' returns 32.

+'a' returns NaN.

So you may use isNaN() to check if it can be changed into number.

View more solutions

Comments

[Oleg]

Reminded me of the good old approach operator -->

[glglgl]

What now - =+ or +_? Please be clear about what you try to ask.

[Kobi]

There is no "=+" in the question. There's "= +", which is very different. + = would not have worked either, that's a syntax error - you cannot ignore spaces here.

[Pieter Witvoet]

The + here is a unary operator, with _ as its operand.

[Garfield]

It's just the negative or positive value as in maths (+10 - positive number), (-10 - negative number). No fancy!

[Michael Wild]

Looks like a Perl programmer couldn't let go of the default variable ;-)

[MC Emperor]

Just like $ is a valid Java and Javascript identifier, while you just came from PHP programming. Confusing. Ninja it is!

[hugo der hungrige]

A good syntax-highlighting would have helped you to answer the question.

[Starx]

Wonder what the close vote is about?

[Grijesh Chauhan]

Ok, here is good read: Javascript Type-Conversion Second is Type Casting In Javascript

[Konrad Viltersten]

Oh, this was one freaky syntax. Never seen that before and it's got a coolness value of at least 98%. I'm going to use it in the next script just to make people scratch their eyes. :D

[vidya sagar]

I wonder why someone would use just an underscore(_) as a variable. Is it to make people scratch their heads while reading his code or he's being simplistic, to reduce the number of chars of the source code? o.O

[Admin]

i hate this kind of variable naming, he/she should be idiot to give such name.

[tckmn]

You can make a smiley face x= +_+ 0;

[nawfal]

possible duplicate of What does the plus sign do in 'return +new Date'

[colincameron]

Does + really mean cast to a number, or does +_ really mean 0+_, in which case _ must be cast before adding to 0?

[lonesomeday]

@c.cam108 See the MDN page on the unary plus operator.

[Kobi]

@c.cam108 - No, they do not behave similarly. Unary plus casts the value to a number, but binary plus does not: +'12' === 12 (result is a number), but 0 + '12' === '012' (result is a string). Note, however, that 0 - '12' === -12.

[TRiG]

+1, as you're the only one here to explicitly point out that _ is a variable.

[Mathletics]

@TRiG what about Starx' answer, which does so without being tl;dr?

[Izkata]

@juzerali Sure, but that's not good practice. Try using addition ;)

[Matthew Lock]

Looks perlish with perl's default variable $_

[Attila O.]

You forgot a + before the b.

[Aki Suihkonen]

I use often x|0 to convert double to int; however this as well as using '~' has the penalty of restricting to numbers < 2^32. +"2e15" does not.

[Grijesh Chauhan]

@AkiSuihkonen Yes good I believe x|0 is even faster then +. Correct ? nice technique :). (2) I use ~ just to show OP that + is not only a sign can be use (i myself use +).

[Grijesh Chauhan]

@Alvin Wong I think X | 0 will be faster then +, not ?

[Aki Suihkonen]

Hard to say -- there's jsperf though is one want's to measure it. OTOH these two operators have a complete different meaning. If only requiring a number (and not integer) '+' is anyway one character shorter.

[Grijesh Chauhan]

@AkiSuihkonen actually bitwise operators are faster then any other operator. that is the reason!

[Aki Suihkonen]

Actually I just tested it jsperf -- 18M ops for '|0', 19M ops for '+'; the performance will probably vary from browser to browser. jsperf.com/strtoint

[Alvin Wong]

@GrijeshChauhan + is faster because it does not perform any other operations on the number (MDN) Also, bitwise OR | only results in an signed integer.

[Juzer Ali]

@Izkata: I know it will concatenate the strings.

[J.G.Sebring]

@AkiSuihkonen Oh my, just did the jsperf test in Firefox, huge difference.. | alot faster.

[Grijesh Chauhan]

@AlvinWong I don't think so. | is much faster. on most browser. may be slow on IE.

[Alvin Wong]

@GrijeshChauhan Unary + converts to Number but | only result in 32-bit signed integer.

[Grijesh Chauhan]

@AlvinWong Yes true! :)

[Grijesh Chauhan]

I think in first line you wants to say parseInr(_) instead of parseFloat(_) ?

[Brian]

No I meant float, because parseInt will stop at non numeric character, parshFloat will not. EXP: parseFloat(2.4) = 2.4 vs parseInt(2.4) = 2.

[Soner from The Ottoman Empire]

String concatenation and conversion is a special feature of the binary plus +. Other arithmetic operators work only with numbers and always convert their operands to numbers.