What does = +_ mean in JavaScript

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

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

=+ 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.