Question

I have come across a refernce to a JavaScript syntax that I don’t understand and cannot find any references to online.

[+num]

What does this syntax do and when is it used?

Answer 1

It forces a conversion to a number.

  +'0'; // 0
  +[]; // 0
  +true; // 1
  +false; // 0
  +"I'm Not A Number"; // NaN

From Operators: https://developer.mozilla.org/en/JavaScript/Reference/Operators/Arithmetic_Operators#.2b_(Unary_Plus)

+ (Unary Plus)

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. For example, y = +x takes the value of x and assigns that to
y; that is, if x were 3, y would get the value 3 and x would retain
the value 3; but if x were the string “3”, y would also get the value
3. 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.