The unary "+" operator
is the "+" that preceeds an operand
converts its operand to the Number type
WHEN converting a numeric string THEN it returns its value as a number
even WHEN converting the string "-Infinity" THEN it returns the Number
null THEN it returns
true THEN it returns 1
WHEN converting an object with a method
valueOf THEN its value is returned
The original ECMAScript 1 specification, "Unary + Operator" is on page 40, chapter 11.4.6 (in a 110 pages PDF).
The original ES1 spec describes the `Number` type in chapter 8.5 (a 110 pages PDF).
The MDN page.
Tweeting that not-assertions make sense, sometimes.