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  The unary operators + and - can be used to convert some value types to numeric values.
  But not every value can be converted to a Number type: use it with an object, and result will be always NaN (Not A Number).


Noncompliant Code Example

var obj = {x : 1};
doSomethingWithNumber(+obj);    // Noncompliant

function foo(){
  return 1;
}
doSomethingWithNumber(-foo);    // Noncompliant


Compliant Solution

var obj = {x : 1};
doSomethingWithNumber(+obj.x);

function foo(){
  return 1;
}
doSomethingWithNumber(-foo());

var str = '42';
doSomethingWithNumber(+str);


Exceptions

Unary + and - can be used with objects corresponding to primitive types.

var b = new Boolean(true);
doSomethingWithNumber(-b);  // Compliant