Explain +var and -var unary operator in javascript

I'm trying to understand unary operators in javascript, I found this guide here http://wiki.answers.com/Q/What_are_unary_operators_in_javascript most of it makes sense but what I don't understand is how the following examples would be used in an actual code example:

+a;
-a;

To my understanding the +a; is meant to make the variable the positive value of a and the -a; is meant to make the variable the negative value of a. I've tried a number of examples like:

a = -10;
a = +a;
document.writeln(a);

And the output is still -10;

I've also tried:

a = false;
a = +a;
document.writeln(a);

And the output is 0;

What is a practical code example of these unary operators?

Solution 1:

The + operator doesn't change the sign of the value, and the - operator does change the sign. The outcome of both operators depend on the sign of the original value, neither operator makes the value positive or negative regardless of the original sign.

var a = 4;
a = -a; // -4
a = +a; // -4

The abs function does what you think that the + opreator does; it makes the value positive regardless of the original sign.

var a =-4;
a = Math.abs(a); // 4

Doing +a is practically the same as doing a * 1; it converts the value in a to a number if needed, but after that it doesn't change the value.

var a = "5";
a = +a; // 5

The + operator is used sometimes to convert string to numbers, but you have the parseInt and parseFloat functions for doing the conversion in a more specific way.

var a = "5";
a = parseInt(a, 10); //5

Solution 2:

One example is that they can be used to convert a string to a number,

var threeStr = '3.0'
var three = +threeStr
console.log(threeStr + 3) // '3.03'
console.log(three + 3) //6

Solution 3:

I would like to explain this from basic mathematical point:
The multiplying rules:

Positive x Positive = Positive: 3 x 2 = 6  
Negative x Negative = Positive: (-2) x (-8) = 16   
Negative x Positive = Negative: (-3) x 4 = -12  
Positive x Negative = Negative: 3 x (-4) = -12  

Considering you example:

a = -10;
a = +a
document.writeln(a);

+a = +(-10) = Positive x Negative = Negative = -10

a = false;
a = +a;
document.writeln(a);

false == 0, +a = +(+0) = Positive * Positive = Positive = 0 (maybe use true is a better example)