Explain +var and -var unary operator in javascript

Question

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?

You may find the description in the MDN documentation helpful. There it says "The unary negation operator precedes its operand and negates it." and "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." — Felix Kling
– Felix Kling, Commented Aug 25, 2012 at 10:20

Guffa · Accepted Answer · 2012-08-25 10:57:49Z

28

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

edited Aug 25, 2012 at 10:57

answered Aug 25, 2012 at 9:35

Guffa

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3 Comments

fakeguybrushthreepwood Over a year ago

Thanks for the feedback. I know can see how -a would be used, but sorry to sound daft @Guffa, but can you provide a super basic example of +a used in code? I'm trying to visualise how it would be used in a real world example.

fakeguybrushthreepwood Over a year ago

Hmm @guffa, sorry but I still can not see what a really practical example for +a would be - especially since you are saying that parseInt and parseFloat are superior ways to convert a string to a number.

Guffa Over a year ago

@u1sonderzug: Yes, there isn't really much practical use for the + unary operator, but other C-style languages has it, so I guess they wouldn't want to exclude it from Javscript. I have used Javscript for about 20 years, and I can't recall that I ever used the + unary operator.

phenomnomnominal · Accepted Answer · 2012-08-25 09:36:12Z

4

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

answered Aug 25, 2012 at 9:36

phenomnomnominal

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1 Comment

fakeguybrushthreepwood Over a year ago

Now I am curious as to why we see the following behavior: console.log(threeStr + 3) is there a name for adding a number to a string?

Baiyan Huang · Accepted Answer · 2012-08-25 09:49:07Z

2

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)

answered Aug 25, 2012 at 9:49

Baiyan Huang

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Comments

Drazen · Accepted Answer · 2012-08-25 09:35:42Z

0

a = 1
b = -a
console.log(b)

output
-1

answered Aug 25, 2012 at 9:35

Drazen

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Comments

Mohit Pandey · Accepted Answer · 2012-08-25 09:43:02Z

0

'+' operator in a variable 'a' simply means : a

'-' operator in a variable 'a' simply means : -a

Since, in above example 
a=-10;
a= +a; // means a, ie, +(-10) which is -10

but,
a= -a;  // means -a, ie, -(-10) which is +10

answered Aug 25, 2012 at 9:43

Mohit Pandey

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Rupesh Patel · Accepted Answer · 2012-08-25 09:49:13Z

0

+a means a*1 and -a means a*(-1)

Thats it!!!!!!

answered Aug 25, 2012 at 9:49

Rupesh Patel

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Comments

Robin Maben · Accepted Answer · 2012-08-25 09:39:29Z

-1

Try this

false == 0 // returns true

So,

a = false

a = +a //a * 1

console.log(a) // prints 0 as expected

answered Aug 25, 2012 at 9:39

Robin Maben

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