Checking if a variable is defined in SASS - ruby

Question

AS the title says I am trying to check whether a variable is defined in SASS. (I am using compass if that makes any different difference)
I've found the Ruby equivalent which is:
defined? foo
Gave that a shot in the dark but it just gave me the error:
defined": expected "{", was "?
I've found a work around (which is obviously just to define the variable in all cases, which in this case it actually makes more sense) but I'd really like to know if this is possible for the future

Answer 1

For Sass 3.3 and later
As of Sass 3.3 there is a variable-exists() function. From the changelog:
It is now possible to determine the existence of different Sass constructs using these new functions:
variable-exists($name) checks if a variable resolves in the current scope.
global-variable-exists($name) checks if a global variable of the given name exists.
...
Example usage:
$some_variable: 5;
#if variable-exists(some_variable) {
/* I get output to the CSS file */
}
#if variable-exists(nonexistent_variable) {
/* But I don't */
}
For Sass 3.2.x and earlier (my original answer)
I ran into the same problem today: trying to check if a variable is set, and if so adding a style, using a mixin, etc.
After reading that an isset() function isn't going to be added to sass, I found a simple workaround using the !default keyword:
#mixin my_mixin() {
// Set the variable to false only if it's not already set.
$base-color: false !default;
// Check the guaranteed-existing variable. If it didn't exist
// before calling this mixin/function/whatever, this will
// return false.
#if $base-color {
color: $base-color;
}
}
If false is a valid value for your variable, you can use:
#mixin my_mixin() {
$base-color: null !default;
#if $base-color != null {
color: $base-color;
}
}

Answer 2

If you're looking for the inverse, it's
#if not variable-exists(varname)
To check if it is undefined or falsy:
#if not variable-exists(varname) or not $varname
And if you want to set it only if it's undefined or null
$varname: VALUE !default;

Answer 3

Just as a complementary answer - you should have a look on the default keyword for certain use cases. It gives you the possibility to assign a default value to variables in case they are not defined yet.
You can assign to variables if they aren’t already assigned by adding
the !default flag to the end of the value. This means that if the
variable has already been assigned to, it won’t be re-assigned, but if
it doesn’t have a value yet, it will be given one.
Example:
In specific-variables.scss you have:
$brand: "My Awesome Brand";
In default-variables.scss you have:
$brand: company-name !default;
$brand-color: #0074BE !default;
Your project is built like this:
#import "specific-variables.scss";
#import "default-variables.scss";
#import "style.scss";
The value of brand will be My Awesome Brand and the value of brand color will be #0074BE.