'What does !default in a css property value mean?

The twitter bootstrap code has a lot of CSS properties with a !default at the end.

E.g.

p {
  color: white !default;
}

What does !default do?

UPDATE

My bad for not being clear. I am using the SASS part of Bootstrap.

csstwitter-bootstrapsass


Solution 1:[1]

!default is used often in Bootstrap Sass. It is similar to a reverse !important. All of Bootstraps Variables are set using !default to allow the developer to further customize bootstrap. With !default sass will only define a variable if it has not already been set.

This allows for more flexibility. 

//Example1 Dress color = red
$auroras-dress-color: blue;
$auroras-dress-color: red;

//Example2 Dress color = red
$auroras-dress-color: blue !default;
$auroras-dress-color: red;

//Example3 Dress color = blue
$auroras-dress-color: blue;
$auroras-dress-color: red !default;

So Why is this important? Bootstrap is a package. Most people don't edit the Bootstrap source. NEVER UPDATE THE BOOTSTRAP SOURCE. To customize bootstrap you will add your own variable file and compile it with the bootstrap code but never touch the native bootstrap package. Bootstrap sass's page has the full skinny on how to customize and compile it in the documentations.

I don't know why less does not do this. I have not worked much with less and do not know if it has it's own built in variable management.

Example fiddle https://jsfiddle.net/siggysid/344dnnwz/

Solution 2:[2]

You can find the following exact definition and a decent explanation in sass-lang website in its doc section (variable) - default value:

Normally when you assign a value to a variable, if that variable already had a value, its old value is overwritten. But if you’re writing a Sass library, you might want to allow your users to configure your library’s variables before you use them to generate CSS. To make this possible, Sass provides the !default flag. This assigns a value to a variable only if that variable isn’t defined or its value is null. Otherwise, the existing value will be used.

Solution 3:[3]

default-values

if that variable isn’t defined or its value is null. Otherwise, the existing value will be used.

Example

case 1: null

// test.sass
$MySize: null
$MySize: 5rem!default // since MySize is "null" so use default

h1
  font-size: $MySize

output CSS

h1 {
  font-size: 5rem;
}

case 2: undefined

// test.sass
$MySize: 5rem!default // since MySize is "undefined" so use default

h1
  font-size: $MySize

output CSS

h1 {
  font-size: 5rem;
}

case 3: defined already

// test.sass
$MySize: 30rem
$MySize: 5rem!default // since MySize has been defined. So ignore this setting.

h1
  font-size: $MySize

output CSS

h1 {
  font-size: 30rem;
}

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Sarah M Giles
Solution 2 Carson
Solution 3 Carson

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