less語言特性(二)

作爲 CSS 的一種擴展，Less 不僅完全兼容 CSS 語法，而且連新增的特性也是使用 CSS 語法。這樣的設計使得學習 Less 很輕鬆，而且你可以在任何時候回退到 CSS。

變量

舉個簡單的例子

@nice-blue: #5B83AD;
@light-blue: @nice-blue + #111;

#header {
  color: @light-blue;
}

編譯爲：

#header {
  color: #6c94be;
}

注意，由於變量只能定義一次，其本質就是“常量”。

混合（Mixin）

Mixins are a way of including ("mixing in") a bunch of properties from one rule-set into another rule-set. So say we have the following class:

.bordered {
  border-top: dotted 1px black;
  border-bottom: solid 2px black;
}

And we want to use these properties inside other rule-sets. Well, we just have to drop in the name of the class where we want the properties, like so:

#menu a {
  color: #111;
  .bordered;
}

.post a {
  color: red;
  .bordered;
}

The properties of the .bordered class will now appear in both #menu a and .post a. (Note that you can also use #ids as mixins.)

Learn more

• More about mixins
• Parametric Mixins

嵌套規則

Less gives you the ability to use nesting instead of, or in combination with cascading. Let's say we have the following CSS:

#header {
  color: black;
}
#header .navigation {
  font-size: 12px;
}
#header .logo {
  width: 300px;
}

In Less, we can also write it this way:

#header {
  color: black;
  .navigation {
    font-size: 12px;
  }
  .logo {
    width: 300px;
  }
}

The resulting code is more concise, and mimics the structure of your HTML.

You can also bundle pseudo-selectors with your mixins using this method. Here's the classic clearfix hack, rewritten as a mixin (&represents the current selector parent):

.clearfix {
  display: block;
  zoom: 1;

  &:after {
    content: " ";
    display: block;
    font-size: 0;
    height: 0;
    clear: both;
    visibility: hidden;
  }
}

See also

• Parent Selectors

運算

任何數字、顏色或者變量都可以參與運算。下面是一組案例：

@base: 5%;
@filler: @base * 2;
@other: @base + @filler;

color: #888 / 4;
background-color: @base-color + #111;
height: 100% / 2 + @filler;

The output is pretty much what you expect—Less understands the difference between colors and units. If a unit is used in an operation, like in:

@var: 1px + 5;

Less 將會使用出現的單位，在這個例子中，最終輸出是 6px。

函數

Less 內置了多種函數用於轉換顏色、處理字符串、算術運算等。這些函數在函數手冊中有詳細介紹。

函數的用法非常簡單。下面這個例子將介紹如何將 0.5 轉換爲 50%，將顏色飽和度增加 5%，以及顏色亮度降低 25% 並且色相值增加 8 等用法：

@base: #f04615;
@width: 0.5;

.class {
  width: percentage(@width); // returns `50%`
  color: saturate(@base, 5%);
  background-color: spin(lighten(@base, 25%), 8);
}

Namespaces & Accessors

(Not to be confused with CSS @namespace or namespace selectors).

Sometimes, you may want to group your variables or mixins, for organizational purposes, or just to offer some encapsulation. You can do this pretty intuitively in Less, say you want to bundle some mixins and variables under #bundle, for later reuse or distributing:

#bundle {
  .button {
    display: block;
    border: 1px solid black;
    background-color: grey;
    &:hover {
      background-color: white
    }
  }
  .tab { ... }
  .citation { ... }
}

Now if we want to mixin the .button class in our #header a, we can do:

#header a {
  color: orange;
  #bundle > .button;
}

Scope

Scope in Less is very similar to that of programming languages. Variables and mixins are first looked for locally, and if they aren't found, the compiler will look in the parent scope, and so on.

@var: red;

#page {
  @var: white;
  #header {
    color: @var; // white
  }
}

Variables and mixins do not have to be declared before being used so the following Less code is identical to the previous example:

@var: red;

#page {
  #header {
    color: @var; // white
  }
  @var: white;
}

See also

• Lazy Loading

註釋

塊註釋和行註釋都可以使用：

/* 一個註釋塊
style comment! */
@var: red;

// 這一行被註釋掉了！
@var: white;

導入（Importe）

和你預期的工作方式一樣。你可以導入一個 .less 文件，此文件中的所有變量就可以全部使用了。如果導入的文件是 .less 擴展名，則可以將擴展名省略掉：

@import "library"; // library.less
@import "typo.css";

Improve this section.

變量

將常用的值定義在一個地方。

概覽

我們經常在代碼中看到同一個值多次（也可能是上百次）重複出現在樣式表的各個地方：

a,
.link {
  color: #428bca;
}
.widget {
  color: #fff;
  background: #428bca;
}

變量的作用就是把值定義在一個地方，然後在各處使用，這樣能讓代碼更易維護：

// Variables
@link-color:        #428bca; // sea blue
@link-color-hover:  darken(@link-color, 10%);

// 用法
a,
.link {
  color: @link-color;
}
a:hover {
  color: @link-color-hover;
}
.widget {
  color: #fff;
  background: @link-color;
}

Variable Interpolation

The examples above focused on using variables to control values in CSS rules, but they can also be used in other places as well, such as selector names, property names, URLs and @import statements.

Selectors

Version: 1.4.0

// Variables
@mySelector: banner;

// Usage
.@{mySelector} {
  font-weight: bold;
  line-height: 40px;
  margin: 0 auto;
}

Compiles to:

.banner {
  font-weight: bold;
  line-height: 40px;
  margin: 0 auto;
}

URLs

// Variables
@images: "../img";

// 用法
body {
  color: #444;
  background: url("@{images}/white-sand.png");
}

Import statements

Version: 1.4.0

Syntax: @import "@{themes}/tidal-wave.less";

Note that currently, only variables which have been declared in the root or current scope will be considered and that only the current file and calling files will be considered when looking for a variable. This means that this usage is typically limited to when you inject a variable into the compile process or define a variable at the beginning of your root file.

When you are importing a css file and not using the inline option (e.g. the import statement will be kept intact) then the above does not apply.

Example:

// Variables
@themes: "../../src/themes";

// Usage
@import "@{themes}/tidal-wave.less";

屬性

Version: 1.6.0

@property: color;

.widget {
  @{property}: #0ee;
  background-@{property}: #999;
}

編譯爲：

.widget {
  color: #0ee;
  background-color: #999;
}

變量名

It is also possible to define variables with a variable name:

@fnord:  "I am fnord.";
@var:    "fnord";
content: @@var;

Which compiles to:

content: "I am fnord.";

Lazy Loading

Variables are lazy loaded and do not have to be declared before being used.

Valid Less snippet:

.lazy-eval {
  width: @var;
}

@var: @a;
@a: 9%;

this is valid Less too:

.lazy-eval-scope {
  width: @var;
  @a: 9%;
}

@var: @a;
@a: 100%;

both compile into:

.lazy-eval-scope {
  width: 9%;
}

When defining a variable twice, the last definition of the variable is used, searching from the current scope upwards. This is similar to css itself where the last property inside a definition is used to determine the value.

For instance:

@var: 0;
.class1 {
  @var: 1;
  .class {
    @var: 2;
    three: @var;
    @var: 3;
  }
  one: @var;
}

Compiles to:

.class1 .class {
  three: 3;
}
.class {
  one: 1;
}

default variables

We sometimes get requests for default variables - an ability to set a variable only if it is not already set. This feature is not required because you can easily override a variable by putting the definition afterwards.

For instance:

// library
@base-color: green;
@dark-color: darken(@base-color, 10%);

// use of library
@import "library.less";
@base-color: red;

This works fine - base-color is overidden and dark-color is a dark red.

Improve this section.

Extend

Extend is a Less Pseudo-Class which merges the selector it is put on with ones that match what it references.

Released v1.4.0

nav ul {
  &:extend(.inline);
  background: blue;
}

In the rule set above, the :extend selector will apply the "extending selector" (nav ul) onto the .inline class wherever the.inline class appears. The declaration block will be kept as-is, but without any reference to the extend (because extend isn't css).

So the following:

nav ul {
  &:extend(.inline);
  background: blue;
}
.inline {
  color: red;
}

Outputs

nav ul {
  background: blue;
}
.inline,
nav ul {
  color: red;
}

Notice how the nav ul:extend(.inline) selector gets output as nav ul - the extend gets removed before output and the selector block left as-is. If no properties are put in that block then it gets removed from the output (but the extend still may affect other selectors).

Extend syntax

The extend is either attached to a selector or placed into a ruleset. It looks like a pseudoclass with selector parameter optionally followed by the keyword all:

Example:

.a:extend(.b) {}

// the above block does the same thing as the below block
.a {
  &:extend(.b);
}

.c:extend(.d all) {
  // extends all instances of ".d" e.g. ".x.d" or ".d.x"
}
.c:extend(.d) {
  // extends only instances where the selector will be output as just ".d"
}

It can contain one more classes to extend, separated by commas.

Example:

.e:extend(.f) {}
.e:extend(.g) {}

// the above an the below do the same thing
.e:extend(.f, .g) {}

Extend attached to selector

Extend attached to a selector looks like an ordinary pseudoclass with selector as a parameter. A selector can contain multiple extend clauses, but all extends must be at the end of the selector.

• Extend after the selector: pre:hover:extend(div pre).
• Space between selector and extend is allowed: pre:hover :extend(div pre).
• Multiple extends are allowed: pre:hover:extend(div pre):extend(.bucket tr) - Note this is the same aspre:hover:extend(div pre, .bucket tr)
• This is NOT allowed: pre:hover:extend(div pre).nth-child(odd). Extend must be last.

If a ruleset contains multiple selectors, any of them can have the extend keyword. Multiple selectors with extend in one ruleset:

.big-division,
.big-bag:extend(.bag),
.big-bucket:extend(.bucket) {
  // body
}

Extend inside ruleset

Extend can be placed into rulesets body using &:extend(selector) syntax. Placing extend into a body is a shortcut for placing it into every single selector of that ruleset.

Extend inside a body:

pre:hover,
.some-class {
  &:extend(div pre);
}

is exactly the same as adding an extend after each selector:

pre:hover:extend(div pre),
.some-class:extend(div pre) {}

Extending nested Selectors

Extend is able to match nested selectors. Following less:

Example:

.bucket {
  tr { // nested ruleset with target selector
    color: blue;
  }
}
.some-class:extend(.bucket tr) {} // nested ruleset is recognized

Outputs

.bucket tr,
.some-class {
  color: blue;
}

Essentially the extend looks at the compiled css, not the original less.

Example:

.bucket {
  tr & { // nested ruleset with target selector
    color: blue;
  }
}
.some-class:extend(tr .bucket) {} // nested ruleset is recognized

Outputs

tr .bucket,
.some-class {
  color: blue;
}

Exact Matching with Extend

Extend by default looks for exact match between selectors. It does matter whether selector uses leading start or not. It does not matter that two nth-expressions have the same meaning, they need to have to same form in order to be matched. The only exception are quotes in attribute selector, less knows they have the same meaning and matches them.

Example:

.a.class,
.class.a,
.class > .a {
  color: blue;
}
.test:extend(.class) {} // this will NOT match the any selectors above

Leading star does matter. Selectors *.class and .class are equivalent, but extend will not match them:

*.class {
  color: blue;
}
.noStar:extend(.class) {} // this will NOT match the *.class selector

Outputs

*.class {
  color: blue;
}

Order of pseudoclasses does matter. Selectors link:hover:visited and link:visited:hover match the same set of elements, but extend treats them as different:

link:hover:visited {
  color: blue;
}
.selector:extend(link:visited:hover) {}

Outputs

link:hover:visited {
  color: blue;
}

nth expression

Nth expression form does matter. Nth-expressions 1n+3 and n+3 are equivalent, but extend will not match them:

:nth-child(1n+3) {
  color: blue;
}
.child:extend(n+3) {}

Outputs

:nth-child(1n+3) {
  color: blue;
}

Quote type in attribute selector does not matter. All of the following are equivalent.

[title=identifier] {
  color: blue;
}
[title='identifier'] {
  color: blue;
}
[title="identifier"] {
  color: blue;
}

.noQuote:extend([title=identifier]) {}
.singleQuote:extend([title='identifier']) {}
.doubleQuote:extend([title="identifier"]) {}

Outputs

[title=identifier],
.noQuote,
.singleQuote,
.doubleQuote {
  color: blue;
}

[title='identifier'],
.noQuote,
.singleQuote,
.doubleQuote {
  color: blue;
}

[title="identifier"],
.noQuote,
.singleQuote,
.doubleQuote {
  color: blue;
}

Extend "all"

When you specify the all keyword last in an extend argument it tells Less to match that selector as part of another selector. The selector will be copied and the matched part of the selector only will then be replaced with the extend, making a new selector.

Example:

.a.b.test,
.test.c {
  color: orange;
}
.test {
  &:hover {
    color: green;
  }
}

.replacement:extend(.test all) {}

Outputs

.a.b.test,
.test.c,
.a.b.replacement,
.replacement.c {
  color: orange;
}
.test:hover,
.replacement:hover {
  color: green;
}

You can think of this mode of operation as essentially doing a non-destructive search and replace.

Selector Interpolation with Extend

Extend is NOT able to match selectors with variables. If selector contains variable, extend will ignore it.

There is a pending feature request for this but it is not an easy change. However, extend can be attached to interpolated selector.

Selector with variable will not be matched:

@variable: .bucket;
@{variable} { // interpolated selector
  color: blue;
}
.some-class:extend(.bucket) {} // does nothing, no match is found

and extend with variable in target selector matches nothing:

.bucket {
  color: blue;
}
.some-class:extend(@{variable}) {} // interpolated selector matches nothing
@variable: .bucket;

Both of the previous examples compile into:

.bucket {
  color: blue;
}

However, :extend attached to an interpolated selector works:

.bucket {
  color: blue;
}
@{variable}:extend(.bucket) {}
@variable: .selector;

previous less compiles into:

.bucket, .selector {
  color: blue;
}

Scoping / Extend Inside @media

Extend written inside a media declaration should match only selectors inside the same media declaration:

@media print {
  .screenClass:extend(.selector) {} // extend inside media
  .selector { // this will be matched - it is in the same media
    color: black;
  }
}
.selector { // ruleset on top of style sheet - extend ignores it
  color: red;
}
@media screen {
  .selector {  // ruleset inside another media - extend ignores it
    color: blue;
  }
}

compiles into:

@media print {
  .selector,
  .screenClass { /*  ruleset inside the same media was extended */
    color: black;
  }
}
.selector { /* ruleset on top of style sheet was ignored */
  color: red;
}
@media screen {
  .selector { /* ruleset inside another media was ignored */
    color: blue;
  }
}

Extend written inside a media declaration does not match selectors inside nested declaration:

@media screen {
  .screenClass:extend(.selector) {} // extend inside media
  @media (min-width: 1023px) {
    .selector {  // ruleset inside nested media - extend ignores it
      color: blue;
    }
  }
}

compiles into:

@media screen and (min-width: 1023px) {
  .selector { /* ruleset inside another nested media was ignored */
    color: blue;
  }
}

Top level extend matches everything including selectors inside nested media:

@media screen {
  .selector {  /* ruleset inside nested media - top level extend works */
    color: blue;
  }
  @media (min-width: 1023px) {
    .selector {  /* ruleset inside nested media - top level extend works */
      color: blue;
    }
  }
}

.topLevel:extend(.selector) {} /* top level extend matches everything */

compiles into:

@media screen {
  .selector,
  .topLevel { /* ruleset inside media was extended */
    color: blue;
  }
}
@media screen and (min-width: 1023px) {
  .selector,
  .topLevel { /* ruleset inside nested media was extended */
    color: blue;
  }
}

Duplication Detection

Currently there is no duplication detection.

Example:

.alert-info,
.widget {
  /* declarations */
}

.alert:extend(.alert-info, .widget) {}

Outputs

.alert-info,
.widget,
.alert,
.alert {
  /* declarations */
}

Use Cases for Extend

Classic Use Case

The classic use case is to avoid adding a base class. For example, if you have

.animal {
  background-color: black;
  color: white;
}

and you want to have a subtype of animal which overrides the background color then you have two options, firstly change your HTML

<a class="animal bear">Bear</a>

.animal {
  background-color: black;
  color: white;
}
.bear {
  background-color: brown;
}

or have simplified html and use extend in your less. e.g.

<a class="bear">Bear</a>

.animal {
  background-color: black;
  color: white;
}
.bear {
  &:extend(.animal);
  background-color: brown;
}

Reducing CSS Size

Mixins copy all of the properties into a selector, which can lead to unnecessary duplication. Therefore you can use extends instead of mixins to move the selector up to the properties you wish to use, which leads to less css being generated.

Example - with mixin:

.my-inline-block() {
    display: inline-block;
  font-size: 0;
}
.thing1 {
  .my-inline-block;
}
.thing2 {
  .my-inline-block;
}

Outputs

.thing1 {
  display: inline-block;
  font-size: 0;
}
.thing2 {
  display: inline-block;
  font-size: 0;
}

Example (with extends):

.my-inline-block {
  display: inline-block;
  font-size: 0;
}
.thing1 {
  &:extend(.my-inline-block);
}
.thing2 {
  &:extend(.my-inline-block);
}

Outputs

.my-inline-block,
.thing1,
.thing2 {
  display: inline-block;
  font-size: 0;
}

Combining Styles / a more advanced mixin

Another use-case is as an alternative for a mixin - because mixins can only be used with simple selectors, if you have two different blocks on html, but need to apply the same styles to both you can use extends to relate two areas.

Example:

li.list > a {
  // list styles
}
button.list-style {
  &:extend(li.list > a); // use the same list styles
}

Improve this section.

混合（Mixin）

"mix-in" properties from existing styles

You can mix-in class selectors and id selectors, e.g.

.a, #b {
  color: red;
}
.mixin-class {
  .a();
}
.mixin-id {
  #b();
}

which results in:

.a, #b {
  color: red;
}
.mixin-class {
  color: red;
}
.mixin-id {
  color: red;
}

Notice that when you call the mixin, the parenthesis are optional.

.a();   //these lines do the same thing
.a;

Not outputting the mixin

If you want to create a mixin but you do not want that mixin to be output, you can put parenthesis after it.

.my-mixin {
  color: black;
}
.my-other-mixin() {
  background: white;
}
.class {
  .my-mixin;
  .my-other-mixin;
}

outputs

.my-mixin {
  color: black;
}
.class {
  color: black;
  background: white;
}

Selectors in mixins

Mixins can contain more than just properties, they can contain selectors to.

For example:

.my-hover-mixin() {
  &:hover {
    border: 1px solid red;
  }
}
button {
  .my-hover-mixin();
}

Outputs

button:hover {
  border: 1px solid red;
}

Namespaces

If you want to mixin properties inside a more complicated selector, you can stack up multiple id's or classes.

#outer {
  .inner {
    color: red;
  }
}

.c {
  #outer > .inner;
}

and again the > is optional

// all do the same thing
#outer > .inner;
#outer > .inner();
#outer.inner;
#outer.inner();

One use of this is known as namespacing. You can put your mixins under a id selector and this makes sure it won't conflict with another library.

Example:

#my-library {
  .my-mixin() {
    color: black;
  }
}
// which can be used like this
.class {
  #my-library > .my-mixin();
}

The !important keyword

Use the !important keyword after mixin call to mark all properties inherited by it as !important:

Example:

.foo (@bg: #f5f5f5, @color: #900) {
  background: @bg;
  color: @color;
}
.unimportant {
  .foo(1);
}
.important {
  .foo(2) !important;
}

Results in:

.unimportant {
  background: #f5f5f5;
  color: #900;
}
.important {
  background: #f5f5f5 !important;
  color: #900 !important;
}

Improve this section.

帶參數的Mixin

How to pass arguments to mixins

Mixins can also take arguments, which are variables pass to the block of selectors when it is mixed in.

For example:

.border-radius(@radius) {
  -webkit-border-radius: @radius;
     -moz-border-radius: @radius;
          border-radius: @radius;
}

And here's how we can mix it into various rulesets:

#header {
  .border-radius(4px);
}
.button {
  .border-radius(6px);
}

Parametric mixins can also have default values for their parameters:

.border-radius(@radius: 5px) {
  -webkit-border-radius: @radius;
     -moz-border-radius: @radius;
          border-radius: @radius;
}

We can invoke it like this now:

#header {
  .border-radius;
}

And it will include a 5px border-radius.

You can also use parametric mixins which don't take parameters. This is useful if you want to hide the ruleset from the CSS output, but want to include its properties in other rulesets:

.wrap() {
  text-wrap: wrap;
  white-space: -moz-pre-wrap;
  white-space: pre-wrap;
  word-wrap: break-word;
}

pre { .wrap }

Which would output:

pre {
  text-wrap: wrap;
  white-space: -moz-pre-wrap;
  white-space: pre-wrap;
  word-wrap: break-word;
}

Mixins With Multiple Parameters

Parameters are either semicolon or comma separated. It is recommended to use semicolon. The symbol comma has double meaning: it can be interpreted either as a mixin parameters separator or css list separator.

Using comma as mixin separator makes it impossible to create comma separated lists as an argument. On the other hand, if the compiler sees at least one semicolon inside mixin call or declaration, it assumes that arguments are separated by semicolons and all commas belong to css lists:

• two arguments and each contains comma separated list: .name(1, 2, 3; something, else),
• three arguments and each contains one number: .name(1, 2, 3),
• use dummy semicolon to create mixin call with one argument containing comma separated css list: .name(1, 2, 3;),
• comma separated default value: .name(@param1: red, blue;).

It is legal to define multiple mixins with the same name and number of parameters. Less will use properties of all that can apply. If you used the mixin with one parameter e.g. .mixin(green);, then properties of all mixins with exactly one mandatory parameter will be used:

.mixin(@color) {
  color-1: @color;
}
.mixin(@color; @padding:2) {
  color-2: @color;
  padding-2: @padding;
}
.mixin(@color; @padding; @margin: 2) {
  color-3: @color;
  padding-3: @padding;
  margin: @margin @margin @margin @margin;
}
.some .selector div {
  .mixin(#008000);
}

compiles into:

.some .selector div {
  color-1: #008000;
  color-2: #008000;
  padding-2: 2;
}

Named Parameters

A mixin reference can supply parameters values by their names instead of just positions. Any parameter can be referenced by its name and they do not have to be in any special order:

.mixin(@color: black; @margin: 10px; @padding: 20px) {
  color: @color;
  margin: @margin;
  padding: @padding;
}
.class1 {
  .mixin(@margin: 20px; @color: #33acfe);
}
.class2 {
  .mixin(#efca44; @padding: 40px);
}

compiles into:

.class1 {
  color: #33acfe;
  margin: 20px;
  padding: 20px;
}
.class2 {
  color: #efca44;
  margin: 10px;
  padding: 40px;
}

The @arguments variable

@arguments has a special meaning inside mixins, it contains all the arguments passed, when the mixin was called. This is useful if you don't want to deal with individual parameters:

.box-shadow(@x: 0; @y: 0; @blur: 1px; @color: #000) {
  -webkit-box-shadow: @arguments;
     -moz-box-shadow: @arguments;
          box-shadow: @arguments;
}
.big-block {
  .box-shadow(2px; 5px);
}

Which results in:

.big-block {
  -webkit-box-shadow: 2px 5px 1px #000;
     -moz-box-shadow: 2px 5px 1px #000;
          box-shadow: 2px 5px 1px #000;
}

Advanced arguments and the @rest variable

You can use ... if you want your mixin to take a variable number of arguments. Using this after a variable name will assign those arguments to the variable.

.mixin(...) {        // matches 0-N arguments
.mixin() {           // matches exactly 0 arguments
.mixin(@a: 1) {      // matches 0-1 arguments
.mixin(@a: 1; ...) { // matches 0-N arguments
.mixin(@a; ...) {    // matches 1-N arguments

Furthermore:

.mixin(@a; @rest...) {
   // @rest is bound to arguments after @a
   // @arguments is bound to all arguments
}

Pattern-matching

Sometimes, you may want to change the behavior of a mixin, based on the parameters you pass to it. Let's start with something basic:

.mixin(@s; @color) { ... }

.class {
  .mixin(@switch; #888);
}

Now let's say we want .mixin to behave differently, based on the value of @switch, we could define .mixin as such:

.mixin(dark; @color) {
  color: darken(@color, 10%);
}
.mixin(light; @color) {
  color: lighten(@color, 10%);
}
.mixin(@_; @color) {
  display: block;
}

Now, if we run:

@switch: light;

.class {
  .mixin(@switch; #888);
}

We will get the following CSS:

.class {
  color: #a2a2a2;
  display: block;
}

Where the color passed to .mixin was lightened. If the value of @switch was dark, the result would be a darker color.

Here's what happened:

• The first mixin definition didn't match because it expected dark as the first argument.
• The second mixin definition matched, because it expected light.
• The third mixin definition matched because it expected any value.

Only mixin definitions which matched were used. Variables match and bind to any value. Anything other than a variable matches only with a value equal to itself.

We can also match on arity, here's an example:

.mixin(@a) {
  color: @a;
}
.mixin(@a; @b) {
  color: fade(@a; @b);
}

Now if we call .mixin with a single argument, we will get the output of the first definition, but if we call it with two arguments, we will get the second definition, namely @a faded to @b.

Improve this section.

作爲函數使用的Mixin

Return variables from mixins

All variables defined in a mixin are visible and can be used in caller's scope (unless the caller defines its own variable with the same name).

Example:

.mixin() {
  @width:  100%;
  @height: 200px;
}

.caller {
  .mixin();
  width:  @width;
  height: @height;
}

Results in:

.caller {
  width:  100%;
  height: 200px;
}

Thus variables defined in a mixin can act as its return values. This allows us to create a mixin that can be used almost like a function.

Example:

.average(@x, @y) {
  @average: ((@x + @y) / 2);
}

div {
  .average(16px, 50px); // "call" the mixin
  padding: @average;    // use its "return" value
}

Results in:

div {
  padding: 33px;
}

Improve this section.

Import Directives

Import styles from other style sheets

In standard CSS, @import at-rules must precede all other types of rules. But Less.js doesn't care where you put @import statements.

Example:

.foo {
  background: #900;
}
@import "this-is-valid.less";

File extensions

@import statements may be treated differently by Less depending on the file extension:

• If the file has a .css extension it will be treated as CSS and the @import statement left as-is (see the inline option below).
• If it has any other extension it will be treated as Less and imported.
• If it does not have an extension, .less will be appended and it will be included as a imported Less file.

Examples:

@import "foo";      // foo.less is imported
@import "foo.less"; // foo.less is imported
@import "foo.php";  // foo.php imported as a less file
@import "foo.css";  // statement left in place, as-is

The following options can be used to override this behavior.

Import Options

Less offers several extensions to the CSS @import CSS at-rule to provide more flexibility over what you can do with external files.

Syntax: @import (keyword) "filename";

The following import directives have been implemented:

• reference: use a Less file but do not output it
• inline: include the source file in the output but do not process it
• less: treat the file as a Less file, no matter what the file extension
• css: treat the file as a CSS file, no matter what the file extension
• once: only include the file once (this is default behavior)
• multiple: include the file multiple times

reference

Use @import (reference) to import external files, but without adding the imported styles to the compiled output unless referenced.

Released v1.5.0

Example: @import (reference) "foo.less";

reference is one of the most powerful features in the Less language. Imagine that reference marks every directive and selector with a reference flag in the imported file, imports as normal, but when the CSS is generated, "reference" selectors (as well as any media queries containing only reference selectors) are not output. reference styles will not show up in your generated CSS unless the reference styles are used as mixins or extended.

Additionally, reference produces different results depending on which method was used (mixin or extend):

• extend: When a selector is extended, only the new selector is marked as not referenced, and it is pulled in at the position of the reference @import statement.
• mixins: When a reference style is used as an implicit mixin, its rules are are mixed-in, marked "not reference", and appear in the referenced place as normal.

reference example

This allows you to pull in only specific, targeted styles from a library such as Bootstrap by doing something like this:

.navbar:extend(.navbar all) {}

And you will pull in only .navbar related styles from Bootstrap.

inline

Use @import (inline) to include external files, but not process them.

Released v1.5.0

Example: @import (inline) "not-less-compatible.css";

You will use this when a CSS file may not be Less compatible; this is because although Less supports most known standards CSS, it does not support comments in some places and does not support all known CSS hacks without modifying the CSS.

So you can use this to include the file in the output so that all CSS will be in one file.

less

Use @import (less) to treat imported files as Less, regardless of file extension.

Released v1.4.0

Example:

@import (less) "foo.css";

css

Use @import (css) to treat imported files as regular CSS, regardless of file extension. This means the import statement will be left as it is.

Released v1.4.0

Example:

@import (css) "foo.less";

outputs

@import "foo.less";

once

The default behavior of @import statements. It means the file is imported only once and subsequent import statements for that file will be ignored.

Released v1.4.0

This is the default behavior of @import statements.

Example:

@import (once) "foo.less";
@import (once) "foo.less"; // this statement will be ignored

multiple

Use @import (multiple) to allow importing of multiple files with the same name. This is the opposite behavior to once.

Released v1.4.0

Example:

// file: foo.less
.a {
  color: green;
}
// file: main.less
@import (multiple) "foo.less";
@import (multiple) "foo.less";

Outputs

.a {
  color: green;
}
.a {
  color: green;
}

Improve this section.

Mixin Guards

Conditional mixins

Guards are useful when you want to match on expressions, as opposed to simple values or arity. If you are familiar with functional programming, you have probably encountered them already.

In trying to stay as close as possible to the declarative nature of CSS, Less has opted to implement conditional execution via guarded mixins instead of if/else statements, in the vein of @media query feature specifications.

Let's start with an example:

.mixin (@a) when (lightness(@a) >= 50%) {
  background-color: black;
}
.mixin (@a) when (lightness(@a) < 50%) {
  background-color: white;
}
.mixin (@a) {
  color: @a;
}

The key is the when keyword, which introduces a guard sequence (here with only one guard). Now if we run the following code:

.class1 { .mixin(#ddd) }
.class2 { .mixin(#555) }

Here's what we'll get:

.class1 {
  background-color: black;
  color: #ddd;
}
.class2 {
  background-color: white;
  color: #555;
}

Guard comparison operators

The full list of comparison operators usable in guards are: >, >=, =, =<, <. Additionally, the keyword true is the only truthy value, making these two mixins equivalent:

.truth (@a) when (@a) { ... }
.truth (@a) when (@a = true) { ... }

Any value other than the keyword true is falsy:

.class {
  .truth(40); // Will not match any of the above definitions.
}

Guards can be separated with a comma ,, if any of the guards evaluates to true, it's considered as a match:

.mixin (@a) when (@a > 10), (@a < -10) { ... }

Note that you can also compare arguments with each other, or with non-arguments:

@media: mobile;

.mixin (@a) when (@media = mobile) { ... }
.mixin (@a) when (@media = desktop) { ... }

.max (@a; @b) when (@a > @b) { width: @a }
.max (@a; @b) when (@a < @b) { width: @b }

Type checking functions

Lastly, if you want to match mixins based on value type, you can use the is functions:

.mixin (@a; @b: 0) when (isnumber(@b)) { ... }
.mixin (@a; @b: black) when (iscolor(@b)) { ... }

Here are the basic type checking functions:

• iscolor
• isnumber
• isstring
• iskeyword
• isurl

If you want to check if a value is in a specific unit in addition to being a number, you may use one of:

• ispixel
• ispercentage
• isem
• isunit

Conditional mixins

(FIXME) Additionally, the default function may be used to make a mixin match depending on other mixing matches, and you may use it to create "conditional mixins" similar to else or default statements (of if and case structures respectively):

.mixin (@a) when (@a > 0) { ...  }
.mixin (@a) when (default()) { ... } // matches only if first mixin does not, i.e. when @a <= 0

Last but not least, you may use the and keyword to provide additional conditions inside a guard:

.mixin (@a) when (isnumber(@a)) and (@a > 0) { ... }

And finally, the not keyword to negate conditions:

.mixin (@b) when not (@b > 0) { ... }

Improve this section.

CSS Guards

"if"'s around selectors

Released v1.5.0

Guards can also be applied to css selectors, which is syntactic sugar for declaring the mixin and then calling it immediately.

For instance, before 1.5.0 you would have had to do this.

.my-optional-style() when (@my-option = true) {
  button {
    color: white;
  }
}
.my-optional-style();

you can now write the guard straight on to styles

button when (@my-option = true) {
  color: white;
}

You can also achieve an 'if' type statement by combining this with the & feature, allowing you to group multiple guards.

& when (@my-option = true) {
  button {
    color: white;
  }
  a {
    color: blue;
  }
}

Improve this section.

循環

創建循環

在 Less 中，mixin 可以被自己調用。當這種遞歸形式的 mixin 與 Guard Expressions 和 Pattern Matching 一起聯合使用的話，就可以創造出各種迭代/循環結構。

案例：

.loop(@counter) when (@counter > 0) {
  .loop((@counter - 1));    // next iteration
  width: (10px * @counter); // code for each iteration
}

div {
  .loop(5); // launch the loop
}

輸出：

div {
  width: 10px;
  width: 20px;
  width: 30px;
  width: 40px;
  width: 50px;
}

下面就是一個用於生成 CSS 柵格類的遞歸循環的實例：

.generate-columns(4);

.generate-columns(@n, @i: 1) when (@i =< @n) {
  .column-@{i} {
    width: (@i * 100% / @n);
  }
  .generate-columns(@n, (@i + 1));
}

輸出：

.column-1 {
  width: 25%;
}
.column-2 {
  width: 50%;
}
.column-3 {
  width: 75%;
}
.column-4 {
  width: 100%;
}

Improve this section.

合併

合併屬性

merge 特性能夠聚合多個屬性從而形成一個用逗號分隔的單一屬性。merge 對於像 background 和 transform 這類屬性非常有用。

添加於 v1.5.0 版本

案例：

.mixin() {
  box-shadow+: inset 0 0 10px #555;
}
.myclass {
  .mixin();
  box-shadow+: 0 0 20px black;
}

輸出爲

.myclass {
  box-shadow: inset 0 0 10px #555, 0 0 20px black;
}

爲了避免意外的合併，merge 需要在每個需要合併的屬性名後面添加一個 + 以作標示。

注意： although convention on the transform property is to have space separated, comma separation is supported which is why there is no option on this feature for whether to generate space or comma separated.

Improve this section.

父選擇符

Referencing parent selectors with &

The & operator represents the parent selectors of a nested rule and is most commonly used when applying a modifying class or pseudo-class to an existing selector:

a {
  color: blue;
  &:hover {
    color: green;
  }
}

results in:

a {
  color: blue;
}

a:hover {
  color: green;
}

Notice that without the &, the above example would result in a :hover rule (a descendant selector that matches hovered elements inside of <a> tags) and this is not what we typically would want with the nested :hover.

The "parent selectors" operator has a variety of uses. Basically any time you need the selectors of the nested rules to be combined in other ways than the default. For example another typical use of the & is to produce repetitive class names:

.button {
  &-ok {
    background-image: url("ok.png");
  }
  &-cancel {
    background-image: url("cancel.png");
  }

  &-custom {
    background-image: url("custom.png");
  }
}

output:

.button-ok {
  background-image: url("ok.png");
}
.button-cancel {
  background-image: url("cancel.png");
}
.button-custom {
  background-image: url("custom.png");
}

Multiple &

& may appear more then once within a selector. This makes it possible to repeatedly refer to a parent selector without repeating its name.

.link {
  & + & {
    color: red;
  }

  & & {
    color: green;
  }

  && {
    color: blue;
  }

  &, &ish {
    color: cyan;
  }
}

will output:

.link + .link {
  color: red;
}
.link .link {
  color: green;
}
.link.link {
  color: blue;
}
.link, .linkish {
  color: cyan;
}

Note that & represents all parent selectors (not just the nearest ancestor) so the following example:

.grand {
  .parent {
    & > & {
      color: red;
    }

    & & {
      color: green;
    }

    && {
      color: blue;
    }

    &, &ish {
      color: cyan;
    }
  }
}

results in:

.grand .parent > .grand .parent {
  color: red;
}
.grand .parent .grand .parent {
  color: green;
}
.grand .parent.grand .parent {
  color: blue;
}
.grand .parent,
.grand .parentish {
  color: cyan;
}

Changing selector order

It can be useful to prepend a selector to the inherited (parent) selectors. This can be done by putting the & after current selector. For example, when using Modernizr, you might want to specify different rules based on supported features:

.header {
  .menu {
    border-radius: 5px;
    .no-borderradius & {
      background-image: url('images/button-background.png');
    }
  }
}

The selector .no-borderradius & will prepend .no-borderradius to its parent .header .menu to form the.no-borderradius .header .menu on output:

.header .menu {
  border-radius: 5px;
}
.no-borderradius .header .menu {
  background-image: url('images/button-background.png');
}

Combinatorial explosion

& can also be used to generate every possible permutation of selectors in a comma separated list:

p, a, ul, li {
border-top: 2px dotted #366;
  & + & {
      border-top: 0;
  }
}

This expands to all possible (16) combinations of the specified elements:

p,
a,
ul,
li {
  border-top: 2px dotted #366;
}
p + p,
p + a,
p + ul,
p + li,
a + p,
a + a,
a + ul,
a + li,
ul + p,
ul + a,
ul + ul,
ul + li,
li + p,
li + a,
li + ul,
li + li {
  border-top: 0;
}