Less.js 用法
编辑:浏览器知识Edit the markdown source for “command-line-usage”
Compile
.less
files to.css
using the command line
Heads up! If the command line isn’t your thing, learn more about GUIs for Less.
Installing
Install with npm
npm install less -g
The -g
option installs the lessc
command available globally. For a specific version (or tag) you can add @VERSION
after our package name, e.g. npm install less@2.7.1 -g
.
Installing for Node Development
Alternatively, if you don’t want to use the compiler globally, you may be after
npm i less --save-dev
This will install the latest official version of lessc in your project folder, also adding it to the devDependencies
in your project’s package.json
.
Beta releases of lessc
Periodically, as new functionality is being developed, lessc builds will be published to npm, tagged as beta. These builds will not be published as a @latest
official release, and will typically have beta in the version (use lessc -v
to get current version).
Since patch releases are non-breaking we will publish patch releases immediately and alpha/beta/candidate versions will be published as minor or major version upgrades (we endeavour since 1.4.0 to follow semantic versioning).
Server-Side and Command Line Usage
The binary included in this repository, bin/lessc
works with Node.js on *nix, OS X and Windows.
Usage: lessc [option option=parameter ...] <source> [destination]
Command Line Usage
lessc [option option=parameter ...] <source> [destination]
If source is set to `-‘ (dash or hyphen-minus), input is read from stdin.
Examples
Compile bootstrap.less to bootstrap.css
lessc bootstrap.less bootstrap.css
Options specific to lessc
For all other options, see Less Options.
Silent
lessc -s lessc –silent
Stops any warnings from being shown.
Version
lessc -v
lessc --version
Help
lessc --help |
|
lessc -h |
Prints a help message with available options and exits.
Makefile
lessc -M
lessc --depends
Outputs a makefile import dependency list to stdout.
No Color
Deprecated.
lessc --no-color
Clean CSS
In v2 of less, Clean CSS is no longer included as a direct dependency. To use clean css with lessc, use the clean css plugin.
Edit the markdown source for “using-less-in-the-browser”
Using Less.js in the browser is the easiest way to get started and convenient for developing with Less, but in production, when performance and reliability is important, we recommend pre-compiling using Node.js or one of the many third party tools available.
To start off, link your .less
stylesheets with the rel
attribute set to “stylesheet/less
“:
<link rel="stylesheet/less" type="text/css" href="styles.less" />
Next, download less.js and include it in a <script></script>
tag in the <head>
element of your page:
<script src="less.js" type="text/javascript"></script>
Setting Options
You can set options either programmatically, by setting them on a less object before the script tag – this then affects all initial link tags and programmatic usage of less.
<script>
less = {
env: "development",
async: false,
fileAsync: false,
poll: 1000,
functions: {},
dumpLineNumbers: "comments",
relativeUrls: false,
rootpath: ":/a.com/"
};
</script>
<script src="less.js"></script>
The other way is to specify the options on the script tag, e.g.
<script>
less = {
env: "development"
};
</script>
<script src="less.js" data-env="development"></script>
Or for brevity they can be set as attributes on the script and link tags:
<script src="less.js" data-poll="1000" data-relative-urls="false"></script>
<link data-dump-line-numbers="all" data-global-vars='{ "myvar": "#ddffee", "mystr": "\"quoted\"" }' rel="stylesheet/less" type="text/css" href="less/styles.less">
Browser Support
Less.js supports all modern browsers (recent versions of Chrome, Firefox, Safari, IE11+, and Edge). While it is possible to use Less on the client side in production, please be aware that there are performance implications for doing so (although the latest releases of Less are quite a bit faster). Also, sometimes cosmetic issues can occur if a JavaScript error occurs. This is a trade off of flexibility vs. speed. For the fastest performance possible for a static web site, we recommend compiling Less on the server side.
There are reasons to use client-side less in production, such as if you want to allow users to tweak variables which will affect the theme and you want to show it to them in real-time – in this instance a user is not worried about waiting for a style to update before seeing it.
Tips
- Make sure you include your stylesheets before the script.
- When you link more than one
.less
stylesheet each of them is compiled independently. So any variables, mixins or namespaces you define in a stylesheet are not accessible in any other. - Due to the same origin policy of browsers, loading external resources requires enabling CORS
Watch Mode
To enable Watch mode, option env
must be set to development
. Then AFTER the less.js file is included, call less.watch()
, like this:
<script>less = { env: 'development'};</script>
<script src="less.js"></script>
<script>less.watch();</script>
Alternatively, you can enable Watch mode temporarily by appending #!watch
to the URL.
Modify Variables
Enables run-time modification of Less variables. When called with new values, the Less file is recompiled without reloading. Simple basic usage:
less.modifyVars({
'@buttonFace': '#5B83AD',
'@buttonText': '#D9EEF2'
});
Debugging
It is possible to output rules in your CSS which allow tools to locate the source of the rule.
Either specify the option dumpLineNumbers
as above or add !dumpLineNumbers:mediaquery
to the url.
You can use the mediaquery
option with FireLESS (it is identical to the SCSS media query debugging format). Also see FireLess and Less v2. The comment
option can be used to display file information and line numbers in the inline compiled CSS code.
Options
Set options in a global less
object before loading the less.js script:
<!-- set options before less.js script -->
<script>
less = {
env: "development",
logLevel: 2,
async: false,
fileAsync: false,
poll: 1000,
functions: {},
dumpLineNumbers: "comments",
relativeUrls: false,
globalVars: {
var1: '"quoted value"',
var2: 'regular value'
},
rootpath: ":/a.com/"
};
</script>
<script src="less.js"></script>
Options specific to Less.js in the browser
For all other options, see Less Options.
async
Type: Boolean
Default: false
Whether to request the import files with the async option or not. See fileAsync.
env
Type: String
Default: depends on page URL
Environment to run may be either development
or production
.
In production, your css is cached in local storage and information messages are not output to the console.
If the document’s URL starts with file://
or is on your local machine or has a non standard port, it will automatically be set to development
.
e.g.
less = { env: 'production' };
errorReporting
Type: String
Options: html
|console
|function
Default: html
Set the method of error reporting when compilation fails.
fileAsync
Type: Boolean
Default: false
Whether to request the import asynchronously when in a page with a file protocol.
functions (Deprecated – use @plugin)
Type: object
User functions, keyed by name.
less = {
functions: {
myfunc: function() {
return less.Dimension(1);
}
}
};
and it can be used like a native Less function e.g.
.my-class {
border-width: unit(myfunc(), px);
}
logLevel
Type: Number
Default: 2
The amount of logging in the javascript console. Note: If you are in the production
environment you will not get any logging.
2 - Information and errors
1 - Errors
0 - Nothing
poll
Type: Integer
Default: 1000
The amount of time (in milliseconds) between polls while in watch mode.
relativeUrls
Type: Boolean
Default: false
Optionally adjust URLs to be relative. When false, URLs are already relative to the entry less file.
useFileCache
Type: Boolean
Default: true
(previously false
in before v2)
Whether to use the per session file cache. This caches less files so that you can call modifyVars and it will not retrieve all the less files again. If you use the watcher or call refresh with reload set to true, then the cache will be cleared before running.
Edit the markdown source for “less-options”
Cross-Platform Options
Include Paths
lessc --include-path=PATH1;PATH2 |
{ paths: ['PATH1', 'PATH2'] } |
If the file in an @import
rule does not exist at that exact location, Less will look for it at the location(s) passed to this option. You might use this for instance to specify a path to a library which you want to be referenced simply and relatively in the Less files.
Rootpath
lessc -rp=resources/ lessc --rootpath=resources/ |
{ rootpath: 'resources/' } |
Allows you to add a path to every generated import and url in your css. This does not affect Less import statements that are processed, just ones that are left in the output css.
For instance, if all the images the css use are in a folder called resources, you can use this option to add this on to the URL’s and then have the name of that folder configurable.
Rewrite URLs
lessc -ru=off lessc --rewrite-urls=off |
{ rewriteUrls: 'off' } |
lessc -ru=all lessc --rewrite-urls=all |
{ rewriteUrls: 'all' } |
lessc -ru=local lessc --rewrite-urls=local |
{ rewriteUrls: 'local' } |
By default URLs are kept as-is (off
), so if you import a file in a sub-directory that references an image, exactly the same URL will be output in the css. This option allows you to rewrite URLs in imported files so that the URL is always relative to the base file that has been passed to Less. E.g.
/* main.less */
@import "global/fonts.less";
/* global/fonts.less */
@font-face {
font-family: 'MyFont';
src: url('myfont/myfont.woff2') format('woff2');
}
With nothing set or with rewriteUrls: 'off'
, compiling main.less
will output:
/* main.less */
/* global/fonts.less */
@font-face {
font-family: 'MyFont';
src: url('myfont/myfont.woff2') format('woff2');
}
With rewriteUrls: 'all'
, it will output:
/* main.less */
/* global/fonts.less */
@font-face {
font-family: 'MyFont';
src: url('./global/myfont/myfont.woff2') format('woff2');
}
With rewriteUrls: 'local'
, it will only rewrite URLs that are explicitly relative (those starting with a .
):
url('./myfont/myfont.woff2') /* becomes */ url('./global/myfont/myfont.woff2')
url('myfont/myfont.woff2') /* stays */ url('myfont/myfont.woff2')
This can be useful in case you’re combining Less with CSS Modules which use similar resolving semantics like Node.js.
You may also want to consider using the data-uri function instead of this option, which will embed images into the css.
Math
Released v3.7.0
lessc -m=[option] lessc --math=[option] |
{ math: '[option]' } |
Less has re-built math options to offer an in-between feature between the previous strictMath
setting, which required parentheses all the time, and the default, which performed math in all situations.
In order to cause fewer conflicts with CSS, which now liberally uses the /
symbol between values, there is now a math mode that only requires parentheses for division. “Strict math” has also been tweaked to operate more intuitively, although the legacy behavior is supported.
The for options available for math
are:
always
(current default) – Less eagerly does mathparens-division
(future default) – No division is performed outside of parens using/
operator (but can be “forced” outside of parens with./
operator)parens
|strict
– A more intuitive form of legacystrictMath: true
strict-legacy
(deprecated) – As named, operates exactly like currentstrictMath: true
, with the exception thatwidth: -(1);
(single dimension values in parens) will now outputwidth: -1;
vs previous behavior ofwidth: -(1)
always Example:
.math {
a: 1 + 1;
b: 2px / 2;
c: 2px ./ 2;
d: (2px / 2);
}
Outputs:
.math {
a: 2;
b: 1px;
c: 1px;
d: 1px;
}
parens-division
Example:
.math {
a: 1 + 1;
b: 2px / 2;
c: 2px ./ 2;
d: (2px / 2);
}
Outputs:
.math {
a: 2;
b: 2px / 2;
c: 1px;
d: 1px;
}
strict
.math {
a: 1 + 1;
b: 2px / 2;
c: (2px / 2) + (3px / 1);
}
Output:
.math {
a: 1 + 1;
b: 2px / 2;
c: 1px + 3px;
}
strict-legacy
In legacy strictMath
mode, mixed expressions outside of parentheses means entire parentheses won’t evaluate. (Probably not what you want.)
.math {
a: 1 + 1;
b: 2px / 2;
c: (2px / 2) + (3px / 1);
}
Output:
.math {
a: 1 + 1;
b: 2px / 2;
c: (2px / 2) + (3px / 1);
}
Strict Math (Deprecated)
This has been replaced by the math
option.
Relative URLs (deprecated)
lessc -ru lessc --relative-urls |
{ relativeUrls: true } |
Has been replaced by rewriteUrls: "all"
Strict Units
lessc -su=on lessc --strict-units=on |
{ strictUnits: true } |
Defaults to off/false.
Without this option, Less attempts to guess at the output unit when it does maths. For instance
.class {
property: 1px * 2px;
}
In this case, things are clearly not right – a length multiplied by a length gives an area, but css does not support specifying areas. So we assume that the user meant for one of the values to be a value, not a unit of length and we output 2px
.
With strict units on, we assume this is a bug in the calculation and throw an error.
IE8 Compatibility (Deprecated)
lessc --ie-compat |
{ ieCompat: true } |
False by default starting in v3.0.0. Currently only used for the data-uri function to ensure that images aren’t created that are too large for the browser to handle.
Enable Inline JavaScript (Deprecated)
lessc --js |
{ javascriptEnabled: true } |
False by default starting in v3.0.0. Enables evaluation of JavaScript inline in .less
files. This created a security problem for some developers who didn’t expect user input for style sheets to have executable code.
Replaced with the @plugin
option.
Global Variables
lessc --global-var="color1=red" |
{ globalVars: { color1: 'red' } } |
This option defines a variable that can be referenced by the file. Effectively the declaration is put at the top of your base Less file, meaning it can be used but it also can be overridden if this variable is defined in the file.
Modify Variables
lessc --modify-var="color1=red" |
{ modifyVars: { color1: 'red' } } |
As opposed to the global variable option, this puts the declaration at the end of your base file, meaning it will override anything defined in your Less file.
URL Arguments
lessc --url-args="cache726357" |
{ urlArgs: 'cache726357' } |
This option allows you to specify a argument to go on to every URL. This may be used for cache-busting for instance.
Line Numbers (Deprecated)
lessc --line-numbers=comments lessc --line-numbers=mediaquery lessc --line-numbers=all |
{ dumpLineNumbers: 'comments' } |
Generates inline source-mapping. This was the only option before browsers started supporting sourcemaps.
Pre-Loaded Plugin
See: Pre-Loaded Plugins
Lint
lessc --lint -l |
{ lint: true } |
Runs the less parser and just reports errors without any output.
Compress (Deprecated)
lessc --compress -x |
{ compress: true } |
Compress using less built-in compression. This does an okay job but does not utilise all the tricks of dedicated css compression. In general, we recommend looking at third-party tools that clean and compress CSS after your Less has been transformed to CSS.
Allow Imports from Insecure HTTPS Hosts
lessc --insecure |
{ insecure: true } |
Source Map Options
Most of these options are not applicable to using Less.js in the browser, as you should generate a source map with your pre-compiled Less files.
Generate a Source Map
lessc --source-map |
{ sourceMap: {} } |
Tells less to generate a sourcemap.
Source Map Output Filename
lessc --source-map=file.map |
{ sourceMap: { outputFilename: 'file.map' } } |
Source Map Rootpath
lessc --source-map-rootpath=dev-files/ |
{ sourceMap: { sourceMapRootpath: 'dev-files/' } } |
Specifies a rootpath that should be prepended to each of the less file paths inside the sourcemap and also to the path to the map file specified in your output css.
Because the basepath defaults to the directory of the input less file, the rootpath defaults to the path from the sourcemap output file to the base directory of the input less file.
Use this option if for instance you have a css file generated in the root on your web server but have your source less/css/map files in a different folder. So for the option above you might have
output.css
dev-files/output.map
dev-files/main.less
Source Map Basepath
lessc --source-map-basepath=less-files/ |
{ sourceMap: { sourceMapBasepath: 'less-files/' } } |
This is the opposite of the rootpath option, it specifies a path which should be removed from the output paths. For instance if you are compiling a file in the less-files directory but the source files will be available on your web server in the root or current directory, you can specify this to remove the additional less-files
part of the path.
It defaults to the path to the input less file.
Include Less Source in the Source Map
lessc --source-map-include-source |
{ sourceMap: { outputSourceFiles: true } } |
This option specifies that we should include all of the Less files in to the sourcemap. This means that you only need your map file to get to your original source.
This can be used in conjunction with the map inline option so that you do not need to have any additional external files at all.
Source Map Map Inline
lessc --source-map-inline |
{ sourceMap: { sourceMapFileInline: true } } |
This option specifies that the map file should be inline in the output CSS. This is not recommended for production, but for development it allows the compiler to produce a single output file which in browsers that support it, use the compiled css but show you the non-compiled less source.
Source Map URL
lessc --source-map-url=../my-map.json |
{ sourceMap: { sourceMapURL: '../my-map.json' } } |
Allows you to override the URL in the css that points at the map file. This is for cases when the rootpath and basepath options are not producing exactly what you need.
Edit the markdown source for “plugins”
Loading plugins before parsing begins in Less.js
While the easiest way to use a plugin is using the @plugin
at-rule, in a Node.js environment, you can pre-load a global Less.js plugin via the command line or by specifying it in the Less options.
Preprocessing
Pre-loading plugins is necessary if you want to add a Less.js Pre-processor. That is, a plugin that gets called and passed the raw Less source before parsing even starts. An example of this would be a Sass-To-Less Pre-processor plugin.
Note: pre-loading is not necessary for pre-evaluation plugins (after Less source is parsed, but before it is evaluated).
Node.js
Using the Command Line
If you are using lessc, the first thing you need to do is install that plugin. In registries like NPM, we recommend a Less.js plugin is registered with the “less-plugin-” prefix (for easier searching), though that isn’t required. So, for a custom plugin, you might install with:
npm install less-plugin-myplugin
To use the plugin, you can pass this on the command line by simply writing:
lessc --myplugin
Less.js will try to load either the “less-plugin-myplugin” and the “myplugin” modules as plugins whenever there’s an unknown Less option (like “myplugin”).
You can also explicitly specify the plugin with:
lessc --plugin=myplugin
To pass options to the plugin, you can write that in one of two ways.
lessc --myplugin="advanced"
lessc --plugin=myplugin=advanced
Loading a Plugin via Less.js
In Node, require the plugin and pass it to less
in an array as an option plugins. E.g.
var LessPlugin = require('less-plugin-myplugin');
less.render(myCSS, { plugins: [LessPlugin] })
.then(
function(output) { },
function(error) { }
);
Edit the markdown source for “programmatic-usage” The main entry point into less
is the less.render
function. This takes the following format
less.render(lessInput, options)
.then(function(output) {
// output.css = string of css
// output.map = string of sourcemap
// output.imports = array of string filenames of the imports referenced
},
function(error) {
});
// or...
less.render(css, options, function(error, output) {})
The options argument is optional. If you specify a callback then a promise will not be returned, where as if you do not specify a callback a promise will be given. Under the hood, the callback version is used so that less can be used synchronously.
If you wanted to render a file, you would first read it into a string (to pass to less.render) and then set the filename field on options to be the filename of the main file. less will handle all the processing of the imports.
The sourceMap
option is an object which enables you to set sub sourcemap options. Available sub options are: sourceMapURL
,sourceMapBasepath
,sourceMapRootpath
,outputSourceFiles
and sourceMapFileInline
. Notice that the sourceMap
option is not available for the less.js in browser compiler now.
less.render(lessInput)
.then(function(output) {
// output.css = string of css
// output.map = undefined
}
//,
less.render(lessInput, {sourceMap: {}})
.then(function(output) {
// output.css = string of css
// output.map = string of sourcemap
}
//or,
less.render(lessInput, {sourceMap: {sourceMapFileInline: true}})
.then(function(output) {
// output.css = string of css \n /*# sourceMappingURL=data:application/json;base64,eyJ2ZXJ..= */
// output.map = undefined
}
Getting Access to the Log
You can add a log listener with the following code
less.logger.addListener({
debug: function(msg) {
},
info: function(msg) {
},
warn: function(msg) {
},
error: function(msg) {
}
});
Note: all functions are optional. An error will not be logged, but instead is passed back to the callback or promise in less.render
Edit the markdown source for “developing-less” Thanks for thinking about contributing! Please read the contributing instructions carefully to avoid wasted work.
Install These Tools
- node – http://nodejs.org/
- phantomjs – http://phantomjs.org/download.html
make sure the paths are setup. If you start your favourite command line and type node -v
you should see the node compiler. If you run phantomjs -v
you should see the phantomjs version number.
- clone your less.js repository locally
- navigate to your local less.js repository and run
npm install
this installs less’ npm dependencies.
Usage
Grunt is used in order to run development commands such as tests, builds, and benchmarks. You can run them either with grunt [command_name]
if you have grunt-cli
installed globally or with npm run grunt -- [command_name]
.
If you go to the root of the Less repository you should be able to do npm test
(a handy alias for npm run grunt -- test
) – this should run all the tests. For the browser specific only, run npm run grunt -- browsertest
If you want to try out the current version of less against a file, from here do node bin/lessc path/to/file.less
To debug the browser tests, run npm run grunt -- browsertest-server
then go to http://localhost:8088/tmp/browser/ to see the test runner pages.
Optional: To get the current version of the Less compiler do npm -g install less
– npm is the node package manager and “-g” installs it to be available globally.
You should now be able to do lessc file.less
and if there is an appropriate file.less
then it will be compiled and output to the stdout. You can then compare it to running locally (node bin/lessc file.less
).
Other grunt commands
npm run grunt -- benchmark
– run our benchmark tests to get some numbers on performancenpm run grunt -- stable
to create a new releasenpm run grunt -- readme
to generate a new readme.md in the root directory (after each release)
How to Run Less in Other Environments
If you look in the libs folder you will see less
, less-node
, less-browser
. The less
folder is pure javascript with no environment specifics. if you require less/libs/less
, you get a function that takes an environment object and an array of file managers. The file managers are the same file managers that can also be written as a plugin.
var createLess = require("less/libs/less"),
myLess = createLess(environment, [myFileManager]);
The environment api is specified in less/libs/less/environment/environment-api.js and the file manager api is specified in less/libs/less/environment/file-manager-api.js.
For file managers we highly recommend setting the prototype as a new AbstractFileManager – this allows you to override what is needed and allows us to implement new functions without breaking existing file managers. For an example of file managers, see the 2 node implementations, the browser implementation or the npm import plugin implementation.
Guide
If you look at http://www.gliffy.com/go/publish/4784259, This is an overview diagram of how less works. Warning! It needs updating with v2 changes.
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