What is a CSS preprocessor? Get started with LESS (Linear Style Sheet)

CSS is generally used with HTML to style web pages. But CSS is primitive and incomplete as it’s very hard to achieve in CSS to build a function, reuse a definition or inheritance and many more. To achieve these properties, CSS preprocessors come into the picture.


We are considering that you have a good understanding of CSS properties and react js components to continue with this blog.

Getting Started:

The CSS preprocessors are scripting languages that extend CSS  with variables, operators, conditions, functions, mixins and many more other usable properties and then we compile that scripted language to our regular CSS. SASS, LESS and Stylus are the well-known CSS preprocessors.

CSS preprocessors help us to write reusable and maintainable CSS code. 

Popular CSS Preprocessors:

There are currently three of the most popular and stable CSS preprocessors SASS, LESS and Stylus but there are many others as well. All CSS preprocessors accomplish similar tasks, but they do so in a slightly different manner and with different syntax.

Each CSS preprocessor has its own syntax that they compile into regular CSS. And also every one of them has its own feature and ecosystem (framework or library).

Get started with LESS:

LESS stands for (Linear Style Sheet). It was initially released in 2009 and it is influenced by Sass, therefore it implements many of its features, including mixins, variables, and nesting.

LESS uses the standard CSS syntax with the .less file extension. This means that a valid .css file is also a valid .less file. Therefore, it’s really easy to learn LESS if you know CSS, even if there are a few extra elements that are not found in CSS.


Following are some of the important features of LESS:


The variables are used to provide reusability. We use @ sign for creating the variables in LESS.


 @color : green;

 @table_height : 400px;

 @table_width : 700px;

 We can also perform calculations while creating the variables.

//Arithmetical operations +, -, *, / in less

@table_height: 400px;

@table_width: @table_height + 300px//result 700px

@min_height: @table_height - 100px//result 300px
@max_height : @table_height * 2//result 800px

@head_color : #224488 / 2; //results #112244

Parent Selector:

The parent selector lets us access the parent (root) element.

@heading_color: green;

.heading {
 color: @heading_color;
 &:hover {
   color: red;


Mixins allow us to embed all the properties of a class into another class. It behaves like a function and also takes the arguments.

.flex-property(@justify-property, @bordersize:0px, @borderstyle:solid, @bordercolor:white ) {
 display: flex;
 flex-direction: row;
 flex-wrap: wrap;
 justify-content: @justify-property;
 border: @bordersize @borderstyle @bordercolor;

In upper code, we have created mixin which has flex property. We can also pass parameters exactly the same as parameters we pass in functions. 

let’s see how we can use this mixin.

#nav-item {
 ul {
   .flex-property(space-between); //here we have used mixin and also have passed the argument.

   padding: 0px;
   margin: 0px;
   margin-top: 10px;
   text-align: center;
   li {
     display: inline-block;
     color: @light-color;
     font-size: 20px;
     text-decoration: underline;


We can call mixin inside the mixin which creates recursion. Below we have shown how exactly it works.

.generate-columns (4);
.generate-columns (@n, @i: 1) when (@i =< @n) {
 .column-@{i} {
   width: (@i * 100% / @n);
   .generate-columns (@n, (@i + 1));;

After compilation to CSS.

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

Mixin guard:

Mixin guard is useful when we want to put some style on the basis of an expression. let’s see its example.

.mixin (@a) when (lightness(@a) >= 50%) {
 font-size: 24px;
.mixin (@a) when (lightness(@a) < 50%) {
 font-size: 20px;
.mixin (@a) {
 color: @a;
.class1 {
 .mixin(hsl(120, 50%, 80%));
.class2 {
 .mixin(hsl(120, 50%, 40%));

//After compilation
.class1 {
 font-size: 24px;
 color: #b3e6b3;
.class2 {
 font-size: 20px;
 color: #339933;

In the above example, we are applying different font sizes to class1 and class2 based on the lightness of the color which is applied to both of the classes.


The merge property is used to extend the multiple properties into a single property. We can use this property in two ways either through coma (,) or space merge. For coma, we use the “+” sign after every attribute and for space, we use the “+_” sign.

This property is helpful while applying shadow or transition effect.

//less code
.myfunc () {
 box-shadow+: 5px 5px 5px grey;
.class {
 box-shadow+: 0 0 5px #f78181;

//after compilation
 Box-shadow: 5px 5px 5px grey, 0 0 5px #f78181;

//less code
.mixin() {
 transform +_: skew(10deg, 10deg);
.myclass {

 transform+_: rotate(-15deg);

//after compilation
 transform: skew(10deg, 10deg) rotate(10deg, 10deg);

Nested directives:

With the help of the nested rules of less, we can implement the media query code which is nothing but the nested directives.

.myclass {
 @media screen {
   color: blue;
   @media (min-width: 1024px) {
     color: green; //this will apply when screen width 

                     is >= 1024px

Here we have discussed all the features of LESS which are very much used and I believe this is enough to get you started with LESS in your next project. For further reading, you can explore the official documentation of less.

How to use LESS in react js:

If you have created the react project with CRA (create react app) then it’s by default support only saas and CSS.  If we want to use LESS in our react project then either we can run the npm run eject command first and then modify webpack configs or we can also use the concurrently module.

Let’s see how we can use LESS in react project with the help of concurrently module step by step.

1) Create react project:

First of all, we need to create a new react app. Open your terminal create a basic react app with CRA command.

npx create-react-app less-with-react

2) Import all required modules:

We need to import all the required modules. Open your terminal and inside your project run the below command.

npm install less less-watch-compiler concurrently --save-dev

Here we imported less which is obvious and a less-watch compiler which is needed for compiling the LESS file into CSS and concurrently module.

3) Create config file:

Now create a less-watcher.config.json file inside your root directory and set the below configuration.

    "watchFolder": "src/",
    "outputFolder": "src/",
    "runOnce": false,
    "enableJs": true

Here we can change the “watchFolder” and “outputFolder” src according to our .less file location.

4) Change scripts:

Inside your package.json file replace the start script in package.json with the following.


   "start": "concurrently --kill-others \"less-watch-compiler --config    less-watcher.config.json\" \"react-scripts start\"",

 "build": "react-scripts build"



And here is done, now we are ready to use the LESS file inside our react project

For the demo, you can refer to the following link


We can write reusable and maintainable code in CSS with the help of the advanced features of Pre-processors. By using a pre-processor, we can easily increase our productivity and decrease the amount of code in the project.



In this article, we discussed the CSS preprocessors, their types and the features of one of the pre-processor LESS. We have also seen how to use Less inside the react application.

Thank you for reading! In case you have any questions feel free to comment.

Nadeem K

Associate Software Engineer

Nadeem is a front-end developer with 1.5+ years of experience. He has experience in web technologies like React.js, Redux, and UI frameworks. His expertise in building interactive and responsive web applications, creating reusable components, and writing efficient, optimized, and DRY code. He enjoys learning about new technologies.

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