Introduction to Svelte

Introduction to Svelte

Svelte is a javascript framework which is used to build fast web applications. Similar to React or Vue but in svelte we don’t need any dependencies in runtime like react or vue takes some time to interpret our code it means in svelte we can get pure JavaScript.

If you are worried about tons of front-end frameworks and their variations out there, why yet another framework?

The frameworks like Angular, ReactJS or VueJS make it easier, faster to spin up applications. More robust and efficient application development within a team.

They changed the way in which we design, develop, and deliver the applications.

More and more frameworks refine the frameworks and make it, even more, easier to develop simple, fast, and efficient applications.

Angular ❤️

Angular is a complete suite but it is bloated.

React 💙

React is a view only library and uses virtual DOM but React gets complicated.

Vue 💚

VueJS is a simple to write and has the best parts of React and Angular.

Svelte 🧡

Svelte is a new approach to build user interfaces. The Svelte has no virtual DOM.

Svelte makes you write less code and uses the platform.

Svelte has no virtual DOM and uses the platform.

Svelte optimizes the application during the build process that increases your application's performance.

Get the app and start running

Enough of (boring) framework introductions, let us start coding.

Where to start?

"Hello World"

npx degit sveltejs/template hello-world
cd hello-world

You have cloned a template from the sveltejs/template. Now install the dependencies and spin the server using

npm i
npm run dev

You should see a simple Hello World application running on localhost:5000.

Let us dive deeper and look at what is generated.

The Svelte components are defined in .svelte files. The generated application has the App.svelte.

The Svelte components are nothing but a simple HTML file. All Svelte components can have the following three segments:

  • Script - to define the JavaScript
  • Style - to define the CSS
  • plain HTML - just your HTML and Svelte elements

If you have worked with any frameworks listed above then you might know what are props.

Props are attributes that you pass it along the Components.

The export let name inside the script tag says that the name is exported from the component and we can send it the information via the attributes.

The props attached to your components will be first exported out of the component.

<script>
        export let name;
</script>

<style> h1 { color: purple; } </style>

Any variable (including the props) that we defined inside the <script>component can be used in the HTML component with {variable_name}notation. The {} is used to define the value defined in the script tag.

<h1>Hello {name}!</h1>

The application starts in the main.js. We import the App.svelte (where the App component is defined).

import App from './App.svelte';

Then we create an App object from the imported App component. Then we define target and also pass in props to the component's constructor.

The target defines where to render the component in the HTML document.

The props is the place where we will define the attributes.

const app = new App({
        target: document.body,
        props: {
                name: 'world'
        }
});

Finally, export the app from the main.js.

export default app;

There is the rollup.config.js which takes care of bundling and building the application.

import App from './App.svelte';

Want a shorter alternative, fork this codesandbox


Tic Tac Toe

Let us re-create the classic Tic Tac Toe from the react (official) example with Svelte.

Create a components folder, this is where we will define all the Sveltecomponents except the App.svelte.

We will need the following components:

  • Square.svelte - Each square in the Tic Tac Toe will be a separate svelte component.
  • Board.svelte - The Board component will hold all the square components. This component will be responsible for passing the data to its child square components and also decide whether the game is still on or finished.
  • Game.svelte - The Game component is an overall wrapper around the Board component.

Let us first create Game.svelte component. The Game Component will hold the Board component.

<div class="game">
    <Board />
</div>

Now we will need to import the Board component.

<script> 
    import Board from './Board.svelte';
</script>

Let us style the board a little bit.

<style>
.game {
    display: flex;
    flex-direction: row;
}
</style>

The Board component will have three rows of three squares in each.

<div class="board-row">
    <Square />
    <Square />
    <Square />
</div>
<div class="board-row">
    <Square />
    <Square />
    <Square />
</div>
<div class="board-row">
    <Square />
    <Square />
    <Square />
</div>

We will need to import the Square component in the <script> section.

<script>
import Square from './Square.svelte';
</script>

Let us style them a little bit.

<style>
.board-row:after {
    clear: both;
    content: "";
    display: table;
}
</style>

Then we will define the Square component inside the Square.svelte.

<style>
 .square {
    background: #fff;
    border: 1px solid #999;
    float: left;
    font-size: 24px;
    font-weight: bold;
    line-height: 34px;
    height: 34px;
    margin-right: -1px;
    margin-top: -1px;
    padding: 0;
    text-align: center;
    width: 34px;
  }

.square:focus { outline: none; }

.square:focus { background: #ddd; } </style>

<script> let value = ""; </script>

<button class="square"> {value} </button>

We defined value and used that inside the button element.

When we click the button it should change the value into X. We will use the on:click event handler.

<button class="square" on:click={() => handleClick}> {value} </button>

The event handler in Svelte is defined on:<event>.

Let us define the handleClick inside the <script> tag.

function handleClick() {
   value = 'X';
}

Now click on the button, you should see the value of button is changed into X.

There is no this, no complex bindings but a simple variable declaration and changing it.

It is not easy to maintain the state in the child component and then propagating it to the parent. Rather we can move the state to the parent and then make the parent decide on how to maintain the child component. To do this, let us change the Board component and send the value and the on:click event handler through the Board component.

Let us consider both the on:click and value is a prop to the Squarecomponent.

<script> 
export let value; 
export let handleClick;
</script>

<button class="square" on:click={handleClick}> {value} </button>

Now we will modify the Board component. Instead of defining each Boardwe will define an array squares and use it.

<script>
    let squares = Array(9).fill('');
</script>

and change the HTML into

  <div class="board-row">
    <Square value={squares[0]} handleClick={() => handleClick(0)}/>
    <Square value={squares[1]} handleClick={() => handleClick(1)}/>
    <Square value={squares[2]} handleClick={() => handleClick(2)}/>
  </div>

<div class="board-row"> <Square value={squares[3]} handleClick={() => handleClick(3)} /> <Square value={squares[4]} handleClick={() => handleClick(4)} /> <Square value={squares[5]} handleClick={() => handleClick(5)} /> </div>

<div class="board-row"> <Square value={squares[6]} handleClick={() => handleClick(6)} /> <Square value={squares[7]} handleClick={() => handleClick(7)} /> <Square value={squares[8]} handleClick={() => handleClick(8)} /> </div>

We will also need to define the handleClick method.

function handleClick(i) {
    squares[i] = 'X';
}

👏 Congrats that is awesome. Let us build the real game.

The Game is played alternatively, that is one player marks with X and the other with O. Let us add that condition in the Board component.

<!-- Board.svelte -->
let isXTurn = true;

function handleClick(i) { squares[i] = isXTurn ? 'X' : 'O'; isXTurn = !isXTurn; }

Cool, now we have to calculate the winner.

function calculateWinner(squares) {
    const lines = [
      [0, 1, 2],
      [3, 4, 5],
      [6, 7, 8],
      [0, 3, 6],
      [1, 4, 7],
      [2, 5, 8],
      [0, 4, 8],
      [2, 4, 6]
    ];
    for (let i = 0; i < lines.length; i++) {
      const [a, b, c] = lines[i];
      if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
        return squares[a];
      }
    }
    return null;
}

The above function will return X or O if there is a winner or else just returns a null.

We will have to check this every time when the user clicks the square. Also once a player clicks a square we should not allow any other clicks on that square.

function handleClick(i) {
    if (calculateWinner(squares) || squares[i] !== '') {
      return;
    }
   squares[i] = isXTurn ? 'X' : 'O';
   isXTurn = !isXTurn;
}

We will need to show the game status for the players. The game status information is dynamic. That is it will change and there should be someone listen to it and change the view once updated.

In React world we will have the state. In Svelte we bind the components with $. The values will be updated.

$: winner = calculateWinner(squares)

$: status = winner ? "Winner: " + winner :"Next player:" + (xIsNext ? "X" : "O");

We will use the status inside the HTML.

 <div class="status"> {status} </div>

<style> .status { margin-bottom: 10px; } </style>

Now the status is re-computed whenever there is a change.


The Svelte components are slim, no extra boilerplates, no this and it is much closer to the platform. In fact, they are nothing but simple HTML.

Thanks For Visiting, Keep Visiting. If you liked this post, share it with all of your programming buddies!

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