Svelte for the Experienced React Dev

“Hello, World!” Svelte style Let’s start with a quick tour of what a Svelte component looks like.

<script>
  let number = 0;
</script>

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

<h1>Value: {number}</h1>

<button on:click={() => number++}>Increment</button>
<button on:click={() => number--}>Decrement</button> 

That content goes in a .svelte file, and is processed by the Rollup or webpack plugin to produce a Svelte component. There’s a few pieces here. Let’s walk through them.

First, we add a <script> tag with any state we need.

We can also add a <style> tag with any CSS we want. These styles are scoped to the component in such a way that, here, <h1> elements in this component will be blue. Yes, scoped styles are built into Svelte, without any need for external libraries. With React, you’d typically need to use a third-party solution to achieve scoped styling, such as css-modules, styled-components, or the like (there are dozens, if not hundreds, of choices).

Then there’s the HTML markup. As you’d expect, there are some HTML bindings you’ll need to learn, like {#if}, {#each}, etc. These domain-specific language features might seem like a step back from React, where everything is “just JavaScript.” But there’s a few things worth noting: Svelte allows you to put arbitrary JavaScript inside of these bindings. So something like this is perfectly valid:

{#if childSubjects?.length}

If you jumped into React from Knockout or Ember and never looked back, this might come as a (happy) surprise to you.

Also, the way Svelte processes its components is very different from React. React re-runs all components any time any state within a component, or anywhere in an ancestor (unless you “memoize”), changes. This can get inefficient, which is why React ships things like useCallback and useMemo to prevent un-needed re-calculations of data.

Svelte, on the other hand, analyzes your template, and creates targeted DOM update code whenever any relevant state changes. In the component above, Svelte will see the places where number changes, and add code to update the <h1> text after the mutation is done. This means you never have to worry about memoizing functions or objects. In fact, you don’t even have to worry about side-effect dependency lists, although we’ll get to that in a bit.

But first, let’s talk about …

State management In React, when we need to manage state, we use the useState hook. We provide it an initial value, and it returns a tuple with the current value, and a function we can use to set a new value. It looks something like this:

import React, { useState } from "react";

export default function (props) {
  const [number, setNumber] = useState(0);
  return (
    <>
      <h1>Value: {number}</h1>
      <button onClick={() => setNumber(n => n + 1)}>Increment</button>
      <button onClick={() => setNumber(n => n - 1)}>Decrement</button>
    </>
  );
}

Our setNumber function can be passed wherever we’d like, to child components, etc.

Things are simpler in Svelte. We can create a variable, and update it as needed. Svelte’s ahead-of-time compilation (as opposed to React’s just-in-time compilation) will do the footwork of tracking where it’s updated, and force an update to the DOM. The same simple example from above might look like this:

<script>
  let number = 0;
</script>

<h1>Value: {number}</h1>
<button on:click={() => number++}>Increment</button>
<button on:click={() => number--}>Decrement</button>

Also of note here is that Svelte requires no single wrapping element like JSX does. Svelte has no equivalent of the React fragment <></> syntax, since it’s not needed.

But what if we want to pass an updater function to a child component so it can update this piece of state, like we can with React? We can just write the updater function like this:

<script>
  import Component3a from "./Component3a.svelte";
        
  let number = 0;
  const setNumber = cb => number = cb(number);
</script>

<h1>Value: {number}</h1>

<button on:click={() => setNumber(val => val + 1)}>Increment</button>
<button on:click={() => setNumber(val => val - 1)}>Decrement</button>

Now, we pass it where needed — or stay tuned for a more automated solution.

Reducers and stores React also has the useReducer hook, which allows us to model more complex state. We provide a reducer function, and it gives us the current value, and a dispatch function that allows us to invoke the reducer with a given argument, thereby triggering a state update, to whatever the reducer returns. Our counter example from above might look like this:

import React, { useReducer } from "react";

function reducer(currentValue, action) {
  switch (action) {
    case "INC":
      return currentValue + 1;
    case "DEC":
      return currentValue - 1;
  }
}

export default function (props) {
  const [number, dispatch] = useReducer(reducer, 0);
  return (
    <div>
      <h1>Value: {number}</h1>
      <button onClick={() => dispatch("INC")}>Increment</button>
      <button onClick={() => dispatch("DEC")}>Decrement</button>
    </div>
  );
}

Svelte doesn’t directly have something like this, but what it does have is called a store. The simplest kind of store is a writable store. It’s an object that holds a value. To set a new value, you can call set on the store and pass the new value, or you can call update, and pass in a callback function, which receives the current value, and returns the new value (exactly like React’s useState).