Reactive Variables are the BEST feature in Svelte

Published on Mar 13, 2021

Reading time 2 min

Well, maybe I’m exagerating a little bit. But for real, reactive variable are such a good feature I don’t know how other frameworks haven’t copied it to it’s full extent yet.

They work like this, you declare a variable and if the variable changes it triggers another variable to change it’s value if it’s related to the initial variable. For example:


    let a = 5
    $: double = a * 2


    <input type="number" bind:value={a}>

You can see a similar example in the docs . But what this basically does, is that every time we change the value of the input it would change the double variable to be times 2 the a variable.

And you could do lots of things with it.

Changing the document title.


    let name = "Title";

    $: document.title = name


    <input type="text" bind:value={name}>

And even calling functions. For this I have the perfect example, making an svg triangle with the sides being reactive. And also using the reactive variables feature to get the hypotenuse with a neat one liner

Here’s the code :))

    let a = 10;
    let b = 5;

    function getSides(a, b) {
    if (a > b) {
        // Doing a rule of 3 by setting the larger side to be 100 and calculating the other
        b = (b * 100) / a;
        a = 100;
    } else {
        a = (a * 100) / b;
        b = 100;
    return { a, b };

    $: sides = getSides(a, b);
    $: hypotenuse = Math.sqrt(Math.pow(a, 2) + Math.pow(b, 2));

    .triangle {
        width: 150px;
        height: 150px;


    <div class="triangle">
        <svg viewBox="0 0 100 100" xmlns="">
            <polygon points="0 0, {sides.b} {sides.a}, 0 {sides.a}"/>

    <span>Hypotenuse: {hypotenuse}</span>
    <input type="number" bind:value={a} name="a">
    <input type="number" bind:value={b} name="b">

Thanks for reading. If you’ve got any other example of how reactive variables could be used feel free to let me know :))