Write a log line when a variable changes

$: console.log(`the value of myvar is now: ${myvar}`);

Setting multiple CSS classes dynamically

Use a ternary expression if you need to set multiple CSS classes dynamically n an element:

<button type="button" class="class-1 class-2 ....
    {isAvailable ? 'is-available is-very-available' : 'is-not-available'}">

Calling a function with arguments, on an event

For example - invoking some code whenever a button is clicked.

You can’t specify the function in the click handler directly, because it will be invoked immediately. Instead, use an arrow function with no arguments. The function will only be invoked when the click event happens:

<button on:click|preventDefault={() => handleClick("foo", "bar")}>
  Click Me!

And then the function would be defined as:

function handleClick(arg1, arg2) {
    // Do stuff here...


Server quits immediately when using rollup-plugin-dev

  • Dev server seems to exit immediately with a message like: [2021-05-15 11:06:37] waiting for changes…
  • Check whether another process is already running on the same port. If so, the server start might fail silently like this.