Custom JS transitions

While you should generally use CSS for transitions as much as possible, there are some effects that can’t be achieved without JavaScript, such as a typewriter effect:

function typewriter(node, { speed = 50 }) {
    const valid = (
        node.childNodes.length === 1 &&
        node.childNodes[0].nodeType === 3
    );
    if (!valid) {
        throw new Error(`This transition only works on elements with a single text node child`);
    }
    const text = node.textContent;
    const duration = text.length * speed;
    return {
        duration,
        tick: t => {
            const i = ~~(text.length * t);
            node.textContent = text.slice(0, i);
        }
    };
}