Systematically like all YouTube videos in a playlist. Watch out for autoplay

autolike.js

// Skip delay in milliseconds
// Raise if your connection is slow
const delay = 8000;

function autolike() {
    const likeButton = document.querySelector('button[aria-label^=like]')
    const nextButton = document.querySelector('.ytp-next-button')
    if(!isSelected(likeButton)) likeButton.click()
    nextButton.click();
    setTimeout(autolike, delay)
}

function isSelected(button) {
    const icon = button.querySelector('.yt-icon');
    const fill = window.getComputedStyle(icon).fill;
    return fill == 'rgb(62, 166, 255)';
}

autolike()