Suggestions.js

// Ideally you would import the Jump library explicitly, but don't sweat it. ;)
const jump = Jump;
// ⬇ Moved this up here as there is no reason for the variable to be set to undefined first.
const imageArray = [
  {
    image: 'img/mattias@400px.png',
    description:
      'Hemsida för content marketing specialisten och tidigare SVT-sportjournalist Mattias Brännholm. Wordpress.',
  },
  {
    image: 'img/stildans@400px.jpg',
    description:
      'Hemsida för vår dansträningsgrupp. Htlm, css, php. Det finns funktioner för att boka tid för varje danspass.',
  },
  {
    image: 'img/brama@400px.png',
    description: 'Webbredaktör för Brama content & Film. Wordpress.',
  },
  {
    image: 'img/brf-malarglimt@400px.png',
    description: 'Hemsida för vår bostadsrättsförening. Wordpress.',
  },
  {
    image: 'img/christinaak@400px.jpg',
    description: 'Webbredaktör för Christinaakademin. Wordpress.',
  },
];
let activated = false; // Never used. Remove.
let currentIndex;
let currentPicture;
const button1 = document.querySelector('.previous');
const button2 = document.querySelector('.next');

// ⬇ I changed this to avoid potentially confusing reuse of a variable name.
// Also moved it up to keep the "setup" stuff in one place.
const menuBtn = document.querySelector('#menu-btn');
const fadeLayer = document.querySelector('.fade-layer');
menuBtn.addEventListener('click', toggleMenu);
fadeLayer.addEventListener('click', toggleMenu);

function toggleMenu() {
  let element = document.querySelector('nav');
  element.classList.toggle('menu-hidden');
  element = document.querySelector('.fade-layer');
  element.classList.toggle('hidden');
}

function toggleSections() {
  let section = document.querySelector('.content');
  let icon = document.querySelector('.collapsible');
  if (section.style.display === 'block') {
    section.style.display = 'none';
    icon.style.transform = 'rotate(0deg)';
  } else {
    section.style.display = 'block';
    icon.style.transform = 'rotate(180deg)';
  }
}

// function setAttribute(){
//     return image
// }

function formatText(textObject) {
  return textObject;
}

function update() {
  let text = document.querySelector('.description');
  text.innerText = formatText(currentPicture.description);
  let images = document.querySelector('.gallery-box img');
  images.setAttribute('src', currentPicture.image);
}

function fadeOut() {
  let images = document.querySelector('.gallery-box img');
  images.classList.add('fade');
}

function fadeIn() {
  let images = document.querySelector('.gallery-box img');
  images.classList.remove('fade');
}

function previous() {
  currentIndex--;
  if (currentIndex < 0) {
    currentIndex = imageArray.length - 1;
  }
  currentPicture = imageArray[currentIndex];
  fadeOut();
  setTimeout(function() {
    update();
    fadeIn();
  }, 500);
}

function next() {
  // 'image' is never used again. Remove.
  let image = document.querySelector('.gallery-box img');
  currentIndex++;
  if (currentIndex >= imageArray.length) {
    currentIndex = 0;
  }
  currentPicture = imageArray[currentIndex];
  fadeOut();
  setTimeout(function() {
    update();
    fadeIn();
  }, 500);
}

function onClick() {
  jump('.top', {
    duration: 500,
    offset: 0,
    callback: undefined,
    a11y: false,
  });
}

let buttonToTop = document.querySelector('.tothetop');
buttonToTop.addEventListener('click', onClick);

// Now all the setup is above this line, while all the execution is below. Nice and tidy. :)

if (window.location.pathname == '/index.html') {
  // From what I can see this code is never called.
  // Is this working as expected? If so, consider moving the functions outside the if-scope.
  // Ask me if you don't know what and how. :)
  function sendEmail(element, message) {
    alert(message);
    element.focus();
  }
  function isMailReady(form) {
    let passed = false;
    if (
      form.email.value.indexOf('@') == -1 ||
      form.email.value.indexOf('.') == -1
    ) {
      sendEmail(form.email, 'Ange en giltig e-postadress.');
    } else if (form.message.value == '') {
      sendEmail(form.message, 'Du har inte skrivit någonting.');
    } else {
      passed = true;
    }
    return passed;
  }
} else if (window.location.pathname == '/arbetsprover.html') {
  // ⬇ Changed to sexy arrow functions because that's what the cool kids do.
  button2.addEventListener('click', () => next());
  button1.addEventListener('click', () => previous());

  // This variable is never used again? Remove.
  let galleryElements = document.querySelector('.gallerybox');
  currentIndex = 0;
  currentPicture = imageArray[currentIndex];
  update();

  let sections = document.querySelector('.collapsible');
  sections.addEventListener('click', toggleSections);
}

// And finally, all the calls to external scripts go here.

jarallax(document.querySelectorAll('.jarallax'), {
  speed: 0.3,
});

jarallax(document.querySelectorAll('.jarallax-video'), {
  speed: 0.2,
  videoSrc: 'https://vimeo.com/372669055',
});

tippy('button', {
  content: 'Global content',
  animation: 'rotate',
  theme: 'tomato',
  duration: 3,
  arrow: true,
  delay: [500, 200],
});