<header>
  <svg width="20" height="20" viewBox="0 0 240 260" class="svg-check site-logo">
    <polyline class="cls-1" points="16 149 72 225 225 16" pathLength="1" />
  </svg>
  <h1>TODOs</h1>
</header>

<form id="todo-form" class="todo-form">
  <label>
    <span class="screen-reader-text">New TODO</span>
    <input type="text" name="todo">
  </label>
  <button id="button-add-todo" class="button-add-todo">
    <svg viewBox="-20 -20 240 240" class="svg-plus">
      <g>
        <line x1="100" x2="100" y2="200" />
        <line y1="100" x2="200" y2="100" />
      </g>
    </svg>
  </button>
</form>

<ol id="todo-list" class="todo-list"></ol>

<footer>
  <div class="todo-type-toggles">
    <button aria-pressed="true">Active</button>
    <button>Completed</button>
  </div>
  <div class="note">Double-click to edit a todo</div>
</footer>
* {
  box-sizing: border-box;
}
html {
  --gray800: oklch(10% 0% 0);
  --gray600: oklch(40% 0% 0);
  --gray100: oklch(92% 0% 0);
  --brand: oklch(85% 0.3 145);

  font-family: system-ui, sans-serif;
  background: var(--gray100);
}
body {
  margin: 0;
}

header {
  background: var(--gray800);
  color: white;
  padding: 1rem;
  display: flex;
  align-items: center;
  gap: 1rem;
  h1 {
    margin: 0;
    font-variation-settings: "wght" 900, "wdth" 700;
  }
  .svg-check {
    width: 2lh;
    height: 2lh;
    stroke: var(--brand);
  }
}

.svg-check,
.svg-plus {
  fill: none;
  stroke-width: 30;
  stroke-linecap: round;
  stroke-linejoin: round;
  display: block;
  pointer-events: none;
}

.todo-form {
  background: var(--gray600);
  justify-content: center;
  padding: 1rem;
  display: flex;
  gap: 0.5rem;
  align-items: stretch;
  label {
    flex: 1 0;
    max-width: 40ch;
  }
  input {
    background: var(--gray100);
    width: 100%;
    border: 0;
    padding: 0.5rem;
    box-shadow: inset 1px 1px 2px oklch(0% 0% 0 / 0.6);
    border-radius: 5px;
    font: inherit;
    font-size: 2rem;
    &:focus {
      background: white;
    }
  }
  .svg-plus {
    stroke: black;
    width: 66%;
  }
}

.button-add-todo {
  border: 0;
  border-radius: 5px;
  font: inherit;
  font-size: 2rem;
  font-weight: 900;
  background: var(--brand);
  aspect-ratio: 1;
  height: 3.35rem;
  display: inline-grid;
  place-items: center;
  line-height: 0;

  &:hover,
  &:focus {
    background: color-mix(in oklch, var(--brand) 100%, black 10%);
  }

  &.shake {
    rotate: 0deg;
    transform-origin: bottom right;
    animation: shake 0.4s cubic-bezier(0.36, 0.07, 0.19, 0.97) both;
  }
  &.added {
    transform-origin: center center;
    animation: added 0.4s cubic-bezier(0.36, 0.07, 0.19, 0.97) both;
  }
}
@keyframes shake {
  50% {
    rotate: -12deg;
  }
}
@keyframes added {
  50% {
    rotate: 1turn;
    translate: 0 50px;
  }
}

.todo-list {
  padding: 1rem;
  list-style: none;
  li {
    background: white;
    box-shadow: 1px 1px 2px oklch(0% 0% 0 / 0.2);
    padding: 0.5rem;
    margin-block-end: 0.5rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    .button-complete {
      order: -1;
    }
    .svg-check {
      stroke: black;
      position: absolute;
      width: 2rem;
      height: 2rem;
      top: -0.5rem;
      left: 0.1rem;
      opacity: 0;
    }
    &.complete {
      .svg-check {
        opacity: 1;
        stroke-dasharray: 1;
        stroke-dashoffset: 1;
        /* https://css-tricks.com/a-trick-that-makes-drawing-svg-lines-way-easier/ */
        animation: do-check 1s infinite alternate;
      }
    }
    &.editing {
      outline: 1px solid var(--brand);

      .text {
        display: none;
      }
    }

    &[data-complete="true"] {
      .text {
        text-decoration: line-through;
        text-decoration-thickness: 2px;
        text-decoration-color: var(--brand);
        opacity: 0.5;
      }
    }

    &.empty {
      justify-content: center;
      background: none;
      box-shadow: none;
    }
  }
}

@keyframes do-check {
  from {
    stroke-dashoffset: 1;
  }
  to {
    stroke-dashoffset: 0;
  }
}

.form-edit {
  width: 100%;
}
.input-edit {
  font: inherit;
  border: 0;
  width: 100%;
  padding: 0;
  &:focus {
    outline: none;
  }
}

.button-complete {
  border: 0;
  border-radius: 3px;
  background: var(--gray100);
  box-shadow: inset 1px 1px 2px oklch(0% 0% 0 / 0.2);
  padding: 0.25rem;
  width: 1.5rem;
  height: 1.5rem;
  position: relative;
}

.screen-reader-text {
  text-indent: -9999em;
  width: 0;
  height: 0;
  display: block;
}

footer {
  font-size: 0.8rem;

  > .note {
    opacity: 0.5;
  }

  display: flex;
  gap: 1rem;
  justify-content: space-between;
  align-items: center;
  padding: 0 1rem;
}

.todo-type-toggles {
  display: flex;

  button {
    border: 0;
    border-radius: 8px;
    padding: 0.2rem 1rem;
    background: var(--button-bg, white);
    box-shadow: 1px 1px 3px oklch(0% 0% 0 / 0.2);
    &[aria-pressed="true"] {
      --button-bg: var(--brand);
      box-shadow: 0 1px 1px oklch(0% 0% 0 / 0.4);
    }
    &:active {
      position: relative;
      top: 1px;
    }
    &:hover,
    &:focus-visible {
      background: color-mix(in oklch, var(--button-bg), black 10%);
    }
  }

  button:first-child {
    border-radius: 5px 0 0 5px;
  }
  button:last-child {
    border-radius: 0 5px 5px 0;
  }
}
// Useful for nuking data quick if you need to
// localStorage["data"] = JSON.stringify([])

// UI constants
const form = document.querySelector("#todo-form");
const list = document.querySelector("#todo-list");
const buttonAddTodo = document.querySelector("#button-add-todo");
const toggles = document.querySelectorAll(".todo-type-toggles > button");

// Enums
const states = {
  ACTIVE: "Active",
  COMPLETED: "Completed"
};

// Get Data on page load
let TODOs = [];
if (localStorage["data"] !== null && localStorage["data"] !== undefined) {
  TODOs = JSON.parse(localStorage["data"]);
}
// console.log({ TODOs });

function buildUI(state) {
  let HTML = ``;
  let viewTODOs = [];

  if (state === states.COMPLETED) {
    viewTODOs = TODOs.filter((todo) => todo.complete);
  } else {
    viewTODOs = TODOs.filter((todo) => !todo.complete);
  }

  if (viewTODOs.length === 0) {
    HTML = `<li class="empty">Nothing to do!</li>`;
  }

  viewTODOs.forEach((todo) => {
    if (todo !== null) {
      HTML += `<li id="${todo.id}" style="view-transition-name: list-item-${todo.id};" data-complete="${todo.complete}">
      <span class="text">${todo.title}</span>
      <button aria-label="Complete" class="button-complete">
        <svg width="20" height="20" viewBox="0 0 241.44 259.83" class="svg-check">
          <polyline points="16.17 148.63 72.17 225.63 225.17 11.63" pathLength="1" />
        </svg>
      </button>
    </li>`;
    }
  });

  list.innerHTML = HTML;
}

form.addEventListener("submit", (event) => {
  event.preventDefault();
  // Don't allow empty todo
  if (!form[0].value) {
    buttonAddTodo.classList.add("shake");
    return;
  }
  addTodo(event);
  form.reset();
});

buttonAddTodo.addEventListener("animationend", () => {
  buttonAddTodo.classList.remove("shake", "added");
});

function addTodo() {
  // TODO (lol): Sanitize user input.
  TODOs.push({
    title: form[0].value,
    complete: false,
    id: self.crypto.randomUUID()
  });
  localStorage["data"] = JSON.stringify(TODOs);
  buttonAddTodo.classList.add("added");
  buildUI();
}

document.documentElement.addEventListener("click", (event) => {
  if (event.target.classList.contains("button-complete")) {
    toggleTodo(event);
  }
});

list.addEventListener("dblclick", (event) => {
  const listItem = event.target.closest("li");

  // If already editing, let it be.
  if (listItem.classList.contains("editing")) return;

  listItem.classList.add("editing");
  const textItem = listItem.querySelector(".text");
  listItem.insertAdjacentHTML(
    "beforeend",
    `<form onsubmit="updateTodo(event);" class="form-edit"><input onblur="updateTodo(event);" type="text" class="input-edit" value="${textItem.textContent}"></form>`
  );

  const input = listItem.querySelector(".input-edit");
  input.focus();

  // put cursor at end of input
  input.setSelectionRange(input.value.length, input.value.length);
});

function updateTodo(event) {
  event.preventDefault();
  const listItem = event.target.closest("li");
  const textItem = listItem.querySelector(".text");
  const inputItem = listItem.querySelector(".input-edit");
  const form = listItem.querySelector(".form-edit");
  textItem.textContent = inputItem.value;
  listItem.classList.remove("editing");
  form.remove();
  TODOs = TODOs.map((todo) => {
    if (todo.id === listItem.id) {
      todo.title = inputItem.value;
    }
    return todo;
  });
  localStorage["data"] = JSON.stringify(TODOs);
}

function toggleTodo(event) {
  const listItem = event.target.parentElement;
  // Trigger complete animation
  listItem.classList.toggle("complete");
  setTimeout(() => {
    if (listItem.dataset.complete === "true") {
      TODOs = TODOs.filter((todo) => !todo.complete);
      if (!document.startViewTransition) {
        buildUI(states.COMPLETED);
      } else {
        document.startViewTransition(() => {
          buildUI();
        });
      }
    } else {
      TODOs.forEach((todo) => {
        if (todo.id === listItem.id) {
          todo.complete = !todo.complete;
        }
      });
      if (!document.startViewTransition) {
        buildUI(states.ACTIVE);
      } else {
        document.startViewTransition(() => {
          buildUI();
        });
      }
    }

    localStorage["data"] = JSON.stringify(TODOs);
  }, 1000);
}

toggles.forEach((toggle) => {
  toggle.addEventListener("click", (event) => {
    toggles.forEach((toggle) => {
      toggle.setAttribute("aria-pressed", false);
    });
    toggle.setAttribute("aria-pressed", true);

    if (toggle.textContent === states.ACTIVE) {
      buildUI(states.ACTIVE);
    } else {
      buildUI(states.COMPLETED);
    }
  });
});

buildUI();

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.