Center Selected Item Inside a Number Picker Widget in JavaScript

Question

[Precondition]

I am having difficulty figuring out how to make a Number Picker Widget behave similarly to the mobile picker component. If the user assigns a number by default, then the selected number should be automatically snap to the center when page load.

---

[Problems]

• How can I snap this element <div class="cell selected">10</div> to the center of the scrollport by default?
• Do not rely on any plugins.

"use strict";
console.clear();

{
  const selector = "scrollport";
  const selected = "selected";
  const scrollports = document.getElementsByClassName(selector);
  const debouncedFunc = debounce(check, 250);
  for (const scrollport of scrollports) {
    scrollport.addEventListener("scroll", debouncedFunc);
  }

  function check(e) {
    const rect = e.target.getBoundingClientRect();
    const centerCell = document.elementFromPoint(
      rect.left + e.target.offsetWidth / 2,
      rect.top + e.target.offsetHeight / 2
    );
    for (const cell of e.target.getElementsByClassName(selected)) {
      cell.classList.remove(selected);
    }
    centerCell.classList.add(selected);
  }
}

function debounce(func, wait) {
  let timeout;
  return function(...args) {
    const context = this;
    clearTimeout(timeout);
    timeout = setTimeout(() => func.apply(context, args), wait);
  };
}

* {
  box-sizing: border-box;
  font-family: Roboto, sans-serif;
}

.container {
  display: flex;
  flex-direction: row;
  width: 10rem;
  height: 22rem;
  border-radius: 3rem;
  border: solid 0.2rem #b2b2c2;
  background-color: #000000;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scrollport:before,
.scrollport:after {
  content: '';
}

.scrollport {
  display: flex;
  flex-direction: column;
  flex-wrap: nowrap;
  width: 9.4rem;
  height: 22rem;
  overflow: auto;
  scroll-snap-type: y mandatory;
}

.scrollport:before,
.scrollport:after,
.cell {
  display: block;
  scroll-snap-align: center;
  flex-grow: 1;
  flex-shrink: 0;
  flex-basis: 33.3%;
  display: flex;
  justify-content: center;
  align-items: center;
  color: #e9e9f2;
  font-size: 2.4rem;
}

.selected {
  font-size: 3rem;
  font-weight: bold;
  color: #0073e6;
}

<div class="container">
  <div class="scrollport">
    <div class="cell">09</div>
    <div class="cell selected">10</div>
    <div class="cell">11</div>
    <div class="cell">12</div>
    <div class="cell">13</div>
    <div class="cell">14</div>
    <div class="cell">15</div>
    <div class="cell">16</div>
  </div>
</div>

---

[Expect Result]

Spider

Answer 1

You can try setting the parent's scroll position by subtracting the selected value's offsetTop with scrollHeight to get the precise vertically centered position:

"use strict";
console.clear();

{
  const selector = "scrollport";
  const selected = "selected";
  const scrollports = document.getElementsByClassName(selector);
  const initialValue = document.getElementsByClassName(selected);
  const debouncedFunc = debounce(check, 250);
  for (const scrollport of scrollports) {
    scrollport.addEventListener("scroll", debouncedFunc);
    scrollport.scrollTo({
      top: initialValue[0].offsetTop - initialValue[0].scrollHeight,
      behavior: 'smooth'
    });
  }

  function check(e) {
    const rect = e.target.getBoundingClientRect();
    const centerCell = document.elementFromPoint(
      rect.left + e.target.offsetWidth / 2,
      rect.top + e.target.offsetHeight / 2
    );
    for (const cell of e.target.getElementsByClassName(selected)) {
      cell.classList.remove(selected);
    }
    centerCell.classList.add(selected);
  }
}

function debounce(func, wait) {
  let timeout;
  return function(...args) {
    const context = this;
    clearTimeout(timeout);
    timeout = setTimeout(() => func.apply(context, args), wait);
  };
}

* {
  box-sizing: border-box;
  font-family: Roboto, sans-serif;
}

.container {
  display: flex;
  flex-direction: row;
  width: 10rem;
  height: 22rem;
  border-radius: 3rem;
  border: solid 0.2rem #b2b2c2;
  background-color: #000000;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scrollport:before,
.scrollport:after {
  content: '';
}

.scrollport {
  display: flex;
  flex-direction: column;
  flex-wrap: nowrap;
  width: 9.4rem;
  height: 22rem;
  overflow: auto;
  scroll-snap-type: y mandatory;
}

.scrollport:before,
.scrollport:after,
.cell {
  display: block;
  scroll-snap-align: center;
  flex-grow: 1;
  flex-shrink: 0;
  flex-basis: 33.3%;
  display: flex;
  justify-content: center;
  align-items: center;
  color: #e9e9f2;
  font-size: 2.4rem;
}

.selected {
  font-size: 3rem;
  font-weight: bold;
  color: #0073e6;
}

<div class="container">
  <div class="scrollport">
    <div class="cell">09</div>
    <div class="cell selected">10</div>
    <div class="cell">11</div>
    <div class="cell">12</div>
    <div class="cell">13</div>
    <div class="cell">14</div>
    <div class="cell">15</div>
    <div class="cell">16</div>
  </div>
</div>

Seema

Good and reliable answer. That's exactly what I wanted. :) – Spider