<h1>Auto Expand Input Height based on Text Length</h1>

<h2>Inputs</h2>

<p><strong>Problem (doesn't resize horizontally):</strong>
  <input class="input" value="99" type="number"></p>

<p><strong>Solution with span:</strong><span class="input" role="textbox" contenteditable>99</span></p>

<p><strong>Solution with JS:</strong><span class="input-wrap"><span class="width-machine" aria-hidden="true">99</span><input class="input" value="99" type="number"></span></p>
.input {
  border: 1px solid #ccc;
  font-family: inherit;
  font-size: inherit;
  padding: 1px 6px;
}

.input-wrap {
  position: relative;
}
.input-wrap .input {
  position: absolute;
  width: 100%;
  left: 0;
}
.width-machine {
  /*   Sort of a magic number to add extra space for number spinner */
  padding: 0 1rem;
}

/* Just for demo */
* {
  box-sizing: border-box;
}
body {
  font-family: "Heebo", sans-serif;
  max-width: 500px;
  margin: 0 auto;
  padding: 1rem;
}

p strong {
  display: block;
}
h1 {
  border-bottom: 5px solid blue;
}
h2 {
  border-bottom: 2px solid navy;
}

// Dealing with Input width
let el = document.querySelector(".input-wrap .input");
let widthMachine = document.querySelector(".input-wrap .width-machine");
el.addEventListener("keyup", () => {
  widthMachine.innerHTML = el.value;
});

// Dealing with Textarea Height
function calcHeight(value) {
  let numberOfLineBreaks = (value.match(/\n/g) || []).length;
  // min-height + lines x line-height + padding + border
  let newHeight = 20 + numberOfLineBreaks * 20 + 12 + 2;
  return newHeight;
}

External CSS

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External JavaScript

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