marvinhagemeister/input-mask.html

## input-mask.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <label for="foo">Input mask demo</label>
    <input id="foo" type="text" pattern="\d{6}" />
    <script>
      /**
       * @param {Event} ev
       * @param {Array<string | RegExp>} expected - The input mask
       */
      function maskHandler(ev, expected) {
        const value = ev.target.value;

        // Save selection or cursor position
        const selStart = el.selectionStart;
        const selEnd = el.selectionEnd;

        let next = "";
        let clearStart = 0;
        let clearEnd = 0;
        let checkIdx = 0;
        for (let i = 0; i < value.length; i++) {
          const check = expected[checkIdx];
          const ch = value[i];
          if (
            check !== undefined &&
            (typeof check === "string" ? ch === check : check.test(ch))
          ) {
            checkIdx++;
            next += ch;
            continue;
          }

          // Adjust selection/cursor position for removed characters
          if (i < selStart) clearStart++;
          if (i > selStart && i < clearEnd) clearEnd++;
        }

        // Overwrite value if we needed to replace something
        if (value !== next) {
          ev.target.value = next;
          const nextSelStart = selStart - clearStart;
          const nextSelEnd = selEnd - clearStart - clearEnd;
          el.setSelectionRange(nextSelStart, nextSelEnd);
        }
      }

      const el = document.querySelector("input");
      el.addEventListener("input", (ev) => {
        // Usage
        maskHandler(ev, [/\d/, /\d/, /\d/, /\d/, /\d/, /\d/]);
      });
    </script>
  </body>
</html>
	<!DOCTYPE html>
	<html lang="en">
	<head>
	<meta charset="UTF-8" />
	<meta name="viewport" content="width=device-width, initial-scale=1.0" />
	<title>Document</title>
	</head>
	<body>
	<label for="foo">Input mask demo</label>
	<input id="foo" type="text" pattern="\d{6}" />
	<script>
	/**
	* @param {Event} ev
	* @param {Array<string \| RegExp>} expected - The input mask
	*/
	function maskHandler(ev, expected) {
	const value = ev.target.value;

	// Save selection or cursor position
	const selStart = el.selectionStart;
	const selEnd = el.selectionEnd;

	let next = "";
	let clearStart = 0;
	let clearEnd = 0;
	let checkIdx = 0;
	for (let i = 0; i < value.length; i++) {
	const check = expected[checkIdx];
	const ch = value[i];
	if (
	check !== undefined &&
	(typeof check === "string" ? ch === check : check.test(ch))
	) {
	checkIdx++;
	next += ch;
	continue;
	}

	// Adjust selection/cursor position for removed characters
	if (i < selStart) clearStart++;
	if (i > selStart && i < clearEnd) clearEnd++;
	}

	// Overwrite value if we needed to replace something
	if (value !== next) {
	ev.target.value = next;
	const nextSelStart = selStart - clearStart;
	const nextSelEnd = selEnd - clearStart - clearEnd;
	el.setSelectionRange(nextSelStart, nextSelEnd);
	}
	}

	const el = document.querySelector("input");
	el.addEventListener("input", (ev) => {
	// Usage
	maskHandler(ev, [/\d/, /\d/, /\d/, /\d/, /\d/, /\d/]);
	});
	</script>
	</body>
	</html>