The Shopify.formatMoney method extracted from option_selection.js for stand-alone purposes.

shopify-money.js

var Shopify = Shopify || {};
// ---------------------------------------------------------------------------
// Money format handler
// ---------------------------------------------------------------------------
Shopify.money_format = "${{amount}}";
Shopify.formatMoney = function(cents, format) {
  if (typeof cents == 'string') { cents = cents.replace('.',''); }
  var value = '';
  var placeholderRegex = /\{\{\s*(\w+)\s*\}\}/;
  var formatString = (format || this.money_format);

  function defaultOption(opt, def) {
     return (typeof opt == 'undefined' ? def : opt);
  }

  function formatWithDelimiters(number, precision, thousands, decimal) {
    precision = defaultOption(precision, 2);
    thousands = defaultOption(thousands, ',');
    decimal   = defaultOption(decimal, '.');

    if (isNaN(number) || number == null) { return 0; }

    number = (number/100.0).toFixed(precision);

    var parts   = number.split('.'),
        dollars = parts[0].replace(/(\d)(?=(\d\d\d)+(?!\d))/g, '$1' + thousands),
        cents   = parts[1] ? (decimal + parts[1]) : '';

    return dollars + cents;
  }

  switch(formatString.match(placeholderRegex)[1]) {
    case 'amount':
      value = formatWithDelimiters(cents, 2);
      break;
    case 'amount_no_decimals':
      value = formatWithDelimiters(cents, 0);
      break;
    case 'amount_with_comma_separator':
      value = formatWithDelimiters(cents, 2, '.', ',');
      break;
    case 'amount_no_decimals_with_comma_separator':
      value = formatWithDelimiters(cents, 0, '.', ',');
      break;
  }

  return formatString.replace(placeholderRegex, value);
};

Thank you!

Shopify's docs have 5th type: amount_with_apostrophe_separator

https://help.shopify.com/en/manual/payments/currency-formatting#currency-formatting-options

      case "amount_with_apostrophe_separator":
        value = formatWithDelimiters(cents, 2, "'", ".");
        break;

Does this limit you to just dollars? Is it possible to get this working across multiple different stores with different currencies?

It doesn’t actually care what the currency is. The first argument is the unformatted amount. The second argument is the format, which could be whatever you need for the currently displayed currency i.e. “£{{ amount }}”. Then pass it a new amount and new format when you change the currency.