code: function normalize(value, safe) {
      return value == null
        ? ''
        : value === false
          ? ''
          : Array.isArray(value)
            ? value.map(normalize).join('')
            : safe === false ? $.escapeHTML(value) : String(value)
    }
docs: Function that normalizes interpolation substitions.
  It flattens arrays while ignoring falsey values.
  All other values are converted to strings.