ionic-framework/core/src/components/input/input.utils.ts

import { printIonError } from '@utils/logging';

export const getCounterText = (
  value: string | number | null | undefined,
  maxLength: number,
  counterFormatter?: (inputLength: number, maxLength: number) => string
) => {
  const valueLength = value == null ? 0 : value.toString().length;
  const defaultCounterText = defaultCounterFormatter(valueLength, maxLength);

  /**
   * If developers did not pass a custom formatter,
   * use the default one.
   */
  if (counterFormatter === undefined) {
    return defaultCounterText;
  }

  /**
   * Otherwise, try to use the custom formatter
   * and fallback to the default formatter if
   * there was an error.
   */
  try {
    return counterFormatter(valueLength, maxLength);
  } catch (e) {
    printIonError('[ion-input] - Exception in provided `counterFormatter`:', e);
    return defaultCounterText;
  }
};

const defaultCounterFormatter = (length: number, maxlength: number) => {
  return `${length} / ${maxlength}`;
};