**// js**
/**
 * Show an alert if required input is blank
 * @param args Required input values
 * @returns {boolean}
 */
export const isValidRequiredInput = (...args) => {
    let validator = true;
    for (let i=0; i < args.length; i=(i+1)|0) {
        if (args[i] === "") {
            validator = false;
        }
    }
    return validator
};

**// TypeScript**
/**
 * Show an alert if required input is blank
 * @param args Required input values
 */
export const isValidRequiredInput = (...args: string[]): boolean => {
  let validator = true;
  for (let i = 0; i < args.length; i = (i + 1) | 0) {
    if (args[i] === '') {
      validator = false;
    }
  }
  return validator;
};

▼呼び出し側　コード例

import {isValidRequiredInput} from "〜";

// signUpの引数には、inputのvalue（state）をセットする
export const signUp = (username, email, password, confirmPassword) => {
    return async (dispatch) => {
        // Validations
        **if(!isValidRequiredInput(email, password, confirmPassword)) {
            alert('必須項目が未入力です。');
            return false
        }**
〜〜〜