JavaScript代码输出undefined问题排查：Book类方法无返回值

问题根源 #

summary()和isAvailable()这两个方法内部仅编写了模板字符串表达式，但没有通过return语句返回结果。JavaScript中函数如果没有显式返回值，默认会返回undefined，这就是控制台输出undefined undefined的原因。

修复后的代码 #

function Book(title, author, isbn, stock, cost) {
  this.title = title;
  this.author = author;
  this.isbn = isbn;
  this.stock = stock;
  this.cost = cost;
  this.summary = function () {
    // 添加return返回模板字符串结果
    return `${this.title} by ${this.author} (ISBN: ${this.isbn}) costs $${this.cost}`;
  };
  this.isAvailable = function () {
    // 添加return返回三元表达式的结果
    return this.stock > 0
      ? `${this.title} is available`
      : `${this.title} is not available at the moment`;
  };
}

const product = new Book(
  "The Design of Everyday Things",
  "Don Norman",
  "978 - 0 - 465 - 05571 - 5",
  10,
  17.99
);
console.log(product.summary());
console.log(product.isAvailable());

预期输出 #

"The Design of Everyday Things by Don Norman (ISBN: 978 - 0 - 465 - 05571 - 5) costs $17.99"
"The Design of Everyday Things is available"

内容的提问来源于stack exchange，提问作者dheerajSan