What does js built-in object mean?

js built-in objects refer to objects that have been predefined in JavaScript and can be used directly in the code. These objects provide a wealth of functions and methods to facilitate developers to perform various operations and processing.

In JavaScript, there are many built-in objects that can be used, including but not limited to: Math, Date, String, Array, Boolean, etc. The following will take these built-in objects as examples to introduce their specific meaning and usage in detail, and attach corresponding code examples.

1. Math object:
   Math object provides many mathematical operations and methods, such as calculating square roots, logarithms, etc.
   Code Example:

   console.log(Math.sqrt(16)); // 输出4，计算16的平方根
   console.log(Math.abs(-10)); // 输出10，计算-10的绝对值
   console.log(Math.floor(3.7)); // 输出3，向下取整

2. Date Object:
   Date object is used to operate and process dates and times.
   Code example:

   let now = new Date(); // 创建当前时间的Date对象
   console.log(now); // 输出当前时间
   console.log(now.getFullYear()); // 输出当前年份
   console.log(now.getMonth()); // 输出当前月份（注意：月份是从0开始计算的）
   console.log(now.getDate()); // 输出当前日期

3. String object:
   String object is used to operate and process strings.
   Code example:

   let str = "Hello World!";
   console.log(str.length); // 输出12，获取字符串的长度
   console.log(str.toUpperCase()); // 输出HELLO WORLD!，将字符串转换为大写
   console.log(str.indexOf("o")); // 输出4，查找字符'o'第一次出现的位置
   console.log(str.substr(6, 5)); // 输出"World"，截取字符串的一部分

4. Array object:
   The Array object is used to operate and process arrays.
   Code Example:

   let arr = [1, 2, 3, 4, 5];
   console.log(arr.length); // 输出5，获取数组的长度
   console.log(arr.join("-")); // 输出"1-2-3-4-5"，将数组中的元素用指定的分隔符连接起来
   console.log(arr.push(6)); // 输出6，将元素6添加到数组末尾，并返回数组的新长度
   console.log(arr.pop()); // 输出5，删除数组末尾的元素，并返回删除的元素

5. Boolean Object:
   Boolean objects are used to operate and process Boolean values.
   Code example:

   let bool = true;
   console.log(bool.toString()); // 输出"true"，将布尔值转换为字符串
   console.log(Boolean(0)); // 输出false，将0转换为布尔值
   console.log(Boolean("")); // 输出false，将空字符串转换为布尔值
   console.log(Boolean(1)); // 输出true，将非0数字转换为布尔值

Through the above examples, we can see that these built-in objects can be used directly in JavaScript. They provide some commonly used methods and properties to facilitate development. perform various operations and processing. Of course, this is only a small part of the built-in objects. There are many other built-in objects available, and developers can learn and use them according to their specific needs.

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