A brief introduction to js data types

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When I was doing some projects recently, I found that my js foundation was still not solid enough. I read the Rhino book again to deepen my understanding and impression. So starting from this article, the rest is about native js.

In this article, we will introduce one of the data types of js in detail.

1. The data types of javaScript (hereinafter referred to as js) are divided into two categories: Primitive types and object types. The primitive types of js include numbers, strings and Boolean values.

2. js has two special primitive values: null (empty) and undefined (undefined), they are not numbers, strings and Boolean values. They usually each represent a unique member of their special type.

3. In js, in addition to numbers, strings, Boolean values, null and undefined, there are objects. Objects are a collection of attributes. Each attribute They are all composed of "name/value pairs" (the value can be a primitive value, such as a number, a string, or an object).

4. Ordinary js objects are unordered collections of "named values". js also defines a special object - array (array), represents an ordered collection of numbered values. js specifically defines syntax for arrays, which we will explain in detail later. Make arrays have some unique behavioral characteristics that are different from ordinary objects.

5. JS also defines a special object - function. A function has an object with executable code associated with it. The executable code is run by calling the function and the results of the operation are returned. Like arrays, functions behave differently from other objects.

6. If the function is used to initialize (using the new operator) a newly created object, we call it a constructor function. Each constructor defines a class object

Let me explain the first type of data type to you in detail - Number

According to the number format in js, the range of integers that can be represented is from -9007199254740992~9007199254740992 (that is, -253~253) contains boundary values.

In js, when a number appears directly in the js program, we call it a digital direct quantity. js supports digital direct quantities in multiple formats.

Integer literal, using a sequence of numbers to represent a decimal integer, such as: 0 3 133333

Floating-point literal, floating-point literal can contain a decimal point, such as: 3.14. 3333 2.02e23(2.02x1023)e or E represents how many powers

The arithmetic operations in js (+(addition), -(subtraction), x(multiplication), /(division), %(remainder )) In addition to these basic operators, js also supports more complex arithmetic operations, which are implemented through functions and constants defined as properties of the Math object:

Math.pow(2,53)       //2的53次幂也就是8007199254740992
  Math.round(.6)       //1.0  四舍五入
  Math.ceil(.6)        //1.0   向上取整
  Math.floor(.6)       //0.0   向下取整
  Math.abs(-5)        //5    求绝对值
  Math.max(x,y.z)       //返回最大值
  Math.min(x,y.z)       //返回最小值
  Math.random()       //生成一个大于等于0小于1的伪随机数
  Math.PI           //π  圆周率
  Math.E           //e  自然对数的底数
  Math.sqrt(3)        //3的平方根
  Math.pow(3,1/3)        //3的立方根
  Math.sin(0)           //三角函数：还有cos()和atan等

js uses IEEE-754 floating point number representation, which is a binary representation that can accurately represent fractions, such as 1/2, 1/8 and 1/1024, but the fractions we commonly use are decimal. The fractions are 1/10/1/100 etc. Binary floating point number representation cannot accurately represent a simple number like 0.1.

For example

0.3-0.2是不等于0.2-0.1的，在真实的运行环境下
0.3-0.2=0.09999999999999998   而
0.2-0.1=0.1
这是由于舍入误差导致的

The content of numeric types is probably finished. In the next chapter, I will tell you about the second data type- characters. string

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