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What are the specific data types of js and the basic data types of JS?

小云云

Feb 03, 2018 am 11:15 AM

javascript Basic type

This article mainly shares with you the specific data types of JS and the basic data types of JS. I hope it can help everyone.

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.

Let’s first introduce one of the data types of js.

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 addition to numbers, strings, Boolean values, null and undefined, objects in js are objects. Objects are collections of attributes. Each attribute consists of a "name/value pair" ( 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 - an array, which 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. Each constructor defines a class object

I will explain it to you in detail below. Let’s explain the first type of data type - numbers

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

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js adopts IEEE-754 Floating point number representation, which is a binary representation, can accurately represent fractions, such as 1/2, 1/8 and 1/1024, but our commonly used fractions are decimal fractions 1/10/1/100 wait. Binary floating point number representation cannot accurately represent a simple number like 0.1.

For example

3-0.2是不等于0.2-0.1的，在真实的运行环境下
3-0.2=0.09999999999999998   而
2-0.1=0.1
这是由于舍入误差导致的

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What are the basic data types in JS development? The data types of JS include basic data types, complex data types and special data types. Below we will mainly explain the basic data types first.

0. Let me first remind you of the following two knowledge points:

0.1typeof is used to detect variable types.

Writing method: typeof a;

0.2console.log() is used to print the part you need on the console

Generally, after entering the HTML file page, right-click and select Inspect, the console will appear, select console You can see the content you printed

In addition: alert() is the page pop-up box to display the content

document.write() is the way to output the content on the page

1. Variables: Before talking about basic data types, let’s first understand how to define variables in JS.

1.1 Define variables: When defining a variable, you can give the variable an initial value, regardless of type (type of container).

1.2 Naming specifications for variables: a combination of letters, numbers, underscores and $; cannot start with a number; cannot be keywords or reserved words; camel case naming method.

1.3 The initial value can only be the following five types:

Number type Number, which can only be a number or a decimal

var a = 10;
console.log(typeof a);//number
var b = 10.6;
console.log(typeof b);//number

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String type String , any character wrapped in single quotes or double quotes

var c = &#39;hello&#39;;
console.log(typeof c);//string
var d = "world";
console.log(typeof d);//string

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Boolean type Boolean, can only be true or false to represent true or false

var e = true;
console.log(typeof e);//boolean
var f = false;
console.log(typeof f);//boolean

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Undefined, no value is assigned after defining the variable. This variable is undefined

1 2	`var` `g;` `console.log(typeof g);//undefined`

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Empty null is the object type. There are many types of object types, such as array objects, mathematical objects, and date objects. (Later learning)

1 2	`var` `h =` `"";` `console.log(typeof h);//null`

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These five are the five basic data types of JS.

2. Type conversion

Mutual conversion between numeric type, string type and Boolean type

2.1 Convert value—Number()

console.log(Number("123"));//123
console.log(Number("12.3"));//12.3
console.log(Number("12hshs"));//NaN
console.log(Number(&#39;0034&#39;));//34
console.log(Number(""));//0
console.log(Number(true));//1
console.log(Number(false));//0
console.log(Number(null));//0
console.log(Number(undefined));//NaN

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Note: NaN: not a number, others will be explained later

2.2 Convert to String(), convert whatever you write

console.log(String(123));//123
console.log(String(0));//0
console.log(String(true));//true
console.log(String(false));//false
console.log(String(undefined));//undefined
console.log(String(null));//null

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2.3 Convert to Boolean()

Tips:

Convert numbers to boolean non-0 to true

字符串转boolean非空为真

NaN null undefined转字符串为假

console.log(Boolean("123"));//true
console.log(Boolean("0"));//true
console.log(Boolean("山东"));//true
console.log(Boolean(""));//false
console.log(Boolean("true"));//true
console.log(Boolean("false"));//true
console.log(Boolean(14));//true
console.log(Boolean(0));//false
console.log(Boolean(NaN));//false
console.log(Boolean(-100));//true
console.log(Boolean(undefined));//false
console.log(Boolean(null));//false

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