Detailed explanation of data types in JavaScript basics

1. Data Type

1.1 Why do we need data types?

In a computer, different data occupies different amounts of storage space. In order to divide data into data of different required memory sizes and make full use of the storage space, different data types are defined.

1.2 Data Types of Variables

JavaScript is a weakly typed or dynamic language, which means that there is no need to declare the data type of a variable in advance. The type will be automatically determined while the program is running. (The variable type of js is determined only when the program is running, based on the data type of the value on the right side of the equal sign)

var age = 10; //This is a numeric data type var myName = 'lili'; //This is a string data type

1.3 Data Type Classification

JS divides data types into two categories:

Simple data types (Number, String, Boolean, Undefined, Null) Complex data types (object)

2. Simple data types (basic data types)

The simple data types in JavaScript and their descriptions are as follows:

2.1 Number

1. Digital system

Common bases: binary, octal, decimal, hexadecimal

Octal number sequence range: 0~7 starting with 0

Hexadecimal number sequence range: 0~9 and A~F starting with 0x

2. Digital range

Maximum and minimum values ​​of numbers in JavaScript

alert(Number.MAX_VALUE); //1.7976931348623157e+308
alert(Number.MIN_VALUE); //5e-324

• infinity, represents infinity, greater than any data
• -infinity, which means infinitesimal, smaller than any data
• NaN, Not a number, represents a non-numeric value

isNaN() method is used to determine non-numbers and return a value. If it is a number, it returns false, and if it is not a number, it returns true.

2.2 String

1. String escape character

The escape characters all start with \. Commonly used escape characters and their descriptions are as follows:

Escape character explanation\n

The meaning of newline

2. String length

The length of the entire string can be obtained through the length property of the string

var myname = 'my name is andy';
console.log(myname.length);

2.3 Boolean

Boolean values ​​have two values: true and false, where true means true and false means false.

When adding a Boolean value to a number, true is 1 and false is 0.

console.log(true + 1); //2
console.log(false + 1); //1

3. Data type conversion

3.1 Convert to string

3.2 Convert to digital type

number() // Convert to a number number('10') // 10
number('abc') // NaN
number(true) // 1
number(false) // 0
number(null) // 0
number(undefined) // NaN

parseInt() // Convert to a number and round down // Get integers from the converted data from the front to the back. Once one is found, it will not search again. Only the code that starts with an integer will be found:
parseInt('12.345') // 12
parseInt('12abc') // 12
parseInt('abc12') // NaN
parseInt(true) // NaN
parseInt(false) // NaN
parseInt(undefined) // NaN
parseInt(null) // NaN
Note: These characters must contain numbers and start with numbers, otherwise they are all NaN

parseFloat() // Convert to number, integer, decimal code:
parseFloat('12.345') // 12.345
parseFloat('12.345abc') // 12.345
parseFloat('abc12.345') // NaN
parseFloate(true) // NaN
parseFloat(false) // NaN
parseFloat(undefined) // NaN
parseFloat(null) // NaN
Note: These characters must contain numbers and start with numbers, otherwise they are all NaN

Implicit Conversion

1. When one of the left and right sides of + is a string, the other one will be quietly converted into a string for concatenation

2. Mathematical operators convert both sides into numbers for arithmetic operations - When one of the left and right sides is a string, the + sign will concatenate them. When there is no string on either side, the + sign can also convert both sides into numbers.

3. When one of the comparison operator is a number, the other one will be quietly converted to a number for comparison.

3.3 Convert to Boolean

Values ​​that represent empty or negative values ​​will be converted to false, such as '', 0, NaN, null, and undefined. Other values ​​will be converted to true. String to Boolean type, empty string is false, and all others are true.

Summarize

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