From beginner to proficient in JavaScript, you only need to learn so much

The content this article brings to you is about JavaScript entry to proficiency. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

#What is javascript?

JavaScript is a scripting programming language in web development. It is also a universal, cross-platform, object- and event-driven scripting language with security.

The main features of JavaScript:

Interpretive, object-based, event-driven, cross-platform, security

Applications of JavaScript:

Verify user input, animation effects, window applications, text special effects, etc.

Methods for using JavaScript in the web interface:

In the page Directly embed JavaScript code

Language is specified in Which scripting language is used in HTML and its version

Src specifies the path to the external script. External script files usually use javascript scripts with an extension of . js

Type specifies which scripting language and version are used in HTML. This attribute has been used since the HTML4.0 standard. It is recommended to use the type attribute instead of the language attribute.

Defer The function of this attribute is when After the document is loaded, the script is executed. When the script does not need to be executed immediately, after setting the defer attribute, the browser will not have to wait for the script language to be loaded, so that the mask will load faster.

Connecting external JavaScript File

<script language="javascript" src="javascript.js"></script>

JavaScript basic syntax

The Javascript program is executed line by line in the order it appears in the html view

If necessary, To execute in the entire html file, it is best to place the file in the

Javascript is very sensitive to the case of letters

The semicolon at the end of each line is optional None

Identifier: It’s just a name

Keyword: It has a specific meaning in javascript

Constant: A quantity whose value cannot be changed while the program is running

Variable: refers to a named storage unit in the program

The naming of variables

Declaration and assignment of variables

Variables in JavaScript all have var To declare, the format is: var a;

Declare variables and assign values: var b=2;

var You can declare multiple variables at the same time

var a,b, c

var declares multiple variables at the same time and assigns values:

var a=1,b=2,c=3;

The scope of the variable

The lifetime of a variable: refers to the valid time the variable exists in the computer

Global variables: always valid during program execution

Local variables: the valid scope of the variable is only within the current function中

Data type

Number (Number) is the most basic data type. In JavaScript, there is no distinction between integer and floating-point values. All numbers are composed of floating-point values. Represents

When a number appears directly in a javascript program, it is called a numerical direct quantity

The numerical direct two forms supported by JAVASCRIPT are as follows:

integer Type data

Hexadecimal and octal

Floating point data

String type

String (String) is composed of Unicode characters, numbers , a sequence composed of punctuation marks, etc., which is the data type used by JavaScript to represent text

Contained in single quotes or double quotes

Boolean data

The value has only two , true, false are used to indicate whether something is true or false

Special data type

Escape characters

Undefined value undefined

Special The type of numeric constant NaN, that is, not a number, is a number generated due to a calculation error in the program.

Null value

Null means null value

Null is not equivalent The empty string "" or 0

Null means that a variable has been assigned a null value, while undefined means that the variable has not been assigned a value

Operator

here I won’t spell it out in detail, anyone who can program knows that

Operators in Javascript can be divided into single-objective operations, binocular operations, and multi-objective operations according to the operands

According to the operation type, they can Divided into arithmetic operators, comparison operators, assignment operators, logical operators, conditional operators

trinary operations

（a==b）? Correct: Error

A is equal to b, return correct

A is not equal to b, return error

Other operators:

bit operator

Typeof operator

The Typeof operator is used to return the type of data currently held by its operand. Often used to determine whether a variable is defined

<script language="javascript">
var a = 3;
alert（"a的类型为"+（typeof a））
<script>

New operator

Used to create a new object

Expression

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