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This article mainly shows you "JavaScript learning knowledge points", the content is easy to understand, clear, hope to help you solve doubts, the following let the editor lead you to study and learn "what are the knowledge points of JavaScript learning" this article.

First, a simple introduction to JavaScript

1.1. A client scripting language

That runs in the client browser. Every browser has a parsing engine for JavaScript

Scripting language: it can be parsed and executed by the browser without compilation.

Function: you can enhance the interaction between users and html pages, control html elements, make the page have some dynamic effects, and enhance the user experience.

The history of 1.2.JavaScript development

In 1992, Nombase developed the first client-side scripting language, which was specifically used for form validation. Named: Cmure -, and later changed its name to: ScriptEase

In 1995, Netscape developed a client-side scripting language: LiveScript. Later, an expert from SUN was invited to modify the LiveScript and name it JavaScript.

In 1996, Microsoft copied JavaScript to develop the JScript language.

In 1997, ECMA (European Association of computer Manufacturers) developed a standard for client scripting languages: ECMAScript, which unifies the coding of all client scripting languages.

JavaScript = ECMAScript + JavaScript own unique thing (BOM+DOM) 1.3.JavaScript advantage

a. Interpretive language

b. Object-based

c. Event driven

d. Weak type

e. High security

f. Cross platform

1.4.JavaScript reference

To insert JavaScript in a HTML page, use a tag.

And will tell JavaScript where to start and end.

The line of code between and contains JavaScript

Several ways of 1.5.JavaScript output window.alert () [xss_clean] () innerHTMLconsole.log ()

From the point of view of personal use, console.log () is more convenient to use in programming, and the direct F12 console can view the output.

What keywords does 1.6.JavaScript have?

1.7.JavaScript comment

/ / this is the code: a single sentence comment, which is usually ctrl + L in the editor.

/ * this is the code * /: multi-line comments, in the editor is generally ctrl + shift + L key.

What are the common identifiers of 1.8.JavaScript

What are the common HTML events in 1.9.JavaScript

1.10.JavaScript common operators

Common assignment operators for 1.11.JavaScript

1.12.JavaScript common comparison operator

1.13.JavaScript common logical operators

Second, elaborate on the basic grammar of JavaScript 2.1. Variable 2.1.1. Define a variable / / declare a variable named test. Var test;var age, name, sex;// declare age, name, sex three variables / / declare variables and assign values var age = 10, name = "Xiaoqiang", sex = "1"; 2.1.2. Variable naming rules and specifications

1. It consists of letters, numbers, underscores and symbols, and cannot start with a number.

2. Keywords cannot be used, for example: while

3. Case-sensitive

Norm: a conventional way of writing

1. The variable name should be meaningful.

2, follow the hump naming method, hump naming method: the first letter is lowercase, followed by an uppercase word, such as userName.

2.2. Data type

The data types are: Number type, String type, Boolean type, Undefined type, Null type, Object type.

2.2.1.Number

Number numeric type: contains integers and decimals

Can be expressed as: decimal, octal, hexadecimal

Such as:

Var num = 10; / / Decimal var num1 = 012; / / Octal var num2 = ox123; / / hexadecimal

Range of values:

Console.log (Number.MAX_VALUE); / / maximum value is 5e-324console.log (Number.MIN_VALUE); / / minimum value is 1.7976931348623157e+308 infinity: Infinity infinitesimal:-Infinity

Numeric judgment cannot use decimals to verify decimals and do not use NaN to verify whether it is NaN (NaN----not a number), but can use isNaN- is not a number to verify whether the result is NaN.

Such as:

Var num; / / declares a variable with no value console.log (isNaN (num)); / / isn't true a number? The result is true2.2.2.String.

String escape character:

Escape sequence character\ b Backspace\ f Paper feed\ nLine feed\ r enter\ t horizontal jump (Ctrl-I)\ 'single quotation mark\ "double quotation mark\ double slash

Eg:

Var str = "iam a pm\" console.log (str); / / iam a pm\

String concatenation:

Var str1 = "Sir"; var str2 = "Hello"; console.log (str1+str2); / / Hello, sir

If the string and number stitching result is also the string stitching effect as shown above

2.2.3.Boolean

Two attributes true/false for Boolean type

2.2.4.Undefined

Undefined represents a variable that declares no assignment. When the variable is only declared, the default value is undefined.

Such as:

Var num;console.log (num); / / undefined2.2.5.Null

Null represents an empty value. If you want the value of the variable to be null, you must set it manually.

2.2.6.Object2.3. Conversion of data types 2.3.1. Convert to a numeric type

(1), Number () can convert any value to a numeric value. If there is a character in the string that is not a numeric value, return NaN.

Such as:

Var num1 = Number ("10"); console.log (num1); / / 10var num2 = Number ('10adbdn'); console.log (num2); / / NaNvar num3 = Number ("10.78"); console.log (num3); / / 10.78var num4 = Number (' 10.65dd'); console.log (num4); / / NaNvar num5 = Number ("A10"); console.log (num5); / / NaN

(2), parseInt () to integer

Such as:

Var num1 = parseInt ("10"); console.log (num1); / / 10var num2 = parseInt ('10adbdn'); console.log (num2); / / 10var num3 = parseInt ("10.78"); console.log (num3); / / 10var num4 = parseInt (' 10.65dd'); console.log (num4); / / 10var num5 = parseInt ("A10"); console.log (num5); / / NaN

(3), parseFloat () to decimal

Such as:

Var num1 = parseFloat ("10"); console.log (num1); / / 10var num2 = parseFloat ('10adbdn'); console.log (num2); / / 10var num3 = parseFloat ("10.78"); console.log (num3); / / 10var num4 = parseFloat (' 10.65dd'); console.log (num4); / / 10var num5 = parseFloat ("A10"); console.log (num5)

The difference among (4), Number (), parseInt () and parseFloat ()

Number () is stricter than parseInt () and parseFloat ().

ParseInt () is similar to parseFloat (), and parseFloat parses the first one. I met the second one. Or a non-digital end.

2.3.2. Convert to string type

(1), toString ()

Such as:

Var num = 10. Console.log (num.toString ()); / / string 10

(2), String ()

Such as:

Var num1 = 5 position console.log (String (num1)); / / string 5

(2), JSON.stringfy ()

2.3.3. Convert to Boolean type

0, empty string, null, undefined, NaN will be converted to false and the rest will be converted to true

Such as:

Var num1 = Boolean (0); console.log (num1); / / falsevar num2 = Boolean (""); console.log (num2); / / falsevar num3 = Boolean (null); console.log (num3); / falsevar num4 = Boolean (undefined); console.log (num4); / / falsevar num5 = 10 politics var num6;console.log (Boolean (num5+num6); / false2.4. Operator

Types of operators: arithmetic operators, composite operators, relational operators, logical operators

(1) arithmetic operator: "+"-"*" / "%"

Arithmetic expression: expression complex operation connected by arithmetic operator

(2) compound operator: "+ ="-= "* =" / = "% ="

Composite operation expression: an expression connected by a composite operator

(3) Relational operator: ">"="

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