Java Script Foundation One

1. What is Java Script
   

2. How to write, Debug JS

3. Variable

4. Data type

I. What is Java Script

HTML: A language that specializes in writing web content, building an overall framework.

CSS: Specifically beautify the language of the Web page style, with "decoration function."

JS: Programming language specifically written for interaction

Interaction: User Action, program handles user's actions, returns processing result

How to run:

Browser: Content typesetting engine JS interpretation engine

Content typesetting engine: software that specifically parses HTML and renders CSS

JS parsing engine: Software that specializes in interpreting and executing JS programs

What are explanations and scripts?

Explanation: Default from top to bottom, read one line, run a line, read the same content, will overwrite the first read.

Scripts: Programs that do not require precompilation to interpret edges

2.JS History:

1955: Netscape and Sun co-developed the JavaScript scripting language and implemented the first version of the JavaScript scripting specification in its Netscape Navigator 2, ja Vascript version 1.0

1957: Launched the ECMA-262 specification version, which defines a new ECMAScript scripting language based on JavaScript.

Among the differences:

ECMAScript: Standard, regulation

Javascript:netscape: Languages implemented in accordance with the ECMAScript standard

JScript: Microsoft complies with ECMAScript standard language

1998: Established, the standard of change became the international ISO standard

3.JavaScript:

Generalized javascript=ecmascript (JS core syntax) +dom (API dedicated to Web content) +bom (API that specifically operates the browser window)

DOM: An API specifically designed to manipulate Web content based on the ES standard.

API: A programming interface that the browser has implemented that developers use directly.

BOM: An API that specializes in browser windows, and is not standard and is defined by each browser.

Characteristics:

1. The code can be written using any text editing tool, similar to C and Java.

2. Without compiling, there is a JavaScript engine explaining execution.

3. Weakly typed languages

4. Object-based

Two. Write and debug:

4. How to Write and debug:

JS script is plain text: written in Deamwaerver

Run the script in 2 categories:

1. Run in a standalone JS interpreter: For example, node. js

2. browser's JS Interpreter

Note: The *JS program is case-sensitive!

* The statements of each program are used;

When to use console: The console is a special place to debug your program!

---write js:2 in the Web page:

1. In

A. Executing with the page load process sequence interpretation

B.

C. outputting a line to a page: write HTML content directly in the BODY element

Use the output function of the page: Docunment.writ ("xxxxxx");

Popup function: Window.alert ("xxxx"); --no alert once, it pops up once.

Each box blocks the browser's loading and program execution process

2. In the event handler function of the element: only a small amount can be written

Events: Changes in the state of the elements triggered by the browser automatically or manually by the user

Event handler function: On event name

The essence is an attribute of the element, and the attribute value is a JS statement

Cases:

<button onclick= " Alert (' ejwpiobhjfipobhsdpfbn ') " > Click to surprise

3. Use an external JS file to store the JS program separately:

1. Create and write a. js file:

All three files must be saved as UTF-8 encoding

2. Introduce the. js file in the page:

---load and execute processes and

Three. * * * variable

What is a variable: storage space for storing a data in memory, and then a name.

When to use: data that is reused in a program is saved first

How to use: Declare assignment values

1. Disclaimer: Create a new variable in memory

How to declare: VAR variable name (declared only, but not assigned, default value is undefined)

Note: 1. Cannot start with a number

2. Cannot use reserved words: JS and pre-used keywords

3. It is best to see the name: such as the name of the hump (first letter lowercase, followed by the first letter of each word capitalized)

2. Assignment: Save the data to the right of the equal sign to the variable to the left of the equal sign

How to assign a value: variable name = value, * Assign a new value to a variable that already contains a value, and the new value will overwrite the old value *

3. Value: As long as the variable name is used, at run time, it is automatically replaced with the value in the variable.

Special Cases: 1. Only declared, but not assigned variables, can be used arbitrarily, output is undefined.

2. Never declare a variable, cannot use, get error prompt uncaught Referenceerror

3. Variables that have never been declared, are not assignable---and VAR

strongly recommended: variables must be declared with Var

Note: as long as the same page, the previous declaration, assignment, can be used later, but no longer ahead of this script

Example 1:

varhb="Spicy Chicken Leg Fort ";
Console.Log(HB);
Box="steamed Bun ";

Console.Log(HB);
Console.Log(Box);

The output is: "Spicy chicken leg fort" "Spicy Chicken leg fort" "Bun"

Declaration in advance: son Ah, before the program is formally executed: the VAR declared variable is declared in advance to the top of the current script block before the program executes. {* * * assignment left in situ}

Four. Constant: Once initialized, the value cannot be changed by the amount.

When to use: In a program, once you define a value that cannot be changed, you must save it with a constant.

How to create: const constant NAME = value; the name to use as a constant is capitalized

Note:

If you insist on changing the value of the constant, no error, but can not modify the success

pi= 3.14 ;
deg=  - ;

pi= 1 ; // An error? Impact results?

Results: No error, no impact on results

Five. * * * * data type: The way data is stored in memory

Why there are different data types: The different types of data in the real world have to be used specifically.

Category: 2 Categories: Values of "primitive type" and "Reference type" objects

Primitive type: The value is stored directly in the variable local type

Class 5: Number srting Boolean Undefined Null

Reference type: The value is not stored directly in the variable's local type, but the variable only holds the address value pointing to the location of the actual data

1.Number: Indicates all numbers

How to define: all non-quoted numbers are number type data.

When to use: No quotation marks are given for calculating or comparing numbers.

For example: Age, Price

Occupied space: All numbers are stored in a 64-bit binary number 1byte=8bit

Integers with 32-bit binary storage = 4 bytes

---value is independent of the amount of space occupied

Rounding Error: There are also numbers in the computer that can not be accurately calculated, resulting in a precision deviation of the calculated results, inevitably

Solution: Rounding--n.tofixed (2) by the specified number of decimal places and rounding the calculation results by 2 decimal places

2.String: String representing all text information

How to define: All information used for display is quoted, single and double quotes.

When to use: all the information used for display is quoted, there is no character type in JS, so a word should also be quoted

Space occupied: Letters or numbers, each 1 bytes; 2 bytes per kanji

JS memory is used to store characters in the Unicode number of the string

Unicode: An artificial number for each character in the primary language

Once the string is created, the value cannot be changed, and the old string can only be replaced with a new string.

Special:+ operation, if the participating operations are all strings, then addition becomes string concatenation

3.Boolean: Only two values: true, flase

When to use: As long as the save right/wrong, true/false, set up/not established, are represented by the Boolean type.

How to define: True with no quotes and flase is the Boolean type

Java Script Foundation One