JavaScript self-organizing the basics-01

1.JavaScript Introduction:

JavaScript is the most popular scripting language on the Internet, and all modern HTML uses JavaScript. Since it is a scripting language, there are three types of features:

(1) weak type;

(2) interpreted language (no compilation required);

(3) Row by line execution, one line of code error, the subsequent code block will not continue to execute;

(4) <script> tags can be directly embedded in the HTML file, the location is arbitrary, usually placed under the modified content or head tag, but written as a separate JS file is conducive to the separation of structure and behavior

2.JavaScript content (drawings):

Where ECMAScript is the core of JavaScript;

Dom is a Document Object model (using JS to manipulate web pages);

BOM is the Browser object model (using JS to manipulate the browser)

Output of 3.JavaScript information:

(1) Alert () method: In the form of a cue box in the page output

(2) Console.log () method: Output Information in console, example:

Console.log ("Hello,javascript")   

(3) document.write (): Write the content directly in the HTML page, for example:

4.JavaScript variables:

Unlike Java, a variable in ECMAScript has no specific type, the variable is defined with the var operator only, it can be initialized to any value, the initialization format of the variable: var variable name = variable value;

var a = "Hello";   var b = 123;  

If you want to define multiple variables, you can use multiple variables in one line, separated by commas, for example:

var a = "Hello",      = 123,      

Variable rules for variable names:

(1) consisting of letters, numbers, underscores, and $ symbols

(2) can not start with a number, do not suggest the following dash start;

(3) strictly distinguish the case;

(4) Can not be keywords and reserved words

5.JavaScript Data type:

JavaScript can be divided into raw data types, reference data types, two kinds:

(1) Raw data type: Number,string,boolean,undefined,null

Number: Numeric, numeric, contains positive, negative, integer, decimal, 0, NaN, Infinity (positive infinity),-infinity (negative infinity);

Note:NaN: The abbreviation for not a number, indicating that the value is not a numeric (also belongs to #)

string: 0 or more characters wrapped with double quotation marks "" or single quotation marks, if there is nothing in the quotation marks, then this string is called an empty string.

Boolean: Boolean: Contains true: Indicates True (SET) and false: False (not set) two values

undefined: Indicates that the variable is undefined, or the variable is defined, but is not assigned a value

null: Indicates that a variable does not point to any piece of storage space, that is, the variable exists, but the inside is empty, similar to undefined

(Tip: In the chrome console output, you will find the number type is dark blue, string is black, the boolean is light blue, undefined and null are dimmed)

(2) Reference data type:

Object, Array (array), date (date), REGEXP (Regular): Wait a minute

(3) How to view the data type of a variable (typeof operator):

Numeric data: The return value is number

Console.log (typeof 123)   //

String data: The return value is string

Console.log (typeof "Hello")  // output string    

Boolean data: The return value is Boolean

Console.log (typeoftrue/false)    

Undefined: The return value is Undefined

Console.log (typeof undefined)   // output undefined

Null: The return value is object (legacy problem, indicating null is also an object)

Console.log (typeofnull)     //    

NaN: The return value is number

Console.log (typeof NaN)    //

Conversion of 6.JAVASCRIPT data types:

(1) When using the addition (+) operator, any data that is added to the string type data is string type data;

Console.log ("Hello" + 123)    // output "Hello 123"  

Note (simple understanding): in JavaScript Hollow string "" is converted to false, non-empty string is converted to true (except "0", "1");

False is converted to 0 or "0", and true to 1 or "1";

When making logical judgments, null,undefined, "" (empty string), 0,nan all default to false;

= = In comparison can be converted data type, = = = is strictly compared, as long as the type mismatch will return false;

In fact, == the comparison is actually converted to a string to compare, but before the Boolean is converted to a string, it is first converted toNumber

Console.log ("123" = =true)//Output FalseConsole.log ("1" = =true)//Output TrueConsole.log ("" = =true)//Output FalseConsole.log (1 = =true)//Output TrueConsole.log ("" ==false)//Output TrueConsole.log (' 123 ' = =false)//Output FasleConsole.log (' 0 ' = =false)//Output TrueConsole.log (0 = =false)//Output TrueConsole.log (' 1 ' = = 1)//Output TrueConsole.log (' 0 ' = = 0)//Output TrueConsole.log (-true)//Output-1

(2) Parseint: Converts a string to an integer (only the numeric value in the string is recognized):

Note: The front and back spaces in the string are ignored (they will not be recognized when there are several values after the number of spaces);

Can correctly identify the positive and negative sign, that is, retain the sign;

A non-numeric character will stop converting when it is converted;

If the first character of a string is non-numeric, the result of the conversion is Nan;

Console.log (parseint ("123"))    // output 123  Console.log (parseint ("1 2"))    //  Only outputs 1  console.log (parseint ( -123))     // output -123  console.log (parseint ("Hello") )    // output nan  console.log (parseint (true))       // output Nan   Console.log (parseint ("123hello"))    // output 123, the back non-numeric type  does not recognize console.log ( parseint ("1"))     // output 1, ignore spaces  

(3) Parsefloat: Convert String to decimal (identify decimal point, note above)

Console.log (parsefloat ("123.55"))    // output 123.55  console.log (parsefloat (". 1hello"))    // Output 0.1  

(4) Number: Convert other types of data into numerical form, note that the converted data must be a pure numeric composition, otherwise cannot be converted, other considerations as above

Console.log (Number (true))   //1  console.log (number (false))    / /0  console.log (number (null))    //0  Console.log ( Number ("123hello"))    //NaN  console.log (number ("12.22"))    //  12.22  console.log (number (undefined))    //

(5) The Information box in the page:

Alert (), pop up a hint box, only OK;

Window.alert ("The weather is fine today")  

Confirm (), pops up a confirmation box, has OK and cancel;

Window.confirm ("Good mood Today")  

Prompt (), pop up an input box, you can enter the content;

The basics of JavaScript are written here for the time being, and the follow-up will fill up ...

JavaScript self-organizing the basics-01