54, JS First Knowledge

JavaScript overview The history of JavaScript

• 1992 Nombas developed the embedded scripting language for C-minus-minus (c--) (originally bundled in Cenvi software). Rename it scriptease. (The language that the client executes)

• Netscape (Netscape) received Nombas's philosophy, (Brendan Eich) developed a set of Netscape scripting languages in its Navigator livescript 2.0 product. Sun and Netscape are done together. And then the name is JavaScript.

• Microsoft then emulated a JavaScript clone called JScript in its IE3.0 product.

• To unify the three, the ECMA (European Computer Manufacturing Association) defines the ECMA-262 specification. The International Organization for Standardization (ISO/IEC) also adopted ECMAScript as the standard (iso/iec-16262). Since then, Web browsers have struggled (albeit with varying degrees of success and failure) to ECMAScript as the basis for JAVASCRIPT implementations. ECMAScript is the norm.

ECMAScript

Although ECMAScript is an important standard, it is not the only part of JavaScript, and certainly not the only one that is standardized. In fact, a complete JavaScript implementation is made up of the following 3 different parts:

• Core (ECMAScript)
• Document Object Model (DOM) Documents object models (consolidated js,css,html)
• Browser object models (BOM) Broswer object Model (integrated JS and browser)
• The vast majority of Javascript in development is object-based. It is also object-oriented.

To put it simply, ECMAScript describes the following:

• Grammar
• Type
• Statement
• Key words
• Reserved words
• Operator
• Object (encapsulates inherited polymorphism) object-based language. Use an object.

Two JavaScript based 2.1 js Introduction method

1 Direct Write    <script>        alert (' Hello Yuan ')    </script>2 import file    <script src= "Hello.js" ></ Script>

2.2 JS variables, constants and identifiers 2.2.1 JS variables

123	x=5y=6z=x+y

In algebra, we use letters (such as x) to hold values (such as 5). By the above expression Z=x+y, we can calculate the value of Z as 11. In JavaScript, these letters are called variables.

So how to define the use of variables in JS?

1. Do not declare variable types when declaring variables. All use the var keyword;

1	vara;<br>a=3;

2, a row can declare multiple variables. And can be of different types

1	varname="yuan", age=20, job="lecturer";

3 . You can declare variables without var. if you don't use Var then it's a global variable

4, the name of the variable, the first character can only be letters, underscores, $ dollar symbol three select one, the remaining characters can be underscores, dollar signs or any alphanumeric characters and case-sensitive, x and X are two variables

" Hi "  "hi" "Hi";

Naming Conventions2.2.2 Constants and identifiers

Constants : Data values that appear directly in the program

Identifier:

1. Consists of letters, numbers, underscores (_), Dollar signs ($) that do not begin with a number
2. A name commonly used to denote functions, variables, etc.
3. For example: _ABC, $ABC, abc,abc123 is an identifier, and 1ABC is not
4. Words that represent a particular meaning in the JavaScript language are called reserved words and do not allow programs to be redefined as identifiers

2.3 JS Data type

/               *     Number-----The  value        Boolean    -----  boolean string     -----  string        undefined  -----  undefined        null       -----   NULL         *

2.3.1 Numeric type (number)

• Integer and floating-point values are not distinguished;
• All numbers are stored in 64-bit floating-point format, equivalent to the double format in the Java and C languages
• The maximum value that can be represented is ±1.7976931348623157 x 10308
• The minimum value that can be expressed is ±5 x 10-324

Integer:
10 integers in JavaScript consist of sequences of numbers
What is the exact range of the expression?-9007199254740992 (-253) to 9007199254740992 (253)
Integer out of range, accuracy will be affected
Floating point number:
Using decimal points to record data
Example: 3.4,5.6
Use Index to record data
Example: 4.3e23 = 4.3 x 1023

16 binary and 8 decimal expressions:
16 binary data before adding 0x, octal front plus 0;16 is composed of 16 characters, such as 0-9,a-f, 8 binary number consists of 0-7 8 digits

16 binary and 8 binary vs. 2 binary conversions:

12	2进制: 1111 0011 1101 0100   <-----> 16进制:0xF3D4 <-----> 10进制:624202进制: 1 111 001 111 010 100 <-----> 8进制:0171724

2.3.2 The String type (string)

is a sequence of Unicode characters, numbers, punctuation marks, string constants are enclosed in single or double quotation marks, and there are no character types in JavaScript, and expressions of common special characters in strings;
Some special characters in a string must be accompanied by a right dash \; Common escape character \ n: newline \ ': single quote \ ': double quote \ \: Right Dash

2.3.3 Boolean Type (Boolean)

The Boolean type has only two values: True and False, also representing 1 and 0, in the actual operation True=1,false=0
Boolean values can also be considered on/off, yes/no, 1/0 correspondence True/false
The Boolean value is primarily used for JavaScript control statements, such as:

if (x==1) {      y=y+1;} else{      y=y-1;      }

2.3.4 Null & undefined type

Undefined type

The Undefined type has only one value, which is Undefined. When the declared variable is not initialized, the default value of the variable is undefined.

When the function has no definite return value, the returned value is also "undefined";

Null type

Another type with only one value is null, and it has only one private value, NULL, which is its literal. The value undefined is actually derived from the value null, so ECMAScript defines them as equal.

Although the two values are equal, they have different meanings. Undefined is a value that is assigned to a variable when it is declared but not initialized, and null is used to represent an object that does not already exist (this is briefly described when discussing the typeof operator). If the function or method is to return an object, the object returned is usually null when it is not found.

2.4 operator

Arithmetic operator:    +   -    *    /     %       + +-        -comparison operator:    >   >=   < <=!    =    = =    = = = = =   !== logical operator:     &&   | |   ! Assignment operator:    = = = =  *=   /= string operator:    +  join, either side operand has one or two is a string do join operation

2.4.1 Arithmetic operators

Note 1: Self-added self-reduction

If x=2, then the value of the X + + expression after the execution of the 3,x--expression is 1;i++ equivalent to i=i+1,i--equivalent to i=i-1;
Increment and decrement operators can be placed before variables or after variables:----

var i=10;console.log (i++); Console.log (i); Console.log (++i); Console.log (i); Console.log (i--); Console.log (-I.);

NOTE 2: Cell operators

123	- 除了可以表示减号还可以表示负号  例如：x=-y+ 除了可以表示加法运算还可以用于字符串的连接  例如："abc"+"def"="abcdef"

JS differs from Python and is a weakly typed language

'  A ' and integer 3 to get the string '123', then you can think of it as an integer 123 , all of which do not require any display conversion. So Python is both a dynamic type language (because it doesn't use a display data type declaration) and a strongly typed language (because it's actually the same type as long as a variable gets a data type). 

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Note 3: NaN

    var d= "Yuan";    D=+d;    alert (d);//nan: A special value of type number, which gets a NaN data alert (typeof (D)) when it encounters an invalid to convert a string to a digit    ;//number    //nan features:        var n =nan;        alert (n>3);    alert (n<3);    alert (n==3);    alert (N==nan);        alert (N!=nan); All operations that//nan participate in are false, except! =

2.4.2 comparison Operators

1	>   >=   <    <=    !=    ==    ===   !==

When using a control statement:

        if (2>1) {       //  3  0  false null undefined []            console.log ("condition set!")        }

equals and non-equal operators are all equal and non-full equals. The two operators do the same as equals and non-equals, except that they do not perform type conversions until they are checked for equality.

Console.log (2==2), Console.log (2== "2"), Console.log (2=== "2"), Console.log (2!== "2");

Note 1:

var bresult ="Blue"<"Alpha"; alert (bresult);//output True in the example above, the string"Blue"Less than"Alpha"Because the character code for the letter B is 66, and the character code for the letter A is 97.. Comparing numbers and strings another tricky situation occurs when you compare numbers in two-string form, such as: Var bresult=" -"<"3"; alert (bresult);Output"true"The above code compares the string" -"And"3"。 The two operands are all strings, so the comparison is their character code ("2"The character code is 50,"3"The character code is 51). However, if one of the operands is a number, the result is interesting: var bresult=" -"< 3; alert (bresult);Output"false"here, the string" -"will be converted to the number 25, then with the number 3compared, the results were expected. Summary: Comparison operators on both sides if one is a numeric type and one is a different type, its type is converted to a numeric type. If both sides of the comparison operator are of type string, the ASC code of the highest bit is compared, and if the highest bit is equal, continue to take the second bit comparison.

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NOTE 2:

2.4.3 logical operators

if (2>1 && [up]) {    console.log ("Condition and")}//think back content? Console.log (1 && 3); Console.log (0 && 3) ; Console.log (0 | | 3); Console.log (2 | | 3);

2.5 Process Control

• Sequential structure (executed from top to bottom)
• Branching structure
• Loop structure

2.5.1 Sequential structure

    <script>        console.log ("Monday");        Console.log ("Tuesday");        Console.log ("Wednesday");    </script>

2.5.1 Branching structure

If-else structure:

if (expression) {   statement 1;   ......   } else{   statement 2;   .....   }

Function Description: Execute statement 1 if the value of the expression is true, otherwise execute statement 2

Example:

var x=(New Date ()). GetDay (); //get today's week value, 0 for Sunday var y; if((x==6) | | (x==0)) {y="Weekend"; }Else{y="Weekday";        } console.log (y); /*equivalent to Y="Weekday"; if((x==6) | | (x==0)) {y="Weekend";  } console.log (y); */

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If-elif-else structure:

if (expression 1) {    statement 1;} else if (expression 2) {    statement 2;} else if (expression 3) {    statement 3;} else{    statement 4;}

Example:

var score=window.prompt ("Your score:");if(score>90) {ret="Excellent";}Else if(score>80) {ret="Liang";}Else if(score>60) {ret="Pass";}Else{ret="inferior lattice";} alert (ret);

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Switch-case structure

Switch basic format switch (expression) {case    value 1: statement 1;break;    Case value 2: statement 2;break;    Case Value 3: statement 3;break;    Default: statement 4;}

Example:

Switch (x) {case 1:y= "Monday";    Break;case 2:y= "Tuesday";    Break;case 3:y= "Wednesday";    Break;case 4:y= "Thursday";    Break;case 5:y= "Friday";    Break;case 6:y= "Saturday";    Break;case 7:y= "Sunday";    break;default:y= "undefined";}

Switch is more concise and clear than the else if structure, making the program more readable and more efficient.

2.5.2 Cycle Structure

Circular statement Flowchart

For loop:

Syntax rules: for    (initial expression; conditional expression; self-increment or decrement)    {            Execute statement ...    }

Function Description: Implement the condition loop, when the condition is established, EXECUTE statement 1, otherwise jump out of the loop body

Another form of A For loop:

For (variable in array or object)    {        Execute statement ...    }

While loop:

Syntax rules:

while (condition) {    statement 1; ...    }

Function Description: Run function and for similar, when the condition is formed loop execution statement curly braces {} Inside the statement, otherwise jump out of the loop; also support continue and break statements.
Example:

var i=1;  while (i<=7) {    document.write (""+i+"" +i+"");    document.write ("<br>");    I+ +;} Loop output H1 to H7 font size

View Code2.5.3 Exception Handling

try {    //This piece of code runs from top to bottom, where any one of the statements throws an exception the code block ends running}catch (e) {    ///If an exception is thrown in a try code block, the code in the catch code block is executed.    //e is a local variable that is used to point to an Error object or other thrown object}finally {//Whether or not the code in the     try is thrown (even if there is a return statement in the try code block), the finally code block is always executed. }

Note: Throw exception throw error (' xxxx ')

54, JS First Knowledge