JS summarized and basic grammar

 

One. General

JavaScript is often called the Web language. Since the earliest JavaScript is used to create web pages effects and form validation.

 

JavaScript development environment to build

Built environment, and the environment is nothing less than to run the code writing code environment.

 

Common text-based code editor as follows:

l Notepad

the Sublime

l  Atom

l  Brackets

l  VScode

Common integrated development environment as follows:

l  Dreamwaver

l WEBSTRA

 

Node.js installation process

Access https://nodejs.org/en/

After installation, you can view the currently installed version node by node -v command in the console inside

 

NPM introduced

NPM full name Node Package Manager, is supplied by the official Node.js package management and distribution tools.

 

1. NPM installation instructions: typing in the command line

asl install -g cnpm --registry = https: //registry.npm.taobao.org

1. Initialize it after installing

npm init: npm initialization settings

1. NPM After installation, the installation of a common input module, the following instruction

npm install readline-sync: an input module mounted

two. JavaScript basic grammar

1. Notes:

Single-line comments // footnotes

 Multi-line comments / *

*

*

*

*/

2. Statement

JS end with a semicolon, each statement on a separate line. Semicolon can not fill, not an error, but the code compression of time may be a problem, it is recommended to add a semicolon.

let i = 10;

console.log(i);//10

A pair of braces can be combined into a plurality of blocks of code statements which

{

    let i = 10;

    console.log(i);//10

}

Note: JS parsing layer by layer, if a layer of error, the contents of the back will not be resolved.

3. Identifier

Meaning: Represents the symbol of a certain entity. That is, from their own name, the name can be used as variable names, function names, object names and so on.

Mandatory requirements
1. may be numbers, letters, underscores, and dollar symbols, may not contain other special symbols

2. Do not start with a number
3. prohibit the use of JavaScript in the keywords and reserved words to name

4. strictly distinguish between sensitive

soft requirements
  look text EENOW

 

4. The three methods named

1. Hungarian notation is Microsoft following nomenclature Hungarian thought by programmers invention, which is characterized by the identifier name begins with one or more lower case letters representing the data type of the variable.

例如：a_array, o_object, i_userAge, b_isPassed

2. hump nomenclature

Hump ​​nomenclature actually divided into two types, one is the big hump, the other is a small hump.

● large hump has been called Pascal nomenclature, that each - words are capitalized

For example: UserName

● difference between small and large hump that hump, the first letter of the first word of a small hump is lowercase, first letter of the word is the big back

write

For example: userName

3. serpentine nomenclature

This nomenclature is common in the Linux kernel, the characteristics C + + standard library, Boost and Ruby, Rust and other languages ​​serpentine nomenclature is separated with an underscore between words such as: user_ name, my_ first_ name

 

5. Keywords and reserved words

Described in ECMA-262 - a set of keywords having particular use. These keywords can be used to indicate the start or end control statements, or to perform certain operations. As a rule, the keyword is reserved for the language itself, it can not be used as an identifier.

 

ECMA-262 also describes another group of words can not be reserved as an identifier. Although the reserved words in the language are not yet any specific purpose. But they are likely to be used as a key in the future.

 

JavaScript keywords and reserved words as follows:

 

abstract、await、 boolean、 break、 byte、 case、catch、 char、 class、 const, continue、debugger、default、 delete、 do、double、 else、 enum、export、 extends、 false、 final、finally、float、 for、 function、 goto、 if、 implements、 import、 in、instanceof、 int、

 

interface、let、 long、 native、 new、 null、 package、 private、 protected、 public、 return、short、static、 super、 switch、 synchronized、 this、 throw、 throws、 transient、 true、 try、typeof、var、 volatile、 void、 while、 with、 yield

 

6. Data Type Description

In JavaScript, the data types can be divided into two categories as a whole in terms of: simple data types and complex data types

● simple data types

Simple data types There are six kinds:

string, symbol, number, boolean, undefined, null symbol type which is the basic data types in which the newly added ES6

 

● complex data types

Complex data types is only one kind of object

Including arrays in JavaScript, regular, with its object types are Types

 

View Data Types

In JavaScript, we can view the data of a data type by typeof operator

console. log( typeof 10);//number

console. log(typeof true);//booleanconsole.

log(typeof 'Hello');//string

console. log(typeof [1,2,3]);//object

 

7. Variables

 

      Next we need to explore any programming language is very important in terms of a thing, variable. The so-called variable is used to refer to eleven values ​​stored in the memory. Of course, before we use a variable, you also need to do first thing is to declare a variable.

 

Declare variables

 

      JavaScript variables declared in a way there are three kinds: var, let, const. Var which is now not recommended, because there will be a variable lift and other issues. (Later we will discuss specific to this issue)

 

      Const and let the difference is that a variable declared const if it is a simple data type, then that can no longer be changed. And let variables declared whether simple data types or complex data types, in the back can be changed. Examples are as follows:

 

const variable declaration

 

      const name = 'Bill';name = 'Lucy';

 

      //TypeError: Assignment to constant variable. 

 

let declare variables

let name = 'Bill';

name = 'Lucy';

console.log(name);

 

Assignment and initialization of variables

let a = 3;

Multiple variables can also be a one-time initialization, write it on one line

let x = 3,y = 4,z = 5;

If you declare variables not given initial value, the default value is undefined

let a;

console.log(a);//undefined

 

Use var to declare variables

 

      Front mentioned, there are three ways in JavaScript variables declared: var, let, const. Var which now no longer recommended for use. This is because the use var to declare variables will be accompanied by some problems. Of course, these questions are often regarded as some of the features of JavaScript, such as repeated statements and omissions in the statement.

 

      Repeat statement

 

      If you are using the var keyword to declare a variable, then it is allowed to repeat the statement. But this time ignores the statement. If you re-statement and with the assignment, the equivalent is reassigned

 

      Repeat statement with no assignment, JS engine will automatically ignore the back of the variable declaration

 

      was test = 3;

 

each test;

 

      console. log(test);//3

 

      If the re-statement with the assignment, it will be eleven coverage data

 

      was test = 3;

 

was test = 5;

 

      console. log(test);//5

 

Scope

1. The global scope

This is the JS engine - came in in the operating environment. In the global scope of variables declared in called global variables. It features variable global variables that can be accessed anywhere.
let a = 5; // this is a global variable
2. local scope

In JavaScript, a pair of braces can produce a local scope. Inside the local scope variables called local variables, since it is a local variable, then only in the local scope can have access to the inside, the outside is not accessible.
{

Leti = 10;
Console log (I); // 10.
The console.log (I);
// a ReferenceError: Not defined IS I
mention in passing that, in the variable var declared with braces not local variables, and It is a global variable. Actually, this is the first to use var to declare eleven question the legacy of variables.

 

Data Type Description

In JavaScript, the data types can be divided into two categories as a whole in terms of: simple data types and complex data types

● simple data types

Simple data types There are six kinds:

string, symbol, number, boolean, undefined, null symbol type which is the basic data types in which the newly added ES6

 

● complex data types

Complex data types is only one kind of object

Including arrays in JavaScript, regular, with its object types are Types

 

View Data Types

In JavaScript, we can view the data of a data type by typeof operator

console. log( typeof 10);//number

console. log(typeof true);//booleanconsole. log(typeof 'Hello');//stringconsole. log(typeof [1,2,3]);//object

 

Boolean

The so-called Boolean type, also known as boolean type, that is, true and false, the value of this type of only two - one is true, the other is false

 

let i = true;

console.log(i);//true

console. log(typeof i);//boolean

 

It should be noted that these two values ​​are not the same numeric value, so true is not necessarily equal to 1 and false is not necessarily equal to zero. There is also a - point is the Boolean literals true and false are case sensitive. In other words, True and False are not a Boolean value.

 

Although the type Boolean literals only two, but in ECMAScript values ​​of all types can be converted to a Boolean. You can use Boolean () function to convert other types Boolean value.

 

console.log(Boolean("Hello"));//true

console. log (Boolean(42));// true

console. log(Boolean(0));//false

 

Digital Type

1. Integer

Binary begin with 0b, octal begin with 0, hexadecimal beginning with 0x

The results still to decimal

1. Real

Floating-point representation of two ways: decimal and scientific notation type type.

let a = 3.14;

let b = 9.12e+2;

console.log(a,b);//3.14 912

1. Numerical extension

JS minimum value supported console.log (Number.MIN_VALUE); // 5e-324

JS the maximum supported console.log (Number.MAX_VALUE); // 1.7976931348623157e + 308

1. In:

English stands for Not a Number

Any operation involving NaN will return NaN

let A = + 10;

console.log(a);//NaN

 

console.log(isNaN(NaN));//true

console.log(isNaN("123"));//false

console.log(isNaN(123));//false

console.log(isNaN("Hello"));//true

console.log(isNaN(true));//false

 

NaN number belongs to type console.log (typeof NaN); // number

 

Numeric conversion

number () can be converted to non-numeric value

console.log(Number(true));//1

console.log(Number(false));//0

console.log(Number(10));//10

console.log(Number(null));//0

console.log(Number(undefined));//NaN

 

console.log(Number("1"));//1

console.log(Number("012"));//12

console.log(Number("0o10"));//8

console.log(Number("0b111"));//7

console.log(Number("3.14"));//3.14

console.log(Number("0xf"));//15

console.log(Number(""));//0

console.log(Number("123Hello"));//NaN

 

parseInt (): the value of non-integer values ​​into

console.log(parseInt("1"));//1

console.log(parseInt("012"));//12

console.log(Number("0o10"));//8

console.log(Number("0b111"));//7

console.log(parseInt("3.14"));//3

console.log(parseInt("0xf"));//15

console.log(parseInt(""));//NaN

console.log(parseInt("123Hello"));//123

 

console.log(parseInt("012"));//12

console.log(parseInt("012",8));//10

console.log(parseInt("AF"));//NaN

console.log(parseInt("AF",16));//175

 

Static method

Number.isInteger (): used to determine whether a value of an integer.

console.log(Number.isInteger(25));//true

console.log(Number.isInteger(25.0));//true

console.log(Number.isInteger(25.1));//false

console.log(Number.isInteger("15"));//false

console.log(Number.isInteger(true));//false

 

Number.isFinite (): not a number when it returns false, it is, then returns true.

console.log(Number.isFinite(true));//false

console.log(Number.isFinite(7));//true

console.log(Number.isFinite(NaN));//false

console.log(Number.isFinite(Infinity));//false

console.log(Number.isFinite("23"));//false

Examples of methods

toFixed () String Return value rounded to the decimal representation specified

let num = 10.456;

console.log(num.toFixed(2));//10.46

console.log(num.toFixed());//10

console.log(num.toFixed(0));//10

console.log(num.toFixed(undefined));//10

console.log (num.toFixed (-1)); // given

 

String type

let a = "abcd";

let b = 'abcd';

let a = "Hello 'World',welcome";//正确

let b = 'Hello "World" , welcome'; // correct

let c = "Hello \"World\",welcome";//正确

 

Adding strings and other data types will be converted to a string

let a = "abcd";

let b = 13 + a;

let c = 3.14 + a;

let d = true + a;

let e = null + a;

let f = undefined + a;

console.log(typeof b);//string

console.log(typeof c);//string

console.log(typeof d);//string

console.log(typeof e);//string

console.log(typeof f);//string

 

 

toString () the addition null and undefined other than the data types into a string

 

let a = 10,b = true,c = null,d;

console.log(typeof a.toString());//string

console.log(typeof b.toString());//string

console.log (typeof c.toString ()); // given

console.log(typeof d.toString());

 

let i = 10;

console.log(i.toString());//10

console.log(i.toString(2));//1010

console.log(i.toString(8));//12

console.log(i.toString(10));//10

console.log(i.toString(16));//a

 

let a = 10,b = true,c = null,d;

console.log(String(a),typeof String(a));//10 string

console.log(String(b),typeof String(b));//true string

console.log(String(c),typeof String(c));//null string

console.log(String(d),typeof String(d));//undefined string

 

String template

 

Enhanced version of the string

let str = `Hello World`;

console.log(str);//Hello World

console.log(typeof str);//string

console.log(str.length);//11

 

let str = `Hello

World`;

console.log(str);

//Hello

//World

console.log(typeof str);//string

console.log(str.length);//12

 

let name = "xiejie";

console.log(`Hello,${name}`);

// Hello, xiejie

 

let count = 10,price = 0.25;

console.log(`${count} items cost $${(count*price).toFixed(2)}`);

//10 items cost $2.50