General

Three states of files in git

at any point of time a file in git can be in 1 of the following three states
modified a file is just modified.You have changed the contents of the file but the file is not stored in the database.
staged file is modified and you have marked it as to be committed.when you perform commit the next time then the file will be committed.
committed this means that the contents of the file are safely stored in the git database.

There are 2 main concepts to remember when working with git:

• Working directory   This represents a specific version of the files.You work with working directory and modify and commit files in the working directory.
• Repository   This stores all the versions of all the files which are managed by git.Repository resides in the .git subdirectory in the working directory.

Checking status of file or directory in git

to check the status of files of a git repository use the following command:

git status

This will list all the files which have been modified since the last commit.

In the following screen shot below none of the files have been modified since the last commit

In the following screenshot 2 files were modified since the last commit

Now we add one of the files to the staging area as:

If we run the git status command now we can see that one of the files has been staged.

Now we commit the staged files using the following command:
$ git commit -m “initial commit”

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Node Package Manager or NPM is a command line tool used while working with Node.js. NPM provides the following features for Node.js developers:

• As there are lots of packages for Node.js ,NPM makes it easier to manage them.It represents a repository of all the packages available for Node.js. We can install the packages using NPM.
• As all the packages are present in central Node.js repository we  can search these using NPM.We can use NPM to search a particular package as :

npm search "package name"

NPM important commands

Following are useful commands for managing node.js packages

To download a package

npm install Package_Name_To_Install

For example following will download a package named test_package

npm install test_package

This command will download the mentioned package to the node_modules directory.It will create node_modules directory
in the current directory if it doesn’t exist.

Using a package

We can use a package which is in the node_modules directory in our application by using the require() method.

For example we can use a module named test_package in our application as:

var lodash = require('lodash');

To install a package globally we use the following command:

npm install package_name -g

global packages differs from local packages in following ways:

• They are installed in the system directory
• They can be used from the command line
• They can not be used in a package using require

View the installed packages

To view the list of all locally installed modules using the following command

npm ls

Above command will display the list of packages as well as the location where the packages are installed

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Enum is a group of numeric contants.TypeScript supports enum since version 0.9.

Enums are useful for representing values which can have one of a fixed set of values.For example we can represent
employee type with the following enum:

enum EmployeeType{
Permanent = 1,
Contractor,
}

One important thing to understand is the type of values enum members can be assigned.Enum members can be assigned values which can be computed at compile time.In the above example both the enum members are assigned constant values.
Enum member Permanent is assigned value 1 explicitly.The value of enum member Contractor is computed from the value of Permanent enum member.The value of Contractor is 2 ,which is calculated by adding 1 to the previous enum member value.

Though we have assigned contant value in the above example ,but we can assign any numeric expression.The only condition is that the numeric expression should be computed at compile time.

We can declare enum variable of the as:

let employeeType = EmployeeType.Permanent;

Converting Enum value to string

For converting string value of an enum member use the following:

EmployeeType[EmployeeType.Permanent]

The above code will return the value of the enum member Permanent of enum EmployeeType to string.

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TypeScript is used for object oriented programming.ECMAScript 6 also supports object oriented programming.

Like other object oriented languages ,classes in typescript have a constructor.If we don’t explicitly specify constructor
then it will have a default constructor.This default constructor initializes the properties of class with default values.

A constructor is declared in typescript using the following syntax :

class Student{
constructor(private name: string, private rollno: string) {

}
}

If you have worked with other object oriented languages then you can immediately see the difference in the constructor definition above.
1.Unlike other languages constructor is declared using the keyword constructor.
2.Properties are not required to be explicitly initialized with the constructor parameters.These are called Parameter properties
The above constructor declaration is identical to the following:

class Student{

private name: string;
private rollno: string;

constructor(name: string, rollno: string) {
this.name = name;
this.rollno = rollno;
}

If we declare constructor parameter using the access modifier private then it initializes private property.
We can create an object of the class using similar syntax as other languages:

var student = new Student("ajay","A005);

 

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Client such as web browser requests a resource (such as HTML document)at a specific URL.In response ,server returns the resource at the requested URL to the client.When the server returns the resource at the requested URL ,it choses a specific representation of the resource such as HTML.

Different client may need different representations of the same resource.For example a client such as web browser may need HTML representation while another client such as a mobile device may require JSON.

The mechanism that decides which representation of the resource should be returned to the client is called Content Negotiation.

How Content Negotiation works

HTTP headers have an important part in Content Negotiation.

Content Negotiation works through the HTTP headers defined by the HTTP protocol.When a client makes a request for a resource ,it indicates which representation or MIME type  it requires by sending the Accept HTTP header in the request.Client can also set the priorities of these MIME type.This means if one representation is not available then next representation is returned by the server based on the priority.

Accept http header

For example to indicate that HTML representation is required ,a client can set the Accept header as:

Accept: text/html

For XML representation client will set the accept header as:

Accept:Application /XML

Content-Type http header

The server responds with the requested representation and sets the Content-Type header. Content-Type denotes the MIME type of the returned representation of the requested resource.For example if the server is returning JSON then it will set Content-Type header as:

Content-Type: application/json

This will indicate to the client that the returned response is JSON and will render it accordingly.

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